development and application of a technique for three ... · computed tomography (cbct) is a...

Development and Application of a Technique for Three-Dimensional Sialography

Using Cone Beam Computed Tomography

By

Fatima M. Jadu BDS, MSc

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Oral Radiology

Graduate Department of Dentistry

University of Toronto

© Copyright by Fatima M. Jadu BDS, MSc 2012

ii

Development and Application of a Technique for Three-Dimensional Sialography

Using Cone Beam Computed Tomography

Fatima M. Jadu BDS, MSc

Doctor of Philosophy

Graduate Department of Dentistry

University of Toronto

2012

ABSTRACT

Introduction: Salivary gland obstructive conditions are common and may necessitate

imaging of the glands for diagnosis and management purposes. Many imaging options

are available but all have limitations. Sialography is considered the gold standard for

examining obstructive conditions of the parotid and submandibular glands but it is

largely influenced by the imaging technique to which it is coupled. Cone beam

computed tomography (cbCT) is a relatively new and very promising imaging modality

that has overcome many of the inherent limitations of other imaging modalities used in

the past for sialography. Materials and methods: A RANDO®Man imaging phantom

was used to determine the effective radiation doses from the series of plain film images

that represent the current standard of practice for sialography. Similar experiments were

then undertaken to determine the effective radiation doses from cbCT when varying the

field-of-view (FOV) size and center, x-ray tube peak kilovoltage (kVp) and milliamperage

(mA). Next, cbCT image quality, measured using the signal-difference-to-noise-ratio

(SDNR) was used to determine those technical factors that optimized image quality.

iii

Finally, using the optimized image acquisition parameters, a prospective clinical study

was conducted to test the diagnostic efficacy of cbCT sialography compared to plain

film sialography. Results: Effective radiation doses were comparable between the plain

film image series and cbCT examinations of the parotid and submandibular glands

when a 6” FOV was chosen, and when the x-ray tube was operating at 80 kVp and 10

mA. We also found that these exposure settings optimized the image SDNR. Finally, we

demonstrated that the diagnostic capabilities of cbCT sialography were superior to plain

film sialography with regards to detecting sialoliths and strictures, and when

differentiating normal salivary glands from those with changes secondary to

inflammation. Conclusion: We have successfully developed a three dimensional (3D)

sialography technique for imaging the parotid and submandibular salivary glands using

cbCT that balances radiation effective dose with image quality. We also demonstrated

the superior diagnostic capabilities of the new technique in a clinical setting.

iv

ACKNOWLEDGEMENT

I would like to express my sincere gratitude to my supervisor Dr. EWN Lam for his

unwavering support and for trusting me to be his first PhD student.

A special thanks to my research committee members Drs. Pharoah, Yaffe and Jokstad

for their constructive criticism and insightful comments.

My sincere thanks also go to Dr. Baghdady, Perschbacher and Petrikowski for kindly

volunteering their time and expertise to this project.

To my best friends Susanne and Mariam, thank you for being my personal cheer squad.

I am indebted to the residents in the Oral and Maxillofacial Radiology graduate program

for helping with this project, Drs. Alsufyani, Madhavji, Khalifa, Chan, Lukat and

Amintavakoli. Thank you for being amazing stimulating students. I especially wish to

thank Dr. Madhavji for putting his computer genius to my service.

I am grateful to the support staff in the oral radiology department for always, always

helping me out.

Lastly, and most importantly, I wish to thank my husband Ahmed for encouraging me to

soar high in the skies of graduate education and research, and my parents who instilled

in me the love of knowledge.

I dedicate this PhD to my lovely children Saeed, Yousef and Maryam…I hope this

experience was a lesson in the rewards of hard work and dedication.

v

TABLE OF CONTENT

Abstract ............................................................................................................................ii

Acknowledgement ...........................................................................................................iv

Table of content .............................................................................................................. v

List of Tables ................................................................................................................. viii

List of Figure ...................................................................................................................ix

List of Appendices ...........................................................................................................xi

1 Introduction ............................................................................................................... 1

1.1 Review of the literature ...................................................................................... 1

1.1.1 Development of the salivary glands ............................................................. 1

1.1.2 Anatomy of salivary glands .......................................................................... 4

1.1.3 Histology of salivary glands ......................................................................... 8

1.1.4 Physiology of salivary glands ..................................................................... 11

1.1.5 Diseases of salivary glands ....................................................................... 15

1.1.6 Imaging of salivary glands ......................................................................... 25

1.1.7 Cone beam computed tomography ........................................................... 34

1.2 Summary .......................................................................................................... 41

1.3 Statement of the problem ................................................................................. 42

1.4 Aims ................................................................................................................. 42

1.5 Hypotheses ...................................................................................................... 43

1.6 Null hypotheses ................................................................................................ 43

vi

2 A comparative study of the effective radiation doses from cone beam computed

tomography and plain radiography for sialography. ...................................................... 44

2.1 Introduction ...................................................................................................... 44

2.2 Objectives ........................................................................................................ 45

2.3 Null hypotheses ................................................................................................ 45

2.4 Materials and methods ..................................................................................... 46

2.5 Results ............................................................................................................. 50

2.6 Discussion ........................................................................................................ 54

3 Optimization of exposure parameters for cone beam computed tomography

sialography .................................................................................................................... 62

3.1 Introduction ...................................................................................................... 62

3.2 Objectives ........................................................................................................ 64

3.3 Null hypotheses ................................................................................................ 65


3.5 Results ............................................................................................................. 71

3.6 Discussion ........................................................................................................ 75

4 a comparative study of the diagnostic capabilities of 2d plain radiography and 3d

cone beam computed tomography for sialography ....................................................... 80

4.1 Introduction ...................................................................................................... 80

4.2 Objectives ........................................................................................................ 82

4.3 Null Hypothesis ................................................................................................ 82


4.5 Results ............................................................................................................. 88

vii

4.6 Discussion ........................................................................................................ 97

5 General Discussion ............................................................................................... 104

5.1 Conclusion ..................................................................................................... 120

5.2 Future directions ............................................................................................ 122

References .................................................................................................................. 124

Copyright Acknowledgement ....................................................................................... 135

viii

LIST OF TABLES

Table 1: Summery of functions of saliva. ....................................................................... 14

Table 2: The selected locations for measuring the absorbed radiation dose in the head

and neck of the RANDO®Man phantom. ....................................................................... 47

Table 3: Reproducibility of the thermoluminescent dosimeter (TLD) chips

measurements. ............................................................................................................. 53

Table 4: The different combinations of operational parameters that were used for the

cbCT scans. .................................................................................................................. 68

Table 5: The effective radiation doses that were used to calculate the figure-of-merit

(FOM) for the different technique combinations. ........................................................... 71

Table 6: List of the radiographic features and findings that were reviewed. .................. 86

Table 7: Radiologic interpretation and identification of features as determined by the

reviewers. ...................................................................................................................... 91

Table 8: Overall percent agreement and positive and negative percent agreements for

cone beam CT and plain imaging sialography. ............................................................. 95

ix

LIST OF FIGURE

Figure 1: Photomicrograph of a developing salivary gland. ............................................. 3

Figure 2: Diagram of the anatomy of the parotid gland. .................................................. 7

Figure 3: Diagram of the anatomy of the submandibular and sublingual glands. ............ 7

Figure 4: Diagram of the different terminal end pieces found in histologic sections of

salivary glands. ............................................................................................................. 10

Figure 5: Plain films for imaging the major salivary glands. ........................................... 31

Figure 6: Ultrasound (US) image of the right parotid gland. .......................................... 31

Figure 7: Computed tomography (CT) images of the left submandibular gland. ........... 32

Figure 8: Magnetic resonance images (MRI) of the left parotid gland. .......................... 32

Figure 9: Lateral skull radiograph of a left submandibular gland sialogram using plain

film. ................................................................................................................................ 33

Figure 10: Scintigraphy of the major salivary glands. .................................................... 33

Figure 11: Illustration demonstrating the basic principle of cbCT. ................................. 35

Figure 12: Variation of effective radiation dose (E) for the parotid gland. ...................... 51

Figure 13: Variation of effective radiation dose (E) for the submandibular gland. ......... 52

Figure 14: Sialogram of left submandibular gland. ........................................................ 61

Figure 15: Photograph (a) and axial cbCT image (b) of the imaging phantom. ............. 67

Figure 16: Four example cbCT images. ........................................................................ 69

Figure 17: Cone beam CT image showing the regions-of-interest (ROI)....................... 69

Figure 18: Variation of pixel signal-difference-to-noise ratio (SDNRp) with different kVp

and mA settings. ........................................................................................................... 72

x

Figure 19: Variation of pixel signal-difference-to-noise-ratio (SDNRp) relative to iodine

concentrations in the phantom. ..................................................................................... 73

Figure 20: Calculated figure-of-merit (FOM). ................................................................. 74

Figure 21: Plain and cbCT images of a left submandibular gland sialogram. ................ 98

Figure 22: Plain and cbCT images of a left parotid gland sialogram. .......................... 100

xi

LIST OF APPENDICES

Appendix 1: Research ethics board approval letter ..................................................... 136

Appendix 2: Patient consent form ............................................................................... 137

1

1 INTRODUCTION

1.1 REVIEW OF THE LITERATURE

The salivary glands are exocrine glands that play an important role in

maintaining the well-being of the oral cavity through the function of producing

saliva.1 There are three pairs of major salivary glands: parotid, submandibular, and

sublingual.1 These are located outside the oral cavity but have ductal systems that

deliver their salivary secretions into the oral cavity.1 As well, the minor salivary

glands are numerous and scattered throughout the oral submucosa.1

An array of disorders can affect the salivary glands and many require

imaging for the purposes of diagnosis, management, and follow up. The choice of

imaging depends on the clinical presentation and the provisional diagnosis, and

this may include one or more of the following techniques: projection radiography

(i.e. plain imaging), computed tomography (CT), magnetic resonance imaging

(MRI), sialography, and scintigraphy. As well, a variant of CT, cone beam

computed tomography (cbCT), has been recently used to image the salivary

glands.

1.1.1 DEVELOPMENT OF THE SALIVARY GLANDS

The parotid glands are the first of the major salivary glands to develop at

four to six weeks in utero. They are followed by the submandibular glands which

develop late in the sixth week and the sublingual glands which develop at seven

to eight weeks.1,2 The minor salivary glands develop later during the 12th week of

intrauterine life.1,2 Development of all salivary glands follows the same process

2

which starts with a focal thickening of oral epithelium. This epithelium proliferates

into the underlying ectomesenchyme forming an epithelial bud.1 Proliferative

condensation of the underlying ectomesenchyme around the epithelial bud is a

vital step in the development process because it will promote the differentiation of

the oral epithelium to glandular epithelium (Figure 1).1 During this time, the

epithelial bud remains connected to the surface epithelium via an epithelial cord

and undergoes further clefting to form more buds. This process of clefting and

continued bud formation is termed branching morphogenesis .1

With time, the epithelial cord undergoes canalization to develop a lumen.

The process of canalization occurs as a result of differential mitotic activity

between the outer cell layers of the cord which are rapidly dividing and the more

slowly dividing inner cell layers.1,2 Lumen development starts in the distal portion

of the cord, then in the proximal portion and finally in the central portion. When

luminal development approaches the terminal buds, they subdivide into two cell

thick terminal end pieces.1,2 The innermost layer of the terminal end piece

differentiates into secretory cells and the outermost layer differentiates into

contractile myoepithelial cells.1

The major salivary glands undergo a further developmental step; that of

encapsulation with fibroconnective tissue.2 The connective tissue not only

surrounds and supports the gland but it also forms the septa between its lobes

and lobules, and carries the nerve, vascular, and lymphatic supply to and from

the parenchyma of the gland.1,2 Interestingly, encapsulation of the parotid gland

occurs after the submandibular and sublingual glands, but before the

3

development of the lymphatic system.2 This discrepancy in timing leads to

entrapment of lymph tissue (nodes and channels) in the parotid but not in the

submandibular and sublingual glands.2 Approximately ten lymph nodes are

usually found in close association with the parotid gland, most of which are found

in the superficial lobe.2 These nodes drain the ipsilateral upper and midface skin

and the palatine tonsils, and in turn drain into the internal jugular chain of lymph

nodes.1

Figure 1: Photomicrograph of a developing salivary gland.

The image demonstrates the proliferation of the surface oral epithelium

(arrowheads) into the underlying mesenchyme (MES) which, in response, is

condensing around it.1

(Copied with permission from Oral Histology: development, structure, and

function. Mosby, St. Louis, MO)

4

1.1.2 ANATOMY OF SALIVARY GLANDS

The paired parotid glands are the largest salivary glands, weighing

between 14 and 28 grams each (Figure 2).1,2 The facial nerve (cranial nerve VII)

which exits the skull through the stylomastoid foramen, pierces the posterior

surface of the parotid gland and courses through its parenchyma in an anterior

and inferior direction lateral to the retromandibular vein.2 While still in the

parenchyma of the gland, the nerve then divides into its five terminal branches

(temporal, zygomatic, buccal, mandibular, and cervical).1-3 Traditionally, the

plane of the facial nerve and its five terminal branches marks the anatomic plane

that divides the parotid gland into superficial and deep lobes. Glandular tissue

located lateral to this plane is considered to be part of the superficial lobe and

any glandular tissue medial to the plane is considered to be part of the deep

lobe.2 Som et al believe that this landmark is anatomically incorrect. Rather, they

believe the posterior border of the mandibular ramus to be a more accurate

dividing line.2 Using this definition, the larger superficial lobe of the parotid gland

lies lateral to the mandibular ramus and masseter muscles, anterior and inferior

to the external auditory meatus extending from approximately the zygomatic arch

superiorly to the angle of the mandible inferiorly.2 In contrast, the smaller deep

lobe of the parotid gland lies posterior and medial to the mandibular ramus but

anterior to the styloid process and the carotid sheath.2 Both anatomic landmarks

for dividing the parotid lobes are currently in use.2

The parotid duct (Stensen’s) leaves the anterior border of the superficial

lobe and runs an anterior course that is inferior to the zygomatic arch and

5

superficial to the lateral surface of the masseter muscle.1,3 At the anterior border

of the masseter muscle the duct turns sharply medially, pierces the buccal fat

pad and buccinator muscle to open into the oral cavity opposite the maxillary

second molar.1,3 Stensen’s duct measures approximately 6 mm to 7 mm in length

with a lumen caliber of 1 mm to 2 mm.2 Accessory parotid tissue is present in

approximately 20% of the population and is usually found anterior to the

superficial lobe and superior to Stensen’s duct.2

Parasympathetic innervation of the parotid gland which regulates

secretion, is received from the glossopharyngeal nerve (cranial nerve IX) which

has a synapse in the otic ganglion and reaches the gland via the

auriculotemporal nerve (branch of the mandibular division of the trigeminal nerve,

cranial nerve X).1,3 Sympathetic innervation which regulates vasoconstriction, is

derived from the sympathetic plexus on the carotid artery.2 Blood supply is

provided by branches from the external carotid artery.2,3

The submandibular gland is the second largest salivary gland, weighing

between 10 and 15 grams (Figure 3).1 It is divided into two lobes by the posterior

free border of the mylohyoid muscle.3 The larger superficial lobe is located in the

submandibular triangle between the mylohyoid muscle and the mandibular fossa

on the medial aspect of the posterior mandibular body.2,3 The smaller deep lobe,

on the other hand, lies superior to the mylohyoid muscle in the posterior floor of

the mouth, medial to the mandibular body.3 The submandibular duct (Wharton’s)

emerges from the deep lobe and courses anteriorly and superiorly between the

sublingual gland laterally and the genioglossus muscle medially to open

6

immediately lateral to the lingual frenum.2,3 Wharton’s duct is approximately 5

mm long with a lumen caliber that ranges between 1 mm and 3 mm.2 The gland

receives parasympathetic innervation from the chorda tympani branch of the

facial nerve (cranial nerve VII) through the lingual nerve and the submandibular

ganglion.2,3 It receives its sympathetic innervation from the sympathetic plexus

around the carotid artery like the parotid gland.2,3 Blood supply to the

submandibular gland is provided by the external maxillary and lingual arteries.2

The smallest of the major salivary glands is the sublingual weighing

approximately 2 to 4 grams (Figure 3).1,2 It is located in the anterior floor of the

mouth between the sublingual fossa on the medial aspect of the anterior

mandibular body laterally and the genioglossus muscle medially.3 It is separated

from the genioglossus muscle by the lingual nerve and Wharton’s duct.2 Saliva is

secreted into the oral cavity though a number of ducts (the ducts of Rivinus) that

open like pores upwards into the sublingual fold.1-3 Occasionally, these ducts

fuse and form Bartholin’s duct which opens into Wharton’s duct.2 Nerve supply to

the sublingual gland is identical to the submandibular gland but the blood supply

is provided by the sublingual artery.3

The minor salivary glands are many, estimated to be between 600 and

1000 aggregates scattered throughout the oral submucosa with the exception of

the anterior hard palate and gingiva.1 They are also found in the submucosa of

the paranasal sinuses, pharynx, larynx, trachea, and bronchi.2 Depending on

their location, they receive autonomic secretory innervations from several ganglia

including the pterygopalatine, otic, and submandibular.2

7

Figure 2: Diagram of the anatomy of the parotid gland.

(Copied with permission from Head and Neck Imaging. Elsevier, St. Louis, MO)

Figure 3: Diagram of the anatomy of the submandibular and sublingual glands.


8

1.1.3 HISTOLOGY OF SALIVARY GLANDS

The structure of any salivary gland follows the same general pattern; a

main excretory duct which branches into smaller lobar ducts, even smaller

interlobular ducts, and finally intralobular ducts.4 The intralobular ducts consist of

the larger striated ducts and the smaller intercalated ducts.1 The lumen of

intercalated ducts is continuous with the blind terminal secretory end pieces or

acini which are either spherical (serous) or tubular (mucous) in shape.1

Surrounding the terminal end pieces and intercalated ducts are contractile

myoepithelial cells.1 These cells are stellate in shape around the acini to help

expel saliva from the acini and fusiform in shape around the ducts to help

maintain the patency of the duct lumens.1 Myoepithelial cells are also believed to

have other functions such as producing antiangiogenic factors and proteins with

tumour suppressor activity.1 The basic histologic structure of a salivary gland is

presented in Figure 4.

For all salivary glands, the main excretory duct is lined with epithelium that

ranges from stratified squamous near the oral cavity to pseudostratified columnar

near the lobar ducts with scattered mucous (goblet) cells.1 The lobar ducts are

lined with epithelium that ranges from high columnar to stratified cuboidal while

interlobular ducts are lined with high columnar epithelium.4

The intralobular ducts and terminal end pieces differ for each salivary

gland. The terminal end pieces of the parotid glands are all spherical and of the

serous type.1 This type of acinus is made up of pyramidal cells that have

spherical basal nuclei, abundant secretory granules, and surround small central

9

lumens (Figure 4).1 Intercalated ducts are numerous and long in the parotid

glands and they are lined with cuboidal epithelium with round central nuclei and

sparse cytoplasm.1 Striated ducts, however, are lined with columnar epithelium

with centrally located nuclei.1

In the submandibular glands, most of the terminal end pieces are serous,

like the parotid gland.1,2 The remainder of the terminal end pieces are mixed

units with mucous tubules capped by serous demilunes (Figure 4).1 In

comparison to serous acini, mucous tubules have larger lumens surrounded by

pyramidal cells with flat basal nuclei (Figure 4).1 Serous demilunes are similar in

structure to serous acini but they empty their secretions into small intercellular

canaliculi that form finger like projections between the cells of the mucous tubule

(Figure 4).1 The submandibular intercalated and striated ducts are structurally

similar but less numerous than those found in the parotid gland.1

The sublingual gland, like the submandibular gland, is considered a mixed

gland with regard to its terminal end pieces because it is made up of mucous

tubules (these are the most abundant) and mucous tubules capped with serous

demilunes.1 Serous acini are rare in the sublingual gland.1 Both intercalated and

striated ducts are fewer and shorter than in the parotid and submandibular

glands.1

The minor salivary glands are made up of predominantly mucous terminal

end pieces, few of which are capped with serous demilunes.1 The exception to

this rule are the minor salivary glands found in the troughs around the

circumvallate papillae on the dorsum of the tongue and the foliate papillae on the

10

sides of the tongue. These are made-up of purely serous acini.1 The ducts of

minor salivary glands are generally less well developed than those of the major

salivary glands.1

Figure 4: Diagram of the different terminal end pieces found in histologic sections of

salivary glands.


11

1.1.4 PHYSIOLOGY OF SALIVARY GLANDS

The main function of salivary glands, both major and minor, is to produce

saliva.1 However, the contribution and composition of saliva differs between

glands.

Parotid glands are the largest salivary glands and therefore they produce

a significant amount of the total saliva volume (approximately 45% or 450 to 675

mL/day).2 Their secretion is mostly serous, rich in amylase and glycoproteins.1

Submandibular glands are the second largest salivary glands and they also

secrete approximately 45% of the total saliva volume. Submandibular gland

secretions are more mucinous in nature due to some of their terminal end pieces

being mucous tubules with serous demilunes.1,2 Sublingual glands contribute 5%

of the total saliva volume (50 to 75 mL/day) and secrete saliva that is more

viscous because most of their terminal end pieces are mucous tubules. Minor

salivary glands also contribute about 5% of the total saliva volume but their saliva

is purely mucinous and rich in secretory immunoglobulin A (IgA).1,2 A minority of

minor salivary glands, as mentioned earlier, are made up of serous acini and

secrete serous saliva.1 Therefore, the saliva found in the oral cavity is termed

mixed or whole saliva because it is composed of differing amounts of all these

saliva types plus desquamated oral epithelial cells, microorganisms and their

products, serum components and inflammatory cells.1

Production of saliva takes place in the acini. This primary saliva is isotonic,

high in sodium and low in potassium.2 Saliva then undergoes modification in the

striated ducts where sodium is reabsorbed and potassium is excreted, making it

12

hypotonic.1,2 Saliva is predominantly made up of water (99.5%) with a specific

gravity of 1.002 to 1.012.2 The production of saliva, especially from the major

salivary glands, is under the control of the autonomic nervous system and is

prompted by stimulation.4 Therefore, more saliva is produced during the day

when there is more chemical, mechanical, and olfactory stimulation with a total of

approximately 1 to 1.5 L of saliva produced in 24 hours.1

Saliva has many functions (Table 1).1 It protects the oral tissues by

mechanically washing away nonadherent bacteria and other debris, and its

mucin content forms a barrier that protects the delicate oral mucosa from

microbial and mechanical trauma.1 Saliva also has buffering capabilities. It

contains ions such as bicarbonate and phosphate that neutralize the acids

produced by cariogenic bacteria, and protects the enamel from demineralization.1

Saliva also contains urea and ammonia, byproducts of bacteria in the oral cavity,

both of which contribute to increasing the pH in the oral cavity and creating an

environment unfavorable for cariogenic bacteria to grow.1

The major immunoglobulin in saliva is secretory IgA which prevents

certain pathologic bacteria from adhering to the oral tissues by causing them to

agglutinate.1 Several proteins in saliva exhibit antimicrobial properties.1

Lyzoymes and peroxidases are two examples of salivary proteins with

antibacterial properties that prevent the growth of cariogenic bacteria.5 Histatins

are another example of salivary antimicrobials but they are potent antifungals

that restrict the growth of opportunistic fungal infections.5 Saliva also plays a

major role in the post-eruption maturation of the crystalline structure of enamel

13

because it contains vital ions such as calcium, phosphorus, and fluoride that

diffuse into the enamel structure rendering it harder and more resistant to

demineralization.1 The process of ion diffusion from saliva to the structure of

enamel also allows the remineralization of early caries lesions.1

Dissolving food substances and presenting them to the taste receptors on

taste buds is another function of saliva.1 Saliva also plays a role in maintaining

the healthy well-being of taste buds.1 For digestion, saliva’s water and mucin

content help form the food bolus.1 Saliva also contains amylase and lipase which

start the process of digesting starches and triglycerides respectively.1 Although

not yet confirmed, the growth factors and trefoil proteins present in saliva are

believed to play a role in hastening clot formation and in advancing wound

repair.1

14

Table 1: Summary of functions of saliva.1,5

Function Effect Active substance

Protection Mechanical washing

Barrier

Water

Mucin

Buffering Neutralize acids

Increase the pH

Bicarbonate and phosphate

Urea and ammonia

Antimicrobial

Barrier

Antibodies

Antibacterial

Antifungal

Mucin

Secretory IgA

Lysozyme, peroxidase

Histatin

Tooth integrity Enamel maturation

Enamel remineralization

Calcium, phosphate

Fluoride

Taste

Dissolve substances

Maintain taste buds

Water, lipocalins

Epidermal growth factor,

carbonic anhydrase VI

Digestion Form food bolus

Digest starch and triglycerides

Water, mucin

Amylase, lipase

Tissue repair Promote wound healing and clot

formation

Growth factors

Trefoil proteins

15

1.1.5 DISEASES OF SALIVARY GLANDS

Pathologic conditions of the salivary glands are many but they are broadly

categorized into inflammatory conditions, non-inflammatory conditions, and

space occupying masses.6 Inflammatory conditions are the most common

abnormalities to affect the salivary glands, and may involve the parenchyma of

the gland (sialadenitis),or the ductal structures (sialodochitis), or both.6 An array

of causes result in inflammation of the salivary glands but infections (bacterial or

viral) are the most common cause of acute inflammation.1,2 Bacterial infections of

the salivary glands are more often than not the result of a retrograde infection

from the oral cavity by one of the following organisms: Staphylococcus aureus,

Streptococcus viridans, Streptococcus pneumoniae, Haemophilus influenzae,

Streptococcus pyogenes, and Escherichia coli.2 Retrograde infections are the

direct result of a decrease in salivary flow which is a serious consequence of

many conditions including dehydration, bulimia, salivary gland obstruction,

therapeutic radiation to the head and neck, certain systemic diseases (diabetes

mellitus, Sjögren syndrome) and some medications (diuretics, antihypertensives,

antidepressants).2 These infections more commonly involve the parotid glands

because Stensen’s duct orifice is larger than Wharton’s duct orifice, and the

saliva produced by the parotid glands is more serous in nature as opposed to the

more viscous mucinous saliva produced by the submandibular and sublingual

glands. Mucin rich saliva contains antibacterial substances such as IgA

antibodies and lysosomes that agglutinate bacteria and prevent their adherence

to the epithelial cells of the ducts.1 The typical clinical presentation of acute

16

inflammation due to bacterial infection is tender swelling of the infected salivary

gland, swelling of the adjacent lymph nodes, and pus at the orifice of the gland.2

Treatment of choice is antibiotics but caution must be exercised because

inadequate or delayed treatment may result in formation of intraglandular

abscesses which may requires more extensive management and surgical

drainage.2

Many viral agents can infect the salivary glands but mumps is by far the

most common.2 It is considered the most common condition to affect the salivary

glands in children.7-9 Mumps is caused by a ribonucleic acid (RNA) virus of the

paramyxovirus group that usually affects the parotid glands bilaterally although

the submandibular and sublingual glands may be infected as well.2 Mumps

produces acute painful swelling of the infected glands and some cases of mumps

may be preceded by a prodromal period of general malaise.2 Some cases of

mumps may be subclinical, and a few cases may develop complications such as

thyroiditis.2 Viral infections generally are self-limiting and their management is

supportive rather than curative.

Obstruction is not only the most common cause of chronic inflammation of

the salivary glands but is also the most common condition to affect them.7,9

Obstruction affects nearly 1% of the general population with a peak incidence

during the fourth to sixth decade of life.7-9 Primary causes of obstruction include

sialoliths, ductal strictures, and mucous plugs. In contrast, secondary causes of

obstruction include trauma to the ductal structures and space occupying masses

that impinge on the ductal system. Sialoliths are the most common cause of

17

salivary gland obstruction, accounting for approximately 66% to 73% of all cases

of obstruction.7,9 The mechanism of sialolith development is not fully understood

but is thought to start by the presence of an intraductal inorganic nidus, on which

layers of organic (carbohydrates and amino acids) and inorganic substances

(calcium phosphate, ammonium, carbonate) from the saliva are deposited.2,10

Even though sialoliths can affect both the major and minor salivary glands they

are most common in the submandibular glands.2 This is due to several factors;

the narrower orifice of Wharton’s compared to Stensen’s duct, the upward

sloping of Wharton’s duct, and the thicker mucinous saliva produced by the

submandibular glands.2 Additionally, the saliva produced by the submandibular

glands is rich in hydroxyapatite and phosphate and has a higher pH which helps

facilitate their precipitation around the nidus.2 Management of sialoliths depends

on many factors including their number, location, and the effects they have

exerted on the gland structure. It ranges from non-invasive, radiologically guided

basket retrieval of the stone to complete excision of the gland and its ductal

systems.2,11 Lithotripsy of sialoliths was attempted in the early 1990s but the

results were variably successful and therefore this modality is no longer

considered a management option.2

Non-inflammatory conditions of the salivary glands include any non-

inflammatory, non-neoplastic condition of the salivary glands.6 Most of these

conditions result in secretory abnormalities, namely xerostomia.12 Xerostomia is

the subjective feeling of dry mouth that results from a decrease in the quantity of

saliva.13 It is believed to affect between 17% and 29% of the population with a

18

female predilection.13 The three most common causes of xerostomia are

medications, Sjögren syndrome, external beam radiation therapy to the head and

neck and radioactive iodine treatment.13 The classes of medications that induce

xerostomia as a side effect is extensive and includes antihypertensives,

antidepressants, diuretics, and antihistamines, to name a few.13 Patients with

xerostomia usually complain of a burning sensation or soreness in the mouth,

loss or altered taste, and difficulty swallowing.13 Upon intraoral examination, the

oral tissues are found to be erythematous, the tongue fissured, and the salivary

glands difficult to milk.13 An increased incidence of cervical caries and

candidiasis (caused by the opportunistic fungus Candida albicans) are also

noted.13 Objectively, the diagnosis of xerostomia is made if the amount of

unstimulated whole saliva collected in 15 minutes is less than 1.5mL or the

amount of stimulated whole saliva collected in one minute is less than 0.1 mL.13

Normal flow rates for unstimulated whole saliva range between 0.3 and 0.5

mL/minute and for stimulated whole saliva the range is 1 to 2 mL/minute.14

Salivary gland scintigraphy has also been used to assess salivary gland

function and to confirm the diagnosis of xerostomia based on a delayed uptake,

reduced concentration, and/or delayed excretion of the intravenously injected

radiopharmaceutical technetium-99m (99mTc) pertechnetate (TPT).2,13

Scintigraphy of the salivary glands will be discussed in detail in the next section

on salivary gland imaging. Management of xerostomia involves managing the

underlying cause and alleviating the patient’s symptoms.13 If the cause of

xerostomia is drug induced then an alternate drug may be recommended or the

19

drug dose may be modified.13 Parasympathomimetic drugs such as pilocarpine

hydrochloride have effectively been shown to stimulate salivary secretion in

patients with xerostomia caused by Sjögren syndrome and radiation therapy to

the head and neck.13 Palliative measures to manage xerostomia include frequent

sipping of water and the use of saliva substitutes which are available

commercially in a multitude of formulas such as chewing gum and mouth

rinses.13 To prevent the complication of dental caries, patients with xerostomia

must adhere to a rigorous oral hygiene program, restrict their dietary intake of

sugar, and apply topical fluoride at regular intervals.13 Candidiasis in xerostomic

patients is managed with oral or systemic antifungals.13

Three conditions of the salivary glands, sialadenosis, Sjögren syndrome,

and post-irradiation sialadenitis are of particular interest to us because patients

with these conditions are often referred for sialography. Sialadenosis, also known

as sialosis, is a non-inflammatory, non-neoplastic, non-tender, chronic or

recurrent enlargement of primarily the parotid glands.6 The condition may present

itself in other major salivary glands and is usually bilateral in distribution.2 A

variety of endocrine diseases, especially diabetes mellitus, can cause

sialadenosis, and parotid enlargement is sometimes the first presentation of the

underlying disease.2 Sialadenosis is associated with a number of nutritional

abnormalities but most commonly it is associated with chronic alcoholism and

alcoholic cirrhosis.2 Some medications cause sialadenosis including non-

steroidal anti-inflammatory drugs (NSAID) and certain antibiotics.2 The

enlargement noted in sialadenosis is due to hypertrophy of the salivary gland

20

acini, and this may result in xerostomia. The xerostomia can resolve completely if

the underlying cause is managed before fatty atrophy of the gland parenchyma

has taken place.2 As such, imaging the enlarged gland with CT or MRI reveals

nonspecific enlargement with fibrous or fatty changes, depending on the stage of

the condition.2 In contrast, sialographic changes noted in sialadenosis are

specific and significantly different from those found in cases of sialadenitis and

Sjögren syndrome and allow a definitive interpretation to be made confidently.2

These sialographic changes of sialadenosis are a normal ductal system that may

be slightly splayed due to glandular enlargement and normal parenchymal

blush.2

Sjögren syndrome is the second most common systemic autoimmune

condition after rheumatoid arthritis.2 It is a disease of the exocrine glands that

primarily affects the salivary and lacrimal glands and predominantly affects

females (90% to 95% of patients diagnosed with the syndrome are females

between the fourth and sixth decade of life).2 There are two forms of the

syndrome, a primary form that occurs alone (also known as sicca syndrome) and

a secondary form that is associated with other autoimmune connective tissue

diseases.2 The most common of the connective tissue diseases to be associated

with Sjögren syndrome is rheumatoid arthritis but others including systemic lupus

erythematosus (SLE), progressive systemic sclerosis, and polymyositis may also

occur.2 Histopathologically, Sjögren syndrome is characterized by a periductal

CD4-positive T-lymphocytic infiltrate that destroys the acini of the exocrine

glands and reduces their secretion resulting in dryness.2 The lymphocytic

21

infiltrate may form localized solid or cystic masses in the parenchyma of major

salivary glands that are known as benign lymphoepithelial lesions (BLEL).2 It

must be noted, however, that BLEL can occur in the major salivary glands

independent of the autoimmune condition.2 The criteria for diagnosing Sjögren

syndrome are:

1. Clinical symptoms of dry mouth (xerostomia) and these were discussed

earlier.2

2. Clinical symptoms of dry eyes (xerophthalmia).2 Patients with xerophthalmia

usually complain of dry eyes, grittiness in the eyes, or the need to frequently

use ocular lubricants.2

3. Clinical signs of xerostomia.2 Confirmation of xerostomia was discussed

earlier but for Sjögren syndrome specifically, one must add the

pathognomonic sialographic changes of homogenous collections of contrast

material distributed throughout the parenchyma without evidence of

obstruction.2,6

4. Clinical signs of xerophthalmia. Objective tests for xerophthalmia include the

Schirmer's test without anesthesia (considered positive if <5 mm of tears are

collected in 5 minutes) and the Rose Bengal score (considered positive if the

stain intensity score is >4 according to van Bijsterveld's scoring system).2

5. Elevated levels of autoantibodies in the serum. Specifically antibodies

against the sicca syndrome A (SS-A)/RO and sicca syndrome B (SS-B)/LA

antigens because these are elevated in the serum of 80% of patients with

Sjögren syndrome and their absence excludes the diagnosis.2 These

22

antigens have two names (for example SS-A and RO) because they were

discovered by different researchers at different times but then they were

found to be immunologically identical (RO and LA represent the initials of the

first patients in which these antigens were discovered).15

6. Histologic evidence of lymphocytic infiltrate in a salivary gland (at least 2

lymphocytic foci, each with more than 50 lymphocytes per 4 mm2).2 This can

be obtained from an excisional biopsy of a labial minor salivary gland or an

incisional biopsy of the parotid gland.2

The diagnosis of primary Sjögren syndrome is based on the presence of four of

the six above listed criteria, or three of the objective criteria listed from 3 to 6.

Secondary Sjögren syndrome is diagnosed in the presence of another

autoimmune condition plus one subjective criterion (items 1 and 2) and two

objective criteria (items 3 through 6).

Post-irradiation sialadenitis is inflammation of the salivary glands following

external beam radiation therapy to the head and neck or radioactive iodine 131

(131I) treatment.16 External beam radiotherapy to the head and neck is used to

manage patients with cancers of the oral cavity and pharynx. Patients usually

receive 60 to 70 Gray (Gy) in increments of 2 Gy over 6-7 weeks. The radiation

results in an inflammatory reaction in the salivary glands that impinges on the

ductal structures causing them to obstruct and the gland to swell.16,17 The

glandular swelling is often painful and bilateral, and most evident in the parotid

glands because they are the most radiosensitive of the salivary glands.2 A

decrease in the quantity of saliva ensues due to acinar cell death.16,18 This is

23

followed by alterations in the quality of saliva so that it contains more proteins

and electrolytes and less amylase.16,18 Ultimately, the salivary glands undergo

atrophy and fibrosis and the loss of salivary flow becomes permanent. Therefore,

every effort is made during treatment planning to reduce the total radiation dose

to the parotid glands and/or reduce the volume of parotid tissue irradiated. If the

parotid glands receive a total dose that is less than their tolerance dose of 25 to

30 Gy, then the damaged acinar cells are likely to be replaced and salivary flow

is likely to be restored.19 This reparative process is gradual and may take up to

two years.19 On the other hands, if parts of the parotid glands are removed from

the treatment field or are shielded and spared the tumouricidal dose, then these

parts undergo hyperplasia and an increase in salivary flow is noted

approximately six to twelve months following radiation treatment.2

Radioactive iodine 131 is an effective treatment for some thyroid

abnormalities because it is efficiently absorbed and concentrated by thyroid

cells.20,21 Unfortunately, 131I is also absorbed by salivary gland parenchymal cells

and secreted in saliva.20 The disease mechanism for 131I induced sialadenitis is

similar to that of external beam radiotherapy induced sialadenitis and involves an

inflammatory reaction that causes obstruction and painful swelling of primarily the

parotid glands.20 The parotid glands are most sensitive because they are made

up primarily of serous cells that concentrate 131I better than the mucous cells that

are found in other salivary glands.21 A dose-dependent decrease in salivary flow

rate is seen in 69% of patients who receive 100 to 200 mCi of 131I.20 Initially, 131I

induced sialadenitis was thought to be a transient condition but more evidence is

24

emerging to support the progressive nature of this condition that leads to fibrosis

and atrophy of the salivary glands.18 A sialogram performed in the early stages

after irradiation demonstrates areas of parenchymal fill voids where atrophy of

the acini has started to occur.2 In later stages, sialograms are not helpful

because infusion of a completely fibrotic gland with contrast material is

exceedingly difficult. Salivary gland scintigraphy findings also vary depending on

the time lapse from treatment.18 Early findings of scintigraphy include normal

uptake of TPT but delayed excretion and in later stages even the uptake of TPT

is delayed.18 Advanced imaging of the major salivary glands with CT or MRI

reveals small and dense fibrotic glands.2

Space occupying masses of the salivary glands can be subdivided into

cystic and neoplastic processes.6 Cysts of the salivary glands are rare

accounting for less than 5% of all salivary gland masses. They may be

subdivided into developmental (branchial, lymphoepithelial, dermoid) and

acquired (sialocysts and AIDS related parotid cysts).2,6 Salivary gland tumours

are uncommon and represent less than 3% of all head and neck tumours.2,6 Most

tumours that affect the salivary glands are benign or low grade malignancies.6

They most commonly affect the parotid glands (80%) followed by the minor

salivary glands (10% to 15%), the submandibular glands (5%), and finally the

sublingual glands (1%).6 Fortunately, the likelihood of a benign tumour affecting

the salivary glands is directly related to the size of the gland; larger glands are

more likely to have benign tumours.6 As such, only 20% of tumours that affect

the parotid glands are malignant. This percentage jumps to 50% to 60% for the

25

submandibular glands, 60% to 75% for the minor salivary glands, and 90% for

the sublingual glands.6 Pleomorphic adenoma is by far the most common type of

tumour to affect both the major and minor salivary glands (75% of all salivary

gland tumours).6 It arises from the ductal epithelium and contains both epithelial

and mesenchymal components hence it is also termed a benign mixed tumour.6

The most common malignant salivary gland tumour is mucoepidermoid

carcinoma which accounts for approximately 30% of all salivary gland tumours.6

1.1.6 IMAGING OF SALIVARY GLANDS

Diagnostic imaging plays an important role in the management of patients

presenting with signs and symptoms related to the major salivary glands.

Imaging may be useful in assessing the nature of the abnormality, the extent of

involvement, the effects on the adjacent anatomic structures, and possibly, the

causes.

The major salivary glands can be imaged using one or more of the

following techniques: projection radiography (i.e. plain imaging), ultrasound (US),

computed tomography (CT), magnetic resonance imaging (MRI), sialography,

and scintigraphy. Plain films (Figure 5) whether intraoral such as occlusal

radiographs, or extraoral such as panoramic radiographs provide a relatively

quick and inexpensive way to demonstrate calcified sialoliths.2,6 The clinical

applicability of projection radiography, however, is limited because only

moderately sized and fairly dense calcifications can be identified.2

26

Ultrasound (Figure 6) like plain film is widely available and is a relatively

safe imaging technique because it does not utilize ionizing radiation.2,6 It is

primarily used to guide biopsies, differentiate solid from cystic masses in the

superficial portions of the parotid and submandibular glands, and guide choices

for further imaging.2,6,22 Recent advances in technology have allowed US to be

more specific in interpreting relatively common salivary gland conditions such as

Sjögren syndrome.2 However, its diagnostic accuracy with regards to identifying

sialoliths is still low.2,6 The major shortcoming of US is its inability to penetrate

deep tissues.2

Computed tomography (Figure 7) is an excellent imaging modality for

evaluating the major salivary glands especially when intravenous contrast is

administered.6 It is regarded by many as the modality of choice for imaging

inflammatory conditions of these glands.2 Computed tomography also has a

sensitivity of nearly 100% for detecting masses in the major salivary glands.2

Unfortunately, CT alone cannot differentiate benign from malignant masses

because benign masses have capsules that give them a smooth well-defined

contour when imaged and low grade malignancies have pseudocapsules that

can also give them a smooth well-defined outline.2 Fortunately, when CT findings

are combined with clinical findings, the distinction between benign and malignant

masses can be made in 90% of cases.2 With regard to obstructive conditions, CT

demonstrates large calcified sialoliths with great sensitivity but fails to

demonstrate small and non-calcified ones and it fails to show the ductal changes

that result from chronic obstruction.2

27

Magnetic resonance imaging (Figure 8) has several advantages over CT.

MRI does not use ionizing radiation, eliminates streak artifacts from dental

restorative material, and is better able to differentiate benign from malignant

masses because of its superior soft tissue contrast resolution.2,6 Malignancies of

the salivary glands for example are more cellular and dense than benign lesions,

and thus have low to intermediate signal intensities on all MRI sequences.2 In

contrast, benign masses have a higher water content and thus a lower T1 signal

intensity and a higher T2 signal intensity.2 Signal changes in T1 and T2 weighted

MR images are also helpful in cases of inflammation because they reflect the

degree of edema versus infiltration by inflammatory cells.2 Magnetic resonance

imaging, however, is not the imaging modality of choice for obstructive conditions

of the salivary glands because of its low spatial resolution, long acquisition time,

and the signal voids that are associated with calcified structures such as

sialoliths.2,22

Sialography (Figure 9) is a functional examination of the parotid and

submandibular salivary glands that was first performed in 1902.6 It depicts the

delicate ductal structures of the salivary glands following the introduction of an

iodinated contrast agent through the orifice of the gland duct.6 The gland is then

imaged with ionizing radiation (plain film, CT, fluoroscopy).6 This examination is

not used for the sublingual or minor salivary glands although sometimes the

sublingual gland is incidentally filled during examinations of the submandibular

gland.6 When appropriately performed, the contrast material fills the ductal

system of the gland giving it an appearance that is similar to that of a branching

28

tree.6 If acinar fill is also achieved, then the tree is said to come into bloom and

the appearance is called parenchymal blush.6 Five minutes following the

procedure, the gland is re-imaged to assess for retention of the contrast

material.6 If the contrast material is not completely cleared from the gland it is

indirectly inferred that gland function is reduced and another image at the ten

minute time point may be indicated. The procedure is indicated in cases of

suspected salivary gland obstruction to evaluate the cause and location of the

obstruction and to assess the extent and severity of the resultant changes to the

gland.2,6 Rarely, the procedure is used to dilate mild ductal strictures.6

Contraindications of the procedure include active infection of the gland in

question because the minimally invasive nature of the procedure may introduce

the infectious agent further into the gland.6 The procedure is also contraindicated

in cases of acute inflammation because of the associate pain which becomes

more intense with injection of the contrast material.6 Other contraindications

include an allergy to iodine compounds and an immediately anticipated thyroid

function test because some of the iodine in the contrast agent is taken up by the

thyroid gland and this may interfere with the results of the test.6 Sialography can

also be combined with MRI, A combination that not only eliminates ionizing

radiation but also the need for injecting a contrast material into the salivary gland

ducts. MR sialography involves giving the patient a secretogogue and then

utilizing the patient’s own saliva as a contrast agent. The patient is then imaged

using one of many suggested protocols that generally include heavily weighted

T2 images and a surface coil.2 Although the procedure is painless and

29

noninvasive because it eliminates the need for catheterization, it is time

consuming and usually complicated by patient movement and swallowing which

make producing clinically useful images difficult.2 Therefore, MR sialography is

no longer routinely performed.2

Scintigraphy (Figure 10) is also a functional examination of the major

salivary glands with the added advantage of being able to examine all the major

salivary glands at once.2,6 Unlike the previously mentioned imaging techniques,

scintigraphy is a radiologic examination that does not examine the morphologic

anatomy but rather the physiologic function of a tissue or organ.6 Scintigraphy is

also known as radionuclide imaging because it uses a radioactive molecule that

emits gamma rays.6 When injected intravenously, the radiopharmaceutical

distributes in the body and is selectively concentrated by certain tissues.2 Then

when it starts to decay, a solid state scintillation camera is used to detect gamma

emissions.6 Glandular tissues including the thyroid and salivary glands uptake,

concentrate, and excrete the radiopharmaceutical technetium-99m (99mTc)

pertechnetate (TPT).6 Technetium-99m (99mTc) is a metastable isotope of

technetium with a short half-life of 6 hours, and pertechnetate is the water soluble

ion that carries and distributes it in the body. In the salivary glands the

concentration of TPT reaches a maximum at about 30 to 45 minutes.6 A sialogog

(lemon juice) is then administered to assess the excretory function of the glands.6

Lesions are identified through either an increased, decreased or absent uptake

and excretion of TPT.6 Certain salivary gland tumours, namely Warthin’s tumours

and oncocytomas characteristically concentrate TPT.2,6 Scintigraphy of the

30

salivary glands like all other nuclear medicine imaging studies has high sensitivity

but low specificity which in addition to its low resolution limits its clinical

applicability.6

31

Figure 5: Plain films for imaging the major salivary glands.

Panoramic radiograph (A) and asymmetric mandibular occlusal radiograph (B) of the

same patient demonstrating a large calcified sialolith in the right submandibular gland

duct. Both images can be used to detect calcified sialoliths and to assess the

surrounding osseous structures.

Figure 6: Ultrasound (US) image of the right parotid gland.

The image demonstrates a solid mass in the superficial lobe of the gland.6

(Copied with permission from Oral Radiology: Principles and Interpretation

Mosby/Elsevier, St. Louis, MO)

A

B

32

Figure 7: Computed tomography (CT) images of the left submandibular gland.

Soft tissue algorithm axial CT images at the level of the mandible (A) and at the level of

the submandibular region (B) demonstrating a well-defined, expansile lesion within the

gland with an internal homogeneous low attenuation.

Figure 8: Magnetic resonance images (MRI) of the left parotid gland.

Axial T1 weighted MR image (A) and T2 weighted MR image (B) of the left parotid gland

demonstrating a mass in the tail of the gland. The mass has a low T1 signal and a

higher T2 signal which is suggestive of a benign neoplasm.

A B

B A

33

Figure 9: Lateral skull radiograph of a left submandibular gland sialogram using plain

film.

Figure 10: Scintigraphy of the major salivary glands.

Right and left oblique views demonstrate an increased uptake of technetium-99m

(99mTc) pertechnetate (TPT) by the right parotid gland (arrowhead) which is highly

suggestive of a Warthin’s tumour or an oncocytoma.6



34

1.1.7 CONE BEAM COMPUTED TOMOGRAPHY

Cone beam computed tomography (cbCT) was first developed in 1982 for

angiography.8 It was introduced in 1998 for maxillofacial imaging and since that

time, has found many dental applications.8,23 The technology generates a cone

shaped x-ray beam, and hence the name. The x-ray beam is emitted from a

source that is rigidly connected to a two dimensional (2D) detector both of which

are mounted on a rotating gantry that revolves around the patient’s head 360°

during a single scan.8,24 Based on the type of detector, cbCT units are classified

into those that use an image intensifier tube (IIT) charged coupled device (CCD)

combination and those that use a flat panel imager (FPI).24

1.1.7.1 Principle

During image acquisition, multiple x-ray exposures are made at fixed

intervals around the patient.8 The photons recorded by the detector after each

exposure constitute the “raw data” that is fed into a computer algorithm to

produce a single cross sectional projection image (basis image) made up of

picture elements (pixels).8 Each pixel is assigned a grey shade value that

corresponds to the linear attenuation coefficient of the tissue it represents.8 At

the end of a scan, between 150 and 599 basis images are produced, offset from

one another by an angle depending on the location of the x-ray source and

detector. Together these images form multiple sequential projection images

known as the “projection data”.8,23,24 The projection data are fed into

sophisticated viewing software programs to generate a three dimensional (3D)

35

volumetric data set (primary reconstruction) which can be viewed in its entirety

from any angle.23,24 The volumetric data set can also be reconstructed into the

three orthogonal planes (axial, coronal, sagittal) (secondary reconstruction).23,24

Scan times range between five and forty seconds depending on the cbCT unit

used and the protocol setting chosen.23

Figure 11: Illustration demonstrating the basic principle of cbCT.



1.1.7.2 Image acquisition

Many commercial cbCT units are available, and although they all follow

the same basic principles of image acquisition, significant differences exist

between the different makes and models. Some units scan the patient in the

sitting position while others scan the patient in the standing or supine position.8

Most units use a pulsed x-ray beam to expose the patient but some use a

36

continuous x-ray beam that significantly increases the radiation dose to the

patient.8 One method used by some cbCT units to reduce the radiation dose to

the patient is called “automatic exposure control” where the peak kilovoltage

(kVp) and milliampere (mA) settings are automatically adjusted depending on the

intensity of the detected x-ray beam.8 Alternatively, the kVp and mA settings are

adjusted automatically based on the exposure of a scout image, or manually by

the operator.8 Field-of-view (FOV) size is another very important factor that

affects the radiation dose to the patient because limiting the scan volume

significantly reduces the radiation dose to the patient.8 Cone beam CT units may

be arbitrarily divided into small FOV (8 cm or less) and large FOV (larger than 8

cm) units. As mentioned earlier, the number of basis images in a single scan can

range from 150 to 599.8 Their total number depends on two factors: the rotational

speed of the source and the detector and the frame rate.8 The frame rate is the

speed with which basis images are acquired and is measured in frames per

second.8 Generally, higher frame rates increase the available data for

reconstruction, reduce image noise, reduce metallic artifacts but require a longer

scan time and consequently deliver higher effective radiation doses to the

patient.8

1.1.7.3 Image reconstruction

Image reconstruction is a complex process.8 Typically, an acquisition

computer acquires the image data and corrects the basis images for inherent

pixel imperfections and uneven exposure.8 The corrected images are then

37

transferred to a processing computer where the remainder of the data processing

and image reconstruction takes place.8 The time for image reconstruction varies

depending on acquisition factors such as FOV size, as well as hardware and

software capabilities.8

1.1.7.4 Image display and optimization

The smallest subunit of a volumetric data set is termed the voxel and,

unlike conventional CT, cbCT voxels are isotropic (that is their x, y, and z

dimensions are all equal). Depending on the system, voxel sizes can range from

0.07 to 0.4 mm.23 Smaller voxels result in images with greater resolution.24

Voxels are 3D reconstructs of pixels and so they too are assigned grey shade

values that represent the x-ray attenuation coefficient of the tissue(s) they

represent.8,23 The ability to display attenuation as different shades of grey is

termed bit depth.8 Cone beam CT units are capable of producing 12- and 14-bit

images.23 Twelve bit images display 212 (4,096) shades of gray and 14 bit images

display 214 (16,384) shades of gray.23 However, the display of grey shades is

limited by computer monitors which can display only eight bit images (256

shades of gray) at a time.23 To overcome this limitation, software developers use

a technique called “windowing and leveling”.23 Windowing allows the data to be

visualized eight bits at a time along a spectrum where air and soft tissues (low-

attenuation structures) are at one end of the spectrum, and bone and teeth (high-

attenuation structures) at the other end.23 Leveling is the adjustment of the image

38

contrast and brightness by the operator to reach a subjective, personal

optimum.23

Other display options include multiplanar reformation (MPR), shaded

surface display and volume rendering.23 Multiplanar reformation is the process of

viewing the data as 2D images of slices or slabs along any straight or curved

line.8 Shaded surface display is the process of selecting an attenuation range so

that any structures that fall outside this range are not visualized.23 Volume

rendering allows the operator to assign a level of transparency to all the

structures imaged.23 All cbCT units include viewing software but third party

software is also available depending on the task required.23

1.1.7.5 Image artifacts

As the x-ray beam which is polychromatic in nature passes through the

object to be imaged, lower energy x-ray photons are attenuated by the object

first.8 This results in an increase in the mean energy of the residual x-ray beam

which is said to become “harder”.25 The phenomenon of beam hardening results

in three types of artifacts: streak, dark bands and cupping artifacts.8,25 Streaks

and dark bands occur when the x-ray beam is hardened to different degrees as it

passes through different parts of a heterogeneous object at various tube

positions.26,27 This type of artifact is common in bony regions of patients and

when contrast material is used.26,27 Cupping artifacts result when the x-ray beam

passing through the thicker middle portion of an object becomes harder than the

beam passing through the thinner edges of the object.26,27 The harder beam is

39

attenuated less and the resultant image appears darker in the center.26,27 Beam

hardening software is used by manufacturers to correct for the these artifacts and

sometimes overcorrection is attempted to minimize blurring at the bone-soft

tissue interface of an image.26,27 In some instances, capping artifacts result and

the reverse becomes true with the edges of the image appearing darker than the

center.26,27

Patient movement causes artifacts in the form of blurriness of the resultant

image.8 To overcome this problem, manufacturers provide head rests and straps

to stabilize the patients’ head during a scan.8 Ring artifacts are due to detector

imperfections or poor calibration.8 Partial volume averaging artifact is commonly

seen in cbCT images.8 This artifact occurs when dense parts of a heterogeneous

object protrude partway into the FOV causing shading artifacts to appear.8 Under

sampling is another common cbCT artifact that appears as a noisy reconstructed

image with streaks when the number of basis images is reduced.8 The divergent

nature of the cbCT x-ray beam may also result in artifacts like distortion and

peripheral noise.8

1.1.7.6 Advantages, applications, and limitations

One of the most important advantages of cbCT is its high spatial

resolution. Resolution is inversely proportional to the voxel size and is influenced

by the isotropic nature of cbCT voxels.8 Another important advantage of cbCT is

the reduced radiation dose to the patient compared with medical multidetector

CT.8 Extensive dosimetry studies have been conducted and report an effective

40

radiation dose that ranges between 5.3 µSv and 38.3 µSv for one small FOV

cbCT unit (Kodak 9000 3D, Carestream Health Inc., Rochester, USA) and

between 68 µSv and 1073 µSv for larger FOV cbCT units.25 In comparison,

conventional CT examinations of the jaws deliver an effective radiation dose that

ranges between 474 µSv and 1410 µSv depending on the protocol chosen.25

These two advantages combined with the reduced scan time, equipment size,

and cost compared to conventional CT have made this imaging modality ideal for

the maxillofacial complex including investigation of major salivary gland

conditions.

Presurgical dental implant site assessment is one of the more common

uses of cbCT.8 It is becoming the standard of practice and this is largely due to

the availability of software algorithms that allow the practitioner to view, make

accurate dimensional measurements, and annotate anatomic structures.8,28

Other applications of cbCT include: localizing impacted teeth, identifying

dentoalveolar fractures, assessing the osseous structures of the

temporomandibular joint (TMJ), and assessing maxillofacial growth and

development.8,24

Cone beam CT like any other imaging modality has its limitations and one

of its major limitations is background noise. Background noise in cbCT is

primarily due to the divergent x-ray beam exposing a large volume of tissue.8

This results in more Compton interactions which in turn results in the production

of more scatter photons that carry non-linear attenuation information.8

Unfortunately, these photons are recorded by the detector and they degrade the

41

quality of the resultant image.8 Another cause of background noise in cbCT is the

inhomogeneity of the x-ray beam and the large size of the area detector which

makes the number of photons reaching a unit area of detector variable and

smaller.8 Poor contrast resolution is another significant limitation of cbCT. This is

also due to the divergent geometry of the x-ray beam and the increased noise.8

Inherent imperfections in flat panel detectors also contribute to the poor soft

tissue resolution seen in cbCT images.8

1.2 SUMMARY

The salivary glands are exocrine glands. There are 3 paired major salivary

glands, the parotid, submandibular and sublingual glands, and numerous minor

salivary glands located throughout the submucosal spaces of the head and neck.

Their sole purpose is the production of saliva which aids in the maintenance of the

healthy well-being of the oral tissues. The salivary glands develop in the early

weeks of intrauterine life in a delicately timed process. Many diseases can affect

the salivary glands but the most common are the obstructive conditions. Imaging

plays an important role in diagnosing diseases of the salivary glands and in

planning management. Many imaging techniques are available for imaging the

salivary glands but each has limitations. Cone beam CT is a relatively new and

promising imaging technique that overcomes many of the inherent limitations of

other imaging techniques and offers many advantages. However, it too has its

limitations and one of these limitations, poor soft tissue resolution, has restricted

its clinical applications.

42

1.3 STATEMENT OF THE PROBLEM

Obstruction is the most common condition to affect the salivary glands.

Imaging plays a major role in identifying the condition and in planning

management. The ideal imaging modality should determine the cause of the

obstruction, the location, as well as the secondary changes in the ductal structures

and parenchyma of affected glands since these findings may significantly influence

the choice of management. Sialography is the examination of choice for

obstructive conditions of the parotid and submandibular glands because of its high

specificity and depiction of the delicate ductal structures of these glands. However,

its success in identifying abnormalities is dependent on the imaging modality to

which it is coupled. Sialography combined with plain films poses an interpretation

challenge because of overlapping structures in the resultant 2D images. The

feasibility of CT for sialography is limited by the significant radiation dose delivered

to the patient and by the anisotropic resolution of the images. Limitations that arise

from coupling sialography with MRI include long acquisition times and low spatial

resolution. Cost and accessibility are further limitations of both CT and MRI. These

limitations are overcome by cbCT, a 3D imaging modality that offers advantages

such as short scan times, relatively low radiation doses, and images with isotropic

voxel resolution.

1.4 AIMS

Develop and optimize a protocol for cbCT sialography based on radiation dose

delivery to the patient and image quality.

43

Compare the diagnostic performance of sialography utilizing cbCT and plain

imaging for the parotid and submandibular glands.

1.5 HYPOTHESES

Varying technical factors associated with cbCT imaging will result in effective

radiation doses equal to or less than those associated with plain film imaging of

the parotid and submandibular glands.

Varying technical factors associated with cbCT imaging will result in image

quality improvements to cbCT images.

The diagnostic capability of cbCT sialography of the parotid and submandibular

glands will be superior to that of plain film sialography of the same glands.

1.6 NULL HYPOTHESES

Varying technical factors associated with cbCT imaging will result in no

changes to the effective radiation doses which will be no different than the

doses associated with plain film imaging of the parotid and submandibular

glands.

Varying technical factors associated with cone beam CT imaging will result in

no changes to the image quality of cbCT images.

The diagnostic capability of cbCT sialography of the parotid and submandibular

glands will be no different than those of plain film sialography of the same

glands.

44

2 A COMPARATIVE STUDY OF THE EFFECTIVE RADIATION DOSES FROM CONE BEAM

COMPUTED TOMOGRAPHY AND PLAIN RADIOGRAPHY FOR SIALOGRAPHY.

2.1 INTRODUCTION

Imaging of the major salivary glands has been performed using one of four

techniques: sialography, ultrasound (US), computed tomography (CT), and

magnetic resonance imaging (MRI). Of these methods, sialography has proven to

be the most effective technique to assess salivary gland function and obstructive

conditions of the salivary glands, in particular.6 Sialography was first performed in

1902,8 and depicts the ductal structures of the salivary glands following the

introduction of a contrast agent into the orifice of a salivary gland duct.

Sialography is largely dependent on the imaging modality to which it is

coupled. Traditionally, sialography has been combined with projection radiography

(i.e. plain film), and this has since become the gold standard to which all other

imaging techniques are compared.29 More recently, three dimensional (3D)

depictions of gland ductal anatomy have been possible by combining sialography

with CT or MRI, but these investigations have several limitations including cost

and accessibility. Also, in the case of MRI and single-row CT, prolonged imaging

times may cause the visualization of the uptake of contrast material to be missed

due to its rapid clearance from the gland.30

Today, the rapid acquisition of a 3D image volume using cone beam

computed tomography (cbCT) has enabled us to overcome the temporal

limitations of medical CT and MR image acquisition. Cone beam CT promises to

45

revolutionize the practice of oral and maxillofacial radiology by offering major

advantages such as short scan times and less radiation dose to the patient.31

Consequently, it has found several applications in the field of dentistry such as

localization of impacted teeth and imaging of the temporomandibular joint.12,32

Imaging of the major salivary glands, however, is one application that has not yet

been explored.

As a first step in developing a protocol for 3D cbCT sialography, we

conducted a dose assessment study comparing this novel methodology with two

dimensional (2D) plain film sialography. Dosimetry studies are crucial, especially

when the head and neck area is being examined because of the number of

critically radiosensitive organs that are in the field being imaged.

2.2 OBJECTIVES

Examine the effect of varying the field-of-view (FOV) size and center, peak

kilovoltage (kVp) setting, and milliampere (mA) setting on the effective radiation

dose (E) using cbCT.

Calculate and compare E for cbCT imaging of the parotid and submandibular

glands and sialography of the same glands using plain images.

2.3 NULL HYPOTHESES

There is no difference in E when the following cbCT technical parameters are

varied:

a. FOV size (6”, 9”, 12”) and center (parotid vs. submandibular)

46

b. Peak kilovolt setting (80 kVp, 100 kVp, 120 kVp)

c. Milliampere setting (10 mA and 15 mA)

There is no difference in E between cbCT and plain film examinations of the

parotid or submandibular glands.

2.4 MATERIALS AND METHODS

A head and neck RANDO®Man (Radiation Analogue Dosimeter) Alderson

phantom (Alderson Research Laboratories, Stanford, CT, USA) was used in this

study. The phantom is a human skull covered with isocyanate rubber which has

tissue radiation attenuation characteristics equivalent to human soft tissues with

respect to both atomic number and tissue density.33 Only the first ten axial slices,

each 2.5 cm in thickness, of the phantom extending from the top of the head to the

level of the clavicles were used. Following the methods of Ludlow et al (2006),34

the absorbed radiation dose was measured at 25 selected locations (Table 2) that

represent critical radiosensitive organs and sites of special interest to dental

imaging.

47

Table 2: The selected locations for measuring the absorbed radiation dose in the head

and neck of the RANDO®Man phantom.

Thermoluminescent dosimeter (TLD) chips were placed in these locations to record the

absorbed radiation dose.

Organ Location Phantom Level

Brain Pituitary fossa 3

Mid brain 2

Eyes Right/left orbit 4

Right/left eye lens 4

Salivary glands Right/left parotid 6

Right/left submandibular 7

Right/left sublingual 7

Thyroid Surface 9

Midline 9

Pharynx Pharynx 9

Bone marrow Calvarium anterior 2

Calvarium posterior 2

Calvarium left 2

Cervical spine 6

Right/left mandibular body 7

Right/left ramus 6

Skin Right cheek 5

Left back of neck 7

48

Lithium fluoride thermoluminescent dosimeter (TLD-100) chips (Global

Dosimetry Solutions Inc., Irvine, CA, USA), measuring 3 mm x 3 mm x 1 mm, were

used to record the absorbed dose at the 25 locations. Unexposed TLD chips were

also analyzed for environmental calibration during each exposure of the phantom.

Each measurement was made in triplicate.

Imaging was performed with the CB MercuRay unit (Hitachi Medical

Systems, Tokyo, Japan). The phantom was oriented in the cbCT unit with the left

parotid or submandibular salivary gland centered in the image field. The images

were acquired using 3 field-of-view (FOV) sizes (12”, 9” and 6”), 3 peak kilovolt

settings (120 kVp, 100 kVp and 80 kVp) and 2 milliampere settings (15 mA and 10

mA).

The number of plain radiographs made for a sialography series varied

depending on the gland being imaged. Parotid gland examinations included: a

panoramic radiograph, two anterior-posterior skull radiographs, and four lateral

skull radiographs. For the submandibular gland, the anterior-posterior views were

omitted and replaced with one preoperative standard mandibular occlusal

radiograph. The panoramic radiographs were made with the Sirona OrthophosDS

(Sirona, Munich, Germany) using 60 kVp, 14 mA, and an exposure time of 14 s;

the image receptor was a 12.7 cm x 30.5 cm cassette containing the Kodak Lanex

Medium intensifying screens and Kodak T-Mat G PAN film (Eastman Kodak Co.,

Rochester, NY, USA). The skull and occlusal radiographs were exposed using the

S.S. White Spacemaker x-ray unit (S.S. White, Philadelphia, PA, USA); but the

image receptor and the exposure parameters varied for each type of radiograph.

49

For the skull radiographs, the Kodak Directview CR850 PSP system (Kodak

Medical Systems, Rochester, NY, USA) was the image receptor and the exposure

parameters were 70 kVp and 15 mA but the exposure time varied by gland such

that it was 0.8 s for the parotid gland and 0.5 s for the submandibular gland. The

intraoral occlusal radiographs required 0.32 s of exposure time, using 70 kVp and

8 mA; and the image receptor was the size 4 Ultra-speed film (Eastman Kodak Co,

Rochester, NY, USA).

For each radiographic examination or series, effective radiation dose (E)

calculations were based on the following equation:35

E (µSv) = Σ(WTHT)= ΣWTDT FT

WT: Weighting factor for tissue T, where the sum over all tissues, ΣWT = 1

HT: Equivalent dose to tissue T in µSv

DT: Average absorbed dose in the volume of tissue T

FT: Fraction of tissue type T irradiated in that view

Tissue weighting factors were based on the recent ICRP 2007 recommendation in

Publication 103.36 The average absorbed radiation dose was calculated as a mean

of three independent measurements at the 25 sites in the tissues or organs of

interest. Additionally, the fraction of tissue irradiated within each FOV was

estimated, and included in the calculation of E, as proposed by Ludlow et al.34

50

2.5 RESULTS

The effective radiation dose (E) changed in direct relationship to changes

in field-of-view (FOV) size and center, peak kilovoltage (kVp) and milliamperage

(mA) setting for cbCT. Specifically, E decreased from 466 µSv for the largest 12”

FOV to 97 µSv for the smallest 6” FOV, when kVp and mA were held constant at

100 kVp and 10 mA. When kVp was decreased from 120 kVp to 100 kVp, E also

decreased from 932 µSv to 683 µSv when the FOV and mA were held constant at

12” and 15 mA. Lastly, when mA was decreased from 15 mA to 10 mA, E

decreased as well from 435 µSv to 275 µSv while FOV and kVp remained constant

at 9” and 100 kVp respectively. Of note is that exposure time for the CB MercuRay

unit (Hitachi Medical Systems, Tokyo, Japan) is constant at 10 seconds. More

importantly, we found that E calculated for the collective series of plain radiographs

made during sialography of the parotid and submandibular glands was comparable

to the E for cbCT examinations centered on the respective glands when the 6”

FOV was used in combination with 80 kVp, 10 mA. These results are summarized

in Figures 12 and 13 which demonstrate the variability for each individual

radiographic technique.

51

Figure 12: Variation of effective radiation dose (E) for the parotid gland.

Variation of E with different fields-of-view (FOV) size, milliampere (mA) and peak kilovolt

(kVp) settings. This graph compares the cbCT techniques that were centered on the left

parotid gland with the plain film series made during sialography of the same gland.

932

683

466

256

435

275

153 145

97 60 65

0

200

400

600

800

1000

1200

E (

µS

v)

Technique

52

Figure 13: Variation of effective radiation dose (E) for the submandibular gland.

Variation of E with different fields-of-view (FOV) size, milliampere (mA) and peak

kilovoltage (kVp) settings. This graph compares the cbCT techniques that were

centered on the left submandibular gland with the plain film series made during

sialography of the same gland.

The reproducibility of the TLD readings was assessed by computing the

coefficient of variation and an example of these results is presented for the 12” FOV, 80

kVp and 10 mA setting in Table 3.

932

683

466

256

435

275

153

421

261

148 156

0

200

400

600

800

1000

1200

E (

µS

v)

Technique

53

Table 3: Reproducibility of the thermoluminescent dosimeter (TLD) chips

measurements.

The measurements, which were in units of micro-Gray (Gy), were based on cbCT

images made with the following technical parameters: 12” FOV, 80 kVp and 10 mA.

Reading

1 (µGy)

Reading

2 (µGy)

Reading

3 (µGy) SD Mean CV*

Thyroid midline 3.2 2.63 2.89 0.29 2.91 10 %

Thyroid surface 2.42 2.37 2.59 0.12 2.46 5 %

Esophagus 2.76 2.29 2.66 0.25 2.57 10 %

Right mandibular body 2.37 2.45 1.70 0.41 2.17 19 %

Left mandibular body 1.82 1.81 1.32 0.29 1.65 17 %

Right ramus 2.57 2.73 2.11 0.32 2.47 13 %

Left ramus 2.60 2.62 2.13 0.28 2.45 11 %

Cervical spine 1.79 1.73 1.75 0.03 1.76 2 %

Anterior calvarium 2.10 1.63 1.54 0.30 1.76 17 %

Posterior calvarium 1.29 1.26 1.38 0.06 1.31 5 %

Left calvarium 1.51 2.04 1.98 0.29 1.84 16 %

Right parotid gland 2.19 2.43 3.01 0.42 2.54 17 %

Left parotid gland 3.07 3.04 3.28 0.13 3.13 4 %

Right submandibular gland 2.32 2.29 2.27 0.03 2.29 1 %

Left submandibular gland 1.83 2.20 1.98 0.19 2.00 9 %

Right sublingual gland 2.40 2.39 2.18 0.12 2.32 5 %

Left sublingual gland 2.24 2.31 2.17 0.07 2.24 3 %

54

Neck 2.05 3.05 3.31 0.52 3.47 15 %

Cheek 2.78 2.47 2.63 0.16 2.63 6 %

Right lens 3.32 3.43 3.58 0.13 3.44 4 %

Left lens 3.2 3.23 2.92 0.17 3.12 5 %

Pituitary fossa 1.72 1.64 1.52 0.10 1.63 6 %

Midbrain 1.47 1.21 1.58 0.19 1.42 13 %

Right orbit 2.29 2.38 2.29 0.05 2.32 2 %

Left orbit 1.93 2.44 1.67 0.39 2.01 19 %

Mean CV 9 %

*CV: coefficient of variation

2.6 DISCUSSION

The potential radiation hazard from low doses of radiation associated with

diagnostic investigations remains a major concern for both radiologist and patients

alike. In the oral and maxillofacial region, this is especially true because of the

radiosensitive organs in the area being imaged, and because of sporadic reports

that implicate oral and maxillofacial radiography with intracranial pathoses.37 In a

population-based case control study, Longstreth et al found that more than 5 full-

mouth series (10-22 images) performed during a lifetime doubled the risk of

development of intracranial meningioma.37 Consequently, health care providers,

including oral and maxillofacial radiologists, are constantly developing new

technologies and protocols to address the radiation dose concerns of patients

without compromising diagnostic quality or patient care.

55

This study is a first step in the development and application of a novel,

potentially improved technique for sialography. The overall goal of our work is to

modulate radiation doses to levels that would be comparable to those arising from

plain radiography sialography, without compromising the quality of the cbCT

images. Using thermoluminescent dosimeter (TLD) measurements on a

RANDO®Man phantom (Alderson Research Laboratories, Stanford, CT, USA), the

dependence of effective radiation doses on the exposure factors was estimated for

cbCT examinations.

The CB MercuRay system (Hitachi Medical Systems, Tokyo, Japan)

allows several technical factors to be controlled by the operator. Holding all other

technical factors constant, the effects of varying an individual factor on the effective

radiation dose (E) was explored. Reducing the field-of-view (FOV) from the larger

size to the next available smaller size resulted in a significant reduction in the

calculated E by approximately 40%. This reduction is due to some of the TLD chips

no longer being in the primary radiation field; some of the reduction in E may also

be due to elimination of scatter radiation. Likewise, reducing the milliampere

setting from 15 mA to 10 mA resulted in a 37% reduction in E as is expected due

to the reduced output from the x-ray tube. Finally, E decreased by approximately

30% and 60% when the peak kilovolt setting was reduced from 120 kVp to 100

kVp and 80 kVp, respectively. These findings are in general agreement with the

results of other published studies.34,38 The significance of these reductions in E,

however, is only meaningful if the image quality is not compromised because as

the amount of radiation decreases, the amount of noise increases. This tradeoff

56

between amount of radiation and image quality will be address in a future study as

a next step in the process of developing a new protocol for sialography using

cbCT.

The Palomo et al study,38 like the present study, also examined the

influence of different cbCT technical settings on E using a RANDO® phantom and

the CB MercuRay cbCT system (Hitachi Medical Systems, Tokyo, Japan). These

authors noted a dose reduction of 5% to 10% to the tissues that remained in the

path of the direct beam as the FOV decreased from 12” to 9” and 6”. They also

achieved a 38% reduction in E by reducing the kVp from 120 kVp to 100 kVp.

Milliampere settings varied markedly (2 mA, 5 mA, 10 mA, and 15 mA) and

produced a linear and exponential pattern of dose reduction in the same study.

Consistently, altering the technical parameters significantly reduces the radiation

dose, and certainly altering these technical factors should be considered to

maintain patient radiation doses as low as reasonably achievable.

Our results are consistent with other dosimetry studies.34,38 Some of the

differences we report in E are likely related to variations in the methodology of

measuring the absorbed radiation dose and in the calculations of E. Two prominent

differences in the methodology are the centering of the image field that we utilized

as well as the number of times a single TLD chip was exposed. In the current

study, the image field was asymmetrically centered on the salivary gland of interest

(either the left parotid or left submandibular salivary gland). In previous studies, the

image field was centered in the midline of one or both jaws.34,38,39 Because this

study is the first of its kind to calculate E for a specific application of cbCT, imaging

57

the major salivary glands, we believed it to be more appropriate to localize the

image field where we did. We firmly believe that this technical modification has led

to a more accurate estimate of E for cbCT examinations of the parotid and

submandibular glands.

Regarding the number of exposures, the TLD chips in the present study

were exposed only once as was done in the Palomo et al study.38 In most other

similar studies the TLD chips were exposed a minimum of three times, after which

the measured absorbed radiation doses were divided by the number of

exposures.31,34,39 We believe that exposing the TLD chips only once allowed for a

more realistic simulation of the patient situation since patients will undergo only

one cbCT scan during the procedure. It also allowed us to truly examine the

reproducibility of the dose measurement, which was determined by calculating the

coefficient of variation, the mean of which was calculated to 9% for the 12” FOV,

80 kVp, and 10 mA cbCT protocol.

The effective radiation dose was calculated according to the ICRP

definition but, as per Ludlow et al,34 it was necessary to estimate the fraction of

irradiated tissue in each FOV. A unique feature of this study is that the effect of the

variation of the percentage contribution of each tissue with changes in the FOV

size and image field center was taken into account. For example, the fraction of

brain irradiated in the 12” FOV centered on the left parotid gland was 100% but

decreased to 50% in the 9” FOV and decreased even further to 10% in the

smallest 6” FOV. On the other hand, it was estimated that only 5% of the brain was

irradiated in the 6” FOV when the FOV was centered on the submandibular gland.

58

Tailoring the fraction of tissue irradiated for each FOV size and center for this

imaging task may have yielded more meaningful dose results.

The calculated thyroid effective radiation dose was by far the largest

contributor to the overall E in all examinations centered on the submandibular

gland, whether using cbCT or plain radiographs as the imaging modality. In fact, E

calculated for the thyroid contributed 71% and 76% of the total E for cbCT and

plain radiograph examinations, respectively, when centered on the submandibular

gland. Because of the anatomic proximity of the two glands, it is inevitable that the

thyroid will receive more radiation dose during examinations of the submandibular

gland. This may be a source of concern because of the relative high

radiosensitivity of the thyroid, especially in children. Fortunately, obstructive

conditions of the salivary glands are very rare in children and adolescents,40 and it

is unlikely that an individual from that age group will require a sialogram procedure.

As for the risk of thyroid cancer in adults from diagnostic radiation exposure, the

data are still inconclusive and no studies have been able to prove a causal

relationship or even a statistically significant association.41,42 Nevertheless, the

inconclusive nature of the data creates a dilemma for physicians and dentists

interested in imaging the submandibular gland. This dilemma is an excellent

example of where risk versus benefit judgment comes into play; that is, the excess

risk of imaging versus the anticipated benefit to improved patient care and

management. Other tissues that contributed significantly to the overall calculated

E, especially in examinations centered on the parotid gland were the salivary

glands and the bone marrow.

59

Based on the findings of this study, a preliminary protocol for cbCT

sialography has been developed. Following the introduction of radiographic

contrast, a lateral skull plain image is made to ensure adequate filling of the gland

with contrast material. This is then followed by cbCT image acquisition using the

6” FOV, and x-ray tube factors of 80 kVp and 10 mA. After the completion of the

cbCT scan, another lateral skull plain radiograph is made five minutes after

removal of the catheter to assess contrast material clearance as this is regarded

as an indirect indicator of gland function and saliva production. The 2 lateral skull

radiographs are made with the Kodak Directview CR850 PSP system (Kodak

Medical Systems, Rochester, NY, USA) using 70 kVp, 15 mA, and 0.8 s exposure

time. One of the initial cases that were performed according to this protocol is

presented in Figure 14. The cbCT images demonstrate the secondary ductal

structures of the submandibular gland that are not readily apparent on the two

dimensional (2D) plain image due to extravasation of contrast material into the

gland capsule.

In conclusion, the calculated effective radiation doses from cbCT

examinations centered on the parotid and submandibular glands were comparable

to E from plain radiography sialography of the same glands when a smaller FOV

was chosen in combination with lower kVp and mA settings (6” FOV, 80 kVp, and

10 mA). Having acquired the dosimetric data for site-specific imaging using cbCT,

a future goal is to optimize the exposure parameters for cbCT sialography by

proposing kVp and mA settings for the CB MercuRay unit (Hitachi Medical

60

Systems, Tokyo, Japan) that will balance the quality of the images and the

radiation dose to the patient.

61

(a) (b)

(c) (d)

Figure 14: Sialogram of left submandibular gland.

(a) Lateral skull plain radiograph of the gland following contrast administration. This

image was made prior to the cbCT scan to insure adequate fill of the gland with contrast

material, and it demonstrates the lack of visibility of the secondary and tertiary ductal

structures due to extravasation of the contrast material into the gland capsule. (b)

Reformatted sagittal cbCT image of the same gland with the secondary and tertiary

ducts clearly visible. Maximum intensity projection (MIP) cbCT images in the coronal

plane (c) and in the axial plane (d) demonstrating the three dimensional (3D) and

multiplanar capabilities of cbCT.

62

3 OPTIMIZATION OF EXPOSURE PARAMETERS FOR CONE BEAM COMPUTED TOMOGRAPHY

SIALOGRAPHY

3.1 INTRODUCTION

In a previous study we demonstrated the wide range of radiation doses

that can be delivered to the patient from cone beam computed tomography (cbCT)

scans simply by varying three technical parameters: field-of-view (FOV) size, peak

kilovoltage (kVp) setting, and milliamperage (mA) setting.43 Based on the results of

the dosimetry study, we proposed a preliminary protocol for sialography using

cbCT. The recommended protocol entailed centering the gland of interest in a 6”

FOV and using exposure factors of 80 kVp with 10 mA.44 However, this protocol

was based primarily on minimization of dose and did not consider the effect of

varying the technical parameters on image quality. The purpose of the present

study was to further optimize the protocol by maximizing the quality of images

while delivering radiation doses to the patient that are as low as reasonably

achievable.

Radiographic image quality is defined as the amount of information within

the image that allows the radiologist to make a diagnostic decision with a particular

level of certainty.45 It is therefore believed that increasing the image quality

increases the certainty of the diagnostic decision; most often this can be achieved

simply by increasing the exposure technique such as choosing greater kVp or mA

settings. However, higher exposure techniques come at the expense of greater

radiation dose to the patient.46 Therefore, it is crucial when developing a new

63

imaging protocol to consider both radiation dose and image quality and balance

the trade-off between them to produce images that are diagnostically adequate

while minimizing radiation exposure to the patient.

Recently there has been interest in addressing the issue of image quality

for cbCT. In an in vitro study, Liang et al. compared the image quality and visibility

of anatomic structures in a dry mandible between five cbCT units and one

multislice CT (MSCT) system using a subjective image classification system.47

Thirteen different scan protocols were used, and five independent dentist

observers were asked to rate the visibility of eleven anatomic structures of the

mandible on a five-point scale.47 Although they found great variability among the

different cbCT systems, they concluded that the image quality was comparable to

MSCT even though MSCT had less image noise.47 This study did not investigate

the trade-off between image quality and patient dose and no recommendations

were made for a specific cbCT protocol.

Similarly, in 2008 Kwong et al. subjectively evaluated the quality of images

of cadaver heads and dry skulls made with the CB MercuRay unit (Hitachi Medical

Systems, Tokyo, Japan) taken at different exposure settings.48 Three FOVs were

examined (12”, 9”, and 6”) in addition to two kVp settings (100 kVp and 120 kVp)

and four mA settings (2 mA, 5 mA, 10 mA, and 15 mA). They also examined the

effect of filtering the x-ray beam with copper.48 Image quality was defined as: “the

ability of the image to answer our [the authors’] diagnostic question”; this was

determined by having thirty observers from various dental disciplines fill out a

diagnostic quality questionnaire.48 For the 6” FOV images we are interested in,

64

Kwong’s study showed that the image quality was affected by altering the kVp and

mA settings but not by adding a copper filter.48 Images were considered to be of

superior diagnostic quality when greater kVp and mA settings were chosen.48

Although the subjective findings for the 6” FOV images are consistent with our

objective findings, once again the issue of patient radiation dose was not

addressed.

In this study we are interested in maximizing the detection of the

difference between the signal from an iodinated contrast agent injected into a

salivary gland ductal system and the signal from surrounding normal tissue. This

signal detection task can be quantified by the metric, signal-difference-to-noise-

ratio (SDNR). SDNR describes the relative magnitude of useful information to that

of the noise that impairs its detection.

A common method employed to optimize a radiographic imaging

technique is to maximize the ratio of image SDNR to the radiation dose to the

patient while maintaining some required level of SDNR.49 We used this approach

in the present study to optimize the protocol for cbCT sialography based on the

results of an exploration of how SDNR varies with kVp and mA settings. We also

report on an evaluation of the sensitivity of image signal to the iodine concentration

in the contrast agent.

3.2 OBJECTIVES

Examine the effect of varying the peak kilovolt (kVp) setting, milliampere (mA)

setting, and iodine concentration in the contrast agent on the signal-difference-

to-noise ratio (SDNR) of cbCT images.

65

Compare the relative tradeoff between SDNR and the effective radiation dose

(E) for cbCT when kVp and mA settings are varied.

Propose an optimized protocol for cbCT sialography.

3.3 NULL HYPOTHESES

There is no difference in SDNR when the following cbCT parameters are

varied:

a. Peak kilovolt setting (60 kVp, 80 kVp, 100 kVp, 120 kVp)

b. Milliampere setting (10 mA and 15 mA)

c. Iodine concentration in contrast agent (120, 140 160, 180 mg I/ml)

There is no difference in the tradeoff between SDNR and E when the following

cbCT parameters are varied:

a. Peak kilovolt setting (60 kVp, 80 kVp, 100 kVp, 120 kVp).

b. Milliampere setting (10 mA and 15 mA)


An imaging phantom was constructed using a round (20 cm diameter)

water filled Plexiglas container. A dry, human mandible was immersed alongside

four iodine contrast phantoms containing variable concentrations of iodine within

this container. The iodine phantoms (1 cm X 1 cm X 4 cm) are solid radiographic

test objects composed of iodobenzene embedded in epoxy resin.50 At the x-ray

energies used in this study the epoxy resin has an x-ray attenuation that is the

same as water, and this was confirmed by imaging a phantom containing only

66

epoxy (0 mg I/ml) immersed in water for each imaging technique. The iodine

phantoms imaged with the mandible contained iodine concentrations of 180, 160,

140, and 120 mg I/ml. Figure 15a is a cropped photograph of the imaging phantom

and 15b is the corresponding axial cbCT image of the same area of the phantom

acquired with 80 kVp and 10 mA x-ray tube factors.

67

(a) (b)

Figure 15: Photograph (a) and axial cbCT image (b) of the imaging phantom.

The images show the immersed mandible and the iodine contrast phantoms arranged

adjacent to one another. In the photograph, the mandible is seen resting on an object

with wells that functioned to support the mandible and hold the iodine cubes in place.

The iodine cubes are solid radiographic test objects composed of iodobenzene

embedded in epoxy resin. They contained iodine concentrations of 180, 160, 140, and

120 mg I/ml. The image was acquired using 80 kVp and 10 mA and edge artifacts are

noted along the edges of the square shaped iodine phantoms.

120 mg I/ml

140 mg I/ml

160 mg I/ml

180 mg I/ml

68

The CB MercuRay unit (Hitachi Medical Systems, Tokyo, Japan) was used to

image the phantom. All the images were acquired in a 6” FOV but the kVp and mA

settings were varied in the technique combinations outlined in Table 4. Sample images

produced by different exposure factors are displayed in Figure 16. For each technique

combination, three scans were made on three separate occasions to account for cbCT

system variability. Then the raw unprocessed DICOM images were uploaded into

ImageJ (National Institutes of Health, Bethesda, MD, USA) for analysis. Using the same

axial slice from each scan, equal sized regions of interest (ROI), 3.5 mm in diameter,

were chosen in the center of each iodine phantom and in an adjacent area that

contained only background water while avoiding areas with image artifacts as

demonstrated in Figure 17.

Table 4: The different combinations of operational parameters that were used for the

cbCT scans.

10 mA 15 mA

60 kVp 60 kVp 10mA 60kVp 15 mA

80 kVp 80 kVp 10mA 80 kVp 15 mA

100 kVp 100kVp 10mA 100 kVp 15 mA

120 kVp 120kVp 10mA 120 kVp 15 mA

69

Figure 16: Four example cbCT images.

The images were made with the same mA setting of 10 but different kVp settings that

varied from 60 to 120. The images demonstrate the effect of changing the kVp on the

perceived image noise with the image on the far right made with the highest kVp setting

displaying the least noise.

Figure 17: Cone beam CT image showing the regions-of-interest (ROI).

The one outlined with black is the ROII in the iodine phantom and the one outlined in

white is the ROIW in an adjacent area with only background water.

60 kVp, 10 mA 80 kVp, 10 mA 100 kVp, 10 mA 120 kVp, 10 mA

70

The signal difference (SD) was calculated by subtracting the mean pixel value

(MPV) recorded in an adjacent ROI with only background water (ROIW) from that

recorded in an iodine phantom (ROII).6

SD = MPV in ROII - MPV in adjacent ROIW (1)

Noise for each iodine sample measurement was defined as the standard deviation (σ)

of the pixel values recorded in the adjacent ROIW.6

Noise = σ (ROIW) (2)

Then the pixel signal-difference-to-noise ratio (SDNRp) was calculated as the ratio of

Equation 1 to Equation 2.6

SDNRp = SD/Noise

Finally a figure-of-merit (FOM), which is a numerical expression representing

the efficiency of a given system or procedure, was calculated to relate the image quality

to the radiation dose risk.51 It was computed as the SDNRp squared divided by the

effective radiation doses (E) for the parotid salivary gland. The effective doses were

either calculated in our previous dosimetry study or calculated in the same manner for

the technique combinations used in this study and are listed in Table 5.44 The FOM

values are reported in inverse micro Sieverts (µSv)-1.

FOM = SDNRp2/E

The reason the SDNR was squared in the calculation of FOM is because the SDNR is

proportional to the square root of the dose, or in other words the SDNR squared is

proportional to the dose.

71

Table 5: The effective radiation doses that were used to calculate the figure-of-merit

(FOM) for the different technique combinations.

Technique Effective dose (µSv)

60 kVp, 10 mA 27

60 kVp, 15 mA 33

80 kVp, 10 mA 60

80 kVp, 15 mA 71

100 kVp, 10 mA 97

100 kVp, 15 mA 145

120 kVp, 10 mA 120

120 kVp, 15 mA 167

Statistical analysis was performed using the SAS software Version 9.1 (SAS

Institute Inc., Cary, NC, USA). The paired t-test was used to compare the SDNR and

FOM for all the combinations of exposure techniques. The alpha value (p) was set at

0.05.

3.5 RESULTS

The pixel signal-difference-to-noise ratio (SDNRp) was calculated for the iodine

concentration routinely used in our clinic (180 mg I/ml) and was found to range between

2.3 for the technique using 60 kVp and 10 mA and 13.3 for the highest exposure using

120 kVp and 15 mA. By applying the paired t-test, a statistically significant difference

was noted between the images that were made with 60 kVp and the images that were

72

made with 80 kVp, 100 kVp, and 120 kVp. This difference was noted regardless of the

mA setting. In addition, a statistically significant difference in SDNRp was found between

the images generated with 10 mA and 15 mA but this was only true for the 60 kVp and

120 kVp techniques. These results are outlined in Figure 18.

Figure 18: Variation of pixel signal-difference-to-noise ratio (SDNRp) with different kVp

and mA settings.

The error bars represent the maximum and minimum values for each individual

technique. The graph highlights the statistically significant differences that were found

among the various techniques (*).

2.3

9.0 11.2 12.4

3.3

11.0 12.1 13.3

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

60 kVp 80 kVp 100kVp 120 kVp

SD

NR

p

Technique

10 mA

15 mA

*

*

* *

*p< 0.05

73

The effect of the iodine concentration on the image SDNRp is demonstrated in

Figure 19. The SDNRp ranged from 5.8 for the 120 mg I/ml concentration to 9.0 for the

180 mg I/ml concentration when 80 kVp was chosen with 10 mA. No statistically

significant difference was noted among the concentrations when the paired t-test was

applied.

Figure 19: Variation of pixel signal-difference-to-noise-ratio (SDNRp) relative to iodine

concentrations in the phantom.

Four concentrations are shown 120, 140, 160, and 180 mg I/ml; these were imaged

using 80 kVp and 10 mA. The error bars represent the maximum and minimum values

of SDNRp for each concentration. A linear fit to the data is shown by the solid line.

0

2

4

6

8

10

12

120 mg I/ml 140 mg I/ml 160 mg I/ml 180 mg I/ml

SD

NR

p

Iodine concentration

74

The calculated figure-of-merit (FOM) for the different technique combinations

are plotted in Figure 20. SDNRp was used to calculate the FOM. The technique with the

greatest FOM value is the one that employs 80 kVp and 15 mA.

Figure 20: Calculated figure-of-merit (FOM).

FOM was calculated for 8 different combination techniques using 60 kVp, 80 kVp, 100

kVp and 120 kVp and two mA settings (15 and 10). The error bars represent the

maximum and minimum values for each individual technique.

0.2

1.4

1.3

1.3 0.3

1.7

1.0 1.1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

60 kVp 80 kVp 100kVp 120 kVp

FO

M=

(SD

NR

p)2

/E (

µS

v)-

1

Technique

10 mA

15 mA

75

3.6 DISCUSSION

Production of diagnostic images involves a complex interplay of many

different parameters. A solid understanding of these parameters, how they interact,

and how they influence both image quality and radiation dose is crucial in the

process of developing new imaging techniques. Therefore, the protocol we

propose for three dimensional (3D) sialography using cbCT is based on an

extensive evaluation of the parameters that can be adjusted by the operator and

their effect on both radiation dose and image quality.

The calculated pixel signal-difference-to-noise ratio (SDNRp) increased in

proportion to peak kilovoltage (kVp) when the milliamperage (mA) was held

constant. This was expected because as the kVp increases, so does the x-ray

beam energy, the flux emitted from the tube, the x-ray beam penetrability, and thus

the number of x-ray photons reaching the detector. With greater numbers of x-ray

photons per unit area of image receptor, the noise decreases throughout the

image, which in turn increases the SDNRp. However, in our experimental data the

positive relationship between SDNR and kVp is exaggerated by another source of

image noise, spatial non-uniformity. An inverse cupping artifact was present in our

images, which is sometimes known as “capping”.26 This artifact occurs when there

is overcompensation for a cupping artifact; an image artifact caused by beam

hardening. The capping was more pronounced in the images acquired at lower

kVp, where the beam is less penetrating or “softer”. Therefore, the SDNRp could

be improved at the lower kVp settings by appropriate corrections for spatial non-

uniformity (i.e. flat-fielding). A positive relationship was also noted between

76

increases in mA and SDNRp, where, within the observed range of SDNRp

variability, the SDNRp increased by a factor of 5.1 when mA was increased by a

factor of 1.5. This relationship indicates that quantum noise is likely the dominant

factor affecting image performance for each technique combination considered

here. These quantitative results are consistent with the trends observed in the

subjective image quality ratings published by Kwong et al.48

Figure-of-merit (FOM) values were influenced by the choice of kVp and

mA. At the lower kVp settings of 60 and 80, a direct relationship was noted

between FOM and mA but at the higher kVp settings of 100 and 120, an inverse

relation was demonstrated because the dose increased significantly more than the

SDNRp. The only FOM values that were significantly different than the others were

those at 60 kVp. This indicates that among the other examined techniques, where

image quality was adequate, the one that should be adopted as a clinical protocol

for sialography is the one which imparts the lowest effective radiation dose. This

technique is the one that utilizes 80 kVp with 10 mA. These technique factors are

optimized specifically for the CB MercuRay unit (Hitachi Medical Systems, Tokyo,

Japan), which has a limited range of choices of kVp settings (60, 80, 100, and 120)

and of mA settings (10 and 15). Further refinement of the sialography protocol

could be made following the same approach used in this work for other cbCT

systems with different imaging technique parameters.

Suomalainen et al. published an extensive study that evaluated the

radiation dose and image quality of four cbCT scanners and compared them to two

multislice CT (MSCT) scanners.44 Using appropriate phantoms, radiation doses

77

and contrast values were measured. Then radiation doses were calculated and

reported as effective radiation doses.44 The contrast-to-noise-ratio (CNR) was

used as an indicator of image quality, and the modulation transfer function (MTF)

quantity was used to determine the resolving power of each system.44 The authors

concluded that despite the large variation in results, cbCT scanners provide

adequate image quality with smaller effective radiation doses as compared to

MSCT.44 However, the authors were quick to emphasize the importance of

optimizing imaging factors for the different examinations.44 It is our opinion that this

recommendation can be accomplished only by optimizing these parameters for

specific clinical applications of cbCT, and by objectively measuring the trade-off

between image quality and radiation dose as we have done in this study for cbCT

sialography.

Omnipaque® (Iohexil injection 39%, General Electric Healthcare Canada

Inc., Mississauga, ON, Canada) is a low osmolar, nonionic, iodinated contrast

agent that is produced in five concentrations: 140, 180, 240, 300, and 350 mg I/ml

for different indications of use. At our institution and for the indication of

sialography, the 180 mg I/ml concentration is used. Therefore, in this study, we

examined the effect of the two smaller commercially available concentrations (140

and 180 mg I/ml) in addition to two intermediate concentrations (120 and 160 mg

I/ml) in order to fully understand the impact of the iodine concentration on SDNRp

for cbCT sialography. Greater concentrations of the contrast agent were deemed

unnecessary because the injection is locally confined to the ductal structures of the

salivary glands as opposed to vascular injection, where the agent quickly becomes

78

diluted by distribution throughout the whole body. An increase in SDNRp was noted

with the increase in iodine concentration but there was no statistically significant

difference among the different concentrations for the technique using 80 kVp and

10 mA. This relation is due to the substantial signal difference provided by the

attenuation of iodine in the salivary ducts.52

To our knowledge there are no other studies in the English literature that

examine the effect of varying the iodine concentration in contrast agents on the

quality of the resultant images for sialography studies. However, a similarity can be

drawn to studies of CT angiography. Unfortunately, these studies vary in their

results from demonstrating no significant effect of the iodine concentration53 to

showing a statistically significant difference54,55 on vascular enhancement. The

results of these studies are complicated by many other variables such as imaging

techniques and injection parameters.55 One particular study by Wang et al.

examined the effects of two iodine concentrations of the same contrast agent on

image quality and side effects for dynamic CT of the neck.56 They found that both

concentrations produced images of excellent quality but the lower concentration

resulted in fewer immediate minor complications.56 In the current study, we

demonstrated a positive linear trend between the iodine concentration and SDNRp,

as indicated by the linear fit to the data in Figure 18. This relationship is expected

because the signal difference will increase proportionally to the amount of iodine

present. However, no statistical differences in SDNRp were noted among the four

concentrations examined; the highest of which (180 mg I/ml) is the concentration

currently used at our institution for sialography. The likelihood of complications

79

from contrast media in relation to sialography procedures is minimal because of

the localization of the contrast material in the salivary gland ducts; nevertheless, it

would be more reasonable to use the smaller commercially available concentration

of the contrast agent, which is 140 mg I/ml.

In conclusion, we have developed an optimized protocol for cbCT

sialography using the CB MercuRay unit (Hitachi Medical Systems, Tokyo, Japan).

The protocol involves centering the salivary gland of interest in a 6” FOV and

choosing a kVp of 80 with 10 mA. Our future goal is to examine the diagnostic

efficacy of this optimized cbCT sialography technique in a clinical setting and to

compare it to the standard of practice which is plain film sialography.

80

4 A COMPARATIVE STUDY OF THE DIAGNOSTIC CAPABILITIES OF 2D PLAIN RADIOGRAPHY

AND 3D CONE BEAM COMPUTED TOMOGRAPHY FOR SIALOGRAPHY

4.1 INTRODUCTION

Obstructive conditions of the major salivary glands are the most common

abnormalities affecting nearly 1% of the population. From an imaging standpoint,

the major salivary glands can be imaged using one of four techniques: ultrasound

(US), computed tomography (CT), magnetic resonance imaging (MRI), and

sialography.7 Ultrasound offers many advantages because it is inexpensive, widely

available, and safe.57 However, US does not demonstrate all calculi accurately or

ductal damage caused by obstruction and inflammation.57 CT has a greater

sensitivity for sialoliths, but it cannot demonstrate small sialoliths. Moreover, it

cannot demonstrate ductal damage.57 MRI, unlike US and CT, can demonstrate

changes in the ductal structures but calcified sialoliths may be overlooked because

of the signal void associated with calcified structures.57

Sialography is a functional examination of the major salivary glands that

involves the injection of a radiopaque contrast agent into the ductal system of the

gland prior to imaging. It is the only examination that demonstrates the fine,

delicate anatomy of the ductal system, and most accurately visualizes sialoliths

and strictures, two of the most common causes of obstruction.2,7 These capabilities

make sialography the most suitable examination for investigation of obstructive

conditions of the parotid and submandibular salivary glands. The capabilities of

sialography are, however, restricted by the limitations of the imaging modalities to

which it is coupled. Plain imaging has been used extensively with sialography,

81

however the 2D images that are generated may have limited diagnostic capability.

Sialography has also been combined with medical CT but the anisotropic voxel

resolution may not demonstrate the fine anatomy of the gland ductal structures.30

Sialography has also been combined with fluoroscopy but this modality delivers

relatively significant doses of radiation to the patient.58

Recently, we and others have begun using cone beam CT (cbCT) as the

imaging tool for sialography. Cone beam CT overcomes many of the shortcomings

of other imaging modalities and as isotropic voxel resolution. Previously, we have

demonstrated our ability to achieve comparable effective radiation doses between

plain imaging and cbCT sialography using the CB MercuRay system (Hitachi

Medical Systems, Tokyo, Japan), by centering the gland of interest in a 6” field-of-

view (FOV) and using x-ray tube settings of 80 peak kilovoltage (kVp) and 10

milliamperage (mA).44 Specifically, a parotid CBCT examination using the above

mentioned settings resulted in an effective radiation dose of 60 µSv compared to

65 µSv for the plain image sialography series, for the submandibular gland, the

effective radiation dose from CBCT was 148 µSv while from plain image

sialography it was 156 µSv.44 Moreover, we were able to confirm adequate image

quality when using these technical factors in a previous in-vitro study using a

sialography phantom.43

The purpose of this study is to compare the diagnostic capabilities of cbCT

sialography with sialography using plain images. We hypothesize that cbCT

sialography will have similar or greater diagnostic capabilities than sialography with

plain imaging with regard to the visualization of normal gland structures such as

82

the primary duct, the identification of abnormal findings such as sialoliths, and

finally, image interpretation.

4.2 OBJECTIVES

To compare the abilities of cbCT sialography and plain film sialography in:

o Visualizing the primary and secondary ducts and the parenchyma of the

parotid and submandibular glands.

o Detecting abnormal findings in the same glands.

o Interpreting the sialographic examinations.

To assess the reliability of cbCT sialography relative to plain film sialography.

To assess the reproducibility of cbCT sialography findings.

4.3 NULL HYPOTHESIS

There is no difference in the abilities of cbCT sialography and plain film

sialography in:

o Visualizing the primary and secondary ducts and the parenchyma of the

parotid and submandibular glands.

o Detecting abnormal findings in the same glands.

o Interpreting the sialographic examinations.

There is no difference in the reliability of cbCT sialography relative to plain film

sialography.

Cone beam CT sialography findings are not reproducible.

83


Fourty-seven (47) subjects were recruited into this prospective clinical

study over a two year time period from January, 2009 to December, 2010. Subject

inclusion criteria were adults over 18 years of age with a suspected obstructive

condition of a parotid or submandibular gland as determined by history and clinical

examination. Exclusion criteria included acute inflammation of the major salivary

gland of interest, known or suspected allergy to iodinated contrast agents, or an

immediately anticipated thyroid function test. For each subject, a clinical

examination was performed and the following clinical data were collected prior to

the sialography procedure: subject age, gender, medical history, chief complaint,

subject self-assessment of pain presence and pain quality, swelling, abnormal

taste, mouth dryness and provoking stimulus. As well, extra- and intra-oral

examinations were performed to determine the presence of a swelling, and salivary

quantity and quality (clear or cloudy). Ethical approval was obtained from the

University of Toronto Research Ethics Board (appendix 1) and dictated that each

subject should sign and receive a copy of the consent form (appendix 2).

Sialography was performed by a resident in oral and maxillofacial

radiology closely supervised by a faculty member certified as a specialist in oral

and maxillofacial radiology. The orifice of the primary duct of the salivary gland

under examination was dilated with a series of metal probes, and this was followed

by canulation of the primary duct with a 24G (Pajunk Medizintechnologie,

Geisingen, Germany) or 30G catheter (Cook, Bloomington, IN, USA). Between 1

mL and 10 mL of Omnipaque® 180 mg I/ml (Iohexil injection 39%, General Electric

84

Healthcare Canada Inc., Mississauga, ON, Canada) were injected slowly into the

duct of the gland until the subject reported maximum tolerance to a feeling of

pressure in the gland. A lateral skull plain image was then made using the GE

focus system (General Electric Corporation, Henry Schein Ash Arcona, Niagara-

on-the-Lake, ON, Canada) and a photostimulable phosphor (PSP) sensor (CR850,

Carestream, Rochester, NY, USA) to confirm optimal fill of the ductal structures of

the gland prior to the cone beam computed tomography (cbCT) image acquisition.

If contrast fill was deemed inadequate, additional contrast was injected and

another lateral skull plain image was made.

Cone beam CT imaging was performed with the CB MercuRay cbCT unit

(Hitachi Medical Systems, Tokyo, Japan) using a 6” field-of-view (FOV), 80 peak

kilovoltage (kVp) and 10 milliamperage (mA) with the occlusal plane of the

dentition parallel to the floor, and the imaging volume centered on the gland of

interest. Five minutes following catheter removal, a second lateral skull plain image

was made to evaluate contrast clearance from the gland. The two lateral skull plain

images represented the two-dimensional (2D) part of the study, and these were

used for comparison with the cbCT images. This protocol allowed us to acquire

and then compare the images of the same gland in the same subject using both

modalities.

Three certified specialists in oral and maxillofacial radiology reviewed the

images after undergoing a calibration exercise prior to image analysis. The

calibration exercise included a review of twelve archived cases of sialography

performed with plain images, with the aim of standardizing structural appearances

85

of the gland and the definitions of descriptive terms. Prior to review by the oral and

maxillofacial radiologists, all images were anonymized. The three observers

reviewed the plain and cbCT sialographic images for each subject separately with

a wash out period of at least one week. As all the images were digitally-acquired,

they were viewed using the CBWorks 2.0 software (CyberMed, Seoul, Korea) on a

19 inch Dell® Ultrasharp 1907 flat panel LCD screen with a maximum resolution of

1280 X 1024 pixels in a dimly lit room. The observers were permitted to enhance

the images by manipulating the brightness and contrast as they chose. As well,

they were permitted to review the 3D renderings of each cbCT image dataset in

their entirety. The reviewing oral and maxillofacial radiologists were blinded to the

clinical data and were asked to make observations with respect to the features

listed in Table 6. Agreement between two of the three oral and maxillofacial

radiologists was used to determine the presence or absence of an imaging finding.

No attempt was made to reconcile disagreements. As well, one of the oral and

maxillofacial radiologists reviewed twenty randomly selected cases (including both

plain and cbCT images) twice to determine intra-observer reliability.

86

Table 6: List of the radiographic features and findings that were reviewed.

Radiographic feature Radiographic finding

Normal structures:

Primary duct

Visualization

Presence of abnormalities

Secondary duct Visualization


Parenchyma Visualization


Abnormal features:

Sialoliths

Number

Size

Location

Strictures Number

Size

Location

Ductal dilatation Severity

Location

Acinar pooling Number

Distribution

Size

Mass Location

Borders

Internal structure

Effect on surrounding structures

Interpretation Normal

Inflammatory (sialadenitis/sialodochitis)

Autoimmune

Other

87

Statistical analysis was performed using the SAS software Version 9.1 (SAS

Institute Inc., Cary, NC, USA). Descriptive statistics were performed for the clinical data

and for the radiographic features listed in Table 1, and McNemar’s Chi Square test was

used to determine differences between the two imaging modalities with regard to the

same outcomes. Because there is no gold standard (i.e. histopathological confirmation)

for this work, unbiased estimates of “accuracy”, “sensitivity”, and “specificity” cannot be

calculated and therefore the terms should not be used.59 Instead, the same numerical

calculations were made, but the estimates are called “overall percent agreement”

instead of “accuracy”, “positive percent agreement” instead of “sensitivity”, and

“negative percent agreement” instead of “specificity”.59 This modification reflects that the

estimates are not of accuracy but of agreement of the new test (CBCT) with the non-

reference standard (plain radiographs).59

“Overall percent agreement” = the number of cases agreed upon by both imaging

modalities/ the total number of cases.

“Positive percent agreement” = the number of cases that both imaging modalities

agreed upon as demonstrating the radiographic feature or findings/ the total number of

cases that demonstrated the radiographic feature or finding on plain images.

“Negative percent agreement” = the number of cases that both imaging modalities

agreed upon as not demonstrating the radiographic feature or finding/ the total number

of cases that did not demonstrate the radiographic feature or finding on plain images.

Comparison was performed between the two imaging modalities for visualization of

normal structures, identification of abnormal findings, and interpretation. Cohen’s kappa

was used to calculate inter- and intra-observer agreement and the Landis and Koch

88

guidelines were used to interpret them. The null hypothesis was rejected when the

alpha (p) value was less than 0.05.

4.5 RESULTS

The 47 subjects ranged in age from 21 years to 87 years with a mean of

48 years. Gender distribution was approximately equal with 27 females (57.4%)

and 20 males (42.6%). The majority of subjects (29 cases, 61.7%) were healthy

with non-contributory medical histories. Sixteen subjects (34.0%) reported a non-

contributory health ailment. Two subjects had received previous radioactive iodine

treatment.

In total, 32 parotid glands and 15 submandibular glands were examined.

Referral to our clinic for sialography was primarily from oral and maxillofacial

surgeons (25 cases, 53.2%), followed by oral and maxillofacial pathologists (14

cases, 29.8%), general dentists (6 cases, 12.8%), and otolaryngologists (2 cases,

4.3%). All subjects were symptomatic, and intermittent swelling was the most

common chief complaint of 32 subjects (68.1%). Three subjects (6.4%)

experienced only a single episode of swelling. With regard to pain, most subjects

(26, 55.3%) reported pain. Fourteen subjects (29.8%) reported dull pain and 8

subjects (17.0%) reported sharp pain. Four other subjects (8.5%) reported

“discomfort” and 21 subjects (44.7%) reported no pain. The majority of subjects

denied any abnormal taste in the mouth or dryness, only 29.8% and 21.3%

reported these symptoms respectively. Twenty subjects (42.6%) could relate their

symptoms of swelling and/or pain to meal time, while three subjects (6.4%) noticed

89

their symptoms to be worse in the morning. One subject claimed tongue movement

was the provoking stimulus. At the time of the examination, saliva could be easily

expressed from the gland of interest in the majority of subjects (31 cases, 66.0%).

For 13 subjects (27.7%) saliva was difficult to expel, and for three subjects (6.4%)

we were unable to expel any saliva from the gland of interest. In the majority of

subjects (46 cases, 97.9%), saliva was clear. Cloudy saliva was found in only one

case (2.1%). None of the subjects in this study suffered any adverse reactions to

the contrast agent or a complication following the procedure.

The two imaging modalities agreed on the interpretation of 39 out of 47

subjects. Of these, four subject image sets (8.5%) were interpreted as being within

the range of normal and 35 (74.5%) were interpreted as abnormal. The majority of

the image sets (29 cases), however, were interpreted as being consistent with

changes secondary to inflammation (sialodochitis and sialadenitis). Two image

sets were interpreted as an autoimmune condition (Sjögren syndrome) and two

others as space occupying tumours. One image set was interpreted as gland

fibrosis and another as sialadenosis. These findings are listed in Table 7.

The abnormal findings listed in Table 6 were identified by the observers

more frequently on cone beam CT (cbCT) images than on plain images and are

summarized in Table 7. The only exception to this was strictures which were

identified more frequently on the plain images. The most common location for

sialoliths and strictures was the primary duct. Solitary sialoliths were identified

more often than multiple sialoliths, and these ranged in size from 1.0 mm to 24.0

mm. Solitary strictures were identified more commonly on cbCT while multiple

90

strictures were identified more frequently on plain imaging. Ductal dilatation was

the most common abnormal finding identified on both plain images (57.4%) and

cbCT images (70.2%). The primary and secondary ductal structures were more

commonly involved and the severity of ductal dilatation was evaluated to be severe

in most cases as accessed on both cbCT and plain imaging. As well, the globular

collections of contrast material seen in acinar pooling were most often described

as non-uniform in distribution and size as they ranged from 0.1 mm to 6.5 mm on

both cbCT and plain imaging.

91

Table 7: Radiologic interpretation and identification of features as determined by the reviewers.

Primary outcome (Interpretation) Cone beam CT (%) Plain film (%) Both modalities (%)

Normal 06/47 (12.8) 10/47 (21.3) 04/47 (08.5)

Abnormal

Inflammation (sialadenitis, sialodochitis)

Autoimmune (Sjögren syndrome)

Gland fibrosis

Sialadenosis

Tumour

41/47

33

2

2

1

3

(87.2)

(80.5)

(04.9)

(04.9)

(02.4)

(07.3)

37/47

30

2

1

2

2

(78.7)

(81.1)

(05.4)

(02.7)

(05.4)

(05.4)

35/47

29

02

01

01

02

(74.5)

(82.9)

(05.7)

(02.9)

(02.9)

(05.7)

Secondary outcomes

Visualization of primary duct 46/47 (97.9) 46/47 (97.9) 45/47 (95.7)

Visualization of secondary ducts 43/47 (91.5) 42/47 (89.4) 39/47 (83.0)

Visualization of parenchyma

39/47 (83.0) 21/47 (44.7) 19/47 (40.4)

Presence of sialoliths

Number: Single

Multiple

Location: Primary duct

Secondary ducts

1ry and 2ry ducts

22/47

15

7

15

5

2

(46.8)

(68.2)

(31.8)

(68.2)

(22.7)

(09.1)

15/47

10

5

10

5

0

(31.9)

(66.7)

(33.3)

(66.7)

(33.3)

(00.0)

15/47

(31.9)

92

Presence of strictures

Number: Single

Multiple

Location: Primary duct

Secondary ducts

1ry and 2ry ducts

19/47

11

8

9

3

7

(40.4)

(57.9)

(42.1)

(47.4)

(15.8)

(36.8)

25/47

6

19

12

5

8

(53.2)

(24.0)

(76.0)

(48.0)

(20.0)

(32.0)

19/47

(40.4)

Presence of ductal dilatation

Severity: Mild

Moderate

Severe

Location: Primary

Secondary

1ry and 2ry ducts

33/47

7

4

22

10

2

21

(70.2)

(21.2)

(12.1)

(66.7)

(30.3)

(06.0)

(63.7)

27/47

7

9

11

8

2

17

(57.4)

(26.0)

(33.3)

(40.7)

(29.6)

(07.4)

(63.0)

25/47

(53.2)

Presence of acinar pooling

Size (mm):

Distribution: Homogenous

Heterogeneous

9/47

0.1-6.5

2

7

(19.1)

(22.2)

(77.8)

4/47

0.2- 5.0

1

3

(08.5)

(25.0)

(75.0)

01/47

(02.1)

* McNemar’s chi square test

The overall percent agreement (“accuracy”) between the two imaging

modalities for visualization of the primary duct was high (95.7%). As the gland

structures became finer and more delicate (i.e. secondary ducts and parenchyma),

the overall agreement between the two imaging modalities decreased to 85.1%

and 53.2%, respectively. Differences for visualization of the parenchyma were

statistically significant (p< 0.001) with more cases visualized on cbCT (39 cases,

83.0%) than plain images (21 cases, 44.7%). Inter-observer agreement for both

plain imaging and cbCT ranged from “moderate to very good” for visualization of

the primary and secondary ducts, and ranged from “fair to moderate” for

visualization of the parenchyma. Intra-observer agreement was “good” for

visualization of the normal structures (plain image 0.71, cbCT 0.79). These

findings are summarized in Table 8.

With regard to the identification of abnormal findings (Table 8), the positive

percent agreement (“sensitivity”) between the two imaging modalities was 100%

for the identification of sialoliths and the negative percent agreement (“specificity”)

was 100% for the identification of strictures. Both of these findings were statistically

significantly different (p<0.05) between the two imaging modalities. Inter-observer

agreement for all the abnormal findings listed in Table 1 ranged from “moderate to

very good” for cbCT and “fair to very good” for plain images while intra-observer

agreement was “good” for both imaging modalities (plain image 0.73, cbCT 0.75).

Overall percent agreement between the two imaging modalities was

similar (83.0%) for normal and abnormal glands. The positive percent agreement

was higher (96.7%) for changes secondary to inflammation whereas the negative

94

percent agreement was higher (94.6%) for normal salivary glands. Inter-observer

agreement for interpretation ranged from “fair to good” for plain imaging and “good

to very good” for cbCT. Furthermore, intra-observer agreement was “good” (plain

image 0.80 and cbCT 0.78). These findings are outlined in Table 8.

Table 8: Overall percent agreement and positive and negative percent agreements for cone beam CT and plain imaging

sialography.

Outcomes Overall %

agreement

Positive %

agreement

Negative %

agreement p*

Cohen’s kappa

(inter-observer)

Primary outcome

Interpretation

Normal

Abnormal

Inflammatory

83.0

89.4

40.0

96.7

94.6

76.5

0.3

0.4

cbCT: 0.73 - 0.90

plain film: 0.39 - 0.75

Secondary outcomes

Visualization of

primary duct

95.7

97.8

00.0

0.5

cbCT: 0.52 - 1.00

plain film: 0.56 - 0.97

Visualization of

secondary ducts 85.1 92.9 20.0 1.0

cbCT: 0.61 - 0.83

plain film: 0.63 - 0.83

96

Visualization of

parenchyma

53.2 90.5 23.1 <0.001 cbCT: 0.22 - 0.58

plain film: 0.35 - 0.60

Presence of sialoliths 85.1 100.0 78.1 0.02 cbCT: 0.62 - 0.84

plain film: 0.75 - 0.88

Presence of strictures 87.2 76.0 100.0 0.04 cbCT: 0.82 - 0.91

plain film: 0.34 - 0.64

Presence of ductal

dilatation 78.7 92.6 60.0 0.1

cbCT: 0.82 - 1.00

plain film: 0.77 - 0.91

Presence of acinar

pooling 76.6 25.0 81.4 0.2

cbCT: 0.63 - 0.73

plain film: 0.44 - 0.65

4.6 DISCUSSION

Sialography was first performed in 190212, and is regarded as the gold

standard for depicting the delicate ductal structures of the major salivary

glands7,60,61 and identifying non-calcified sialoliths and ductal strictures.2,7 The

purpose of this study was to determine whether sialography performed with cone

beam computed tomography (cbCT) is superior to sialography performed with plain

imaging.

Interpretation was our primary outcome and we found the overall percent

agreement (“accuracy") between the two imaging modalities to be the same for

interpreting the examinations as normal or abnormal. We also noted that the

positive percent agreement (“sensitivity”) was higher for changes seen secondary

to inflammation and the negative percent agreement (“specificity”) was higher for

normal salivary glands. The high positive percent agreement (96.7%) for the

identification of abnormal glands, particularly those demonstrating changes

secondary to inflammation suggests that inflammatory changes can be confidently

ruled out if these changes are not seen on cbCT images. In contrast, the high

negative percent agreement (94.6%) for normal glands suggests that if an

abnormal finding is detected on cbCT images, then disease can be confidently

ruled in. Figure 21 is an example of a case that was interpreted by all three

observers as normal on plain imaging but was interpreted as demonstrating

changes secondary to sialodochitis when the cbCT images were reviewed.

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(a) (b) (c)

Figure 21: Plain and cbCT images of a left submandibular gland sialogram.

This is a case of a 26 year old female that came to our clinic complaining of a single

episode of painful swelling in the area of the left submandibular salivary gland. Image

(a) is the lateral skull plain film radiograph that was made following contrast

administration. All three observers agreed on the interpretation of normal when this

image was reviewed. The observers dismissed the area of dilatation in the proximal part

of the primary duct (red arrow) as a point of branching rather than abnormal. Images (b)

and (c) are maximum intensity projection (MIP) cbCT images in the sagittal and axial

planes respectively. These images demonstrate that the area of ductal dilatation is not

due to branching and were thus interpreted by all three observers as changes

secondary to sialodochitis.

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Sialadenitis of the major salivary glands especially the chronic type is a

relatively common condition with approximately two-thirds of cases reportedly

being due to ductal obstruction.2,62-64 Ductal obstruction in turn, may have as

primary causes calculi, strictures and fibromucinous plugs, and as secondary

causes mass lesions that may impinge on the ductal structures and cause them to

occlude. In this study, sialoliths were the most common cause of obstruction

identified on both cbCT (46.8%) and plain (31.9%) images. These findings are

consistent with the work of Ngu et al (2007), who reported that the most common

cause of ductal obstruction (73.2%) was salivary calculi, followed by strictures

(22.6%) and mucous plugs (4.2%). Of note was our finding that more sialoliths

were identified on cbCT images than on plain images. Dreiseidler et al suggested

that 2D plain images have limited success in identifying sialoliths because of

overlapping anatomical structures.65 Moreover, Som and Curtin, estimate that

approximately 20% of submandibular gland sialoliths and 40% of parotid gland

sialoliths are missed on plain images due to low calcium content.2 Figure 22 is an

example of one such sialolith that was missed on plain imaging because of

overlapping structures but identified on cbCT.

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(a) (b) (c)

Figure 22: Plain and cbCT images of a left parotid gland sialogram.

This is a case of a 52 year old female that was referred to our clinic to investigate

episodes of intermittent painful swellings in the area of the left parotid gland. Image (a)

is the lateral skull plain film radiograph that was made following contrast administration

and demonstrates severe sialectasia of the primary and secondary ductal structures but

with no obvious cause. Images (b) and (c) are MIP cbCT images of the same gland in

the sagittal and the axial planes respectively identifying a cause for the sialectasia, a

non-calcified sialolith immediately proximal to the duct orifice (red arrow).

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Solitary sialoliths are more common than multiple sialoliths, and these are

more commonly found in the primary ducts of glands.2 Our data are in agreement

with these findings. The high positive percent agreement (100%) between the

cbCT and plain imaging datasets suggests that if no sialoliths are detected on

cbCT images, then they can be confidently ruled out.

Strictures, like sialoliths, are most often single and more commonly found

in the primary duct.7 In the current study, the cbCT results support these findings.

However, more strictures were identified on plain images and they were more often

described as multiple on plain imaging. The high negative percent agreement

(100%) of these data suggests that if a stricture is identified on cbCT images, then

an obstruction can be confidently ruled in.

Ductal obstruction, regardless of cause, results in the classic painful meal

time swelling of the affected gland that is frequently described in the literature and

was the most common complaint of subjects in this study.2,64,66 Upon imaging of

the affected gland, sialectasia of the ductal structures is the most prominent

feature as is demonstrated in the literature and was confirmed in this study.2,64

Cone beam CT sialography, is a novel investigation, and there are few

case reports in the literature. Drage and Brown reported two cases in females in

their sixth decade of life with classic symptoms of salivary obstruction.67 The

authors indicated that the primary duct, secondary ducts, and obstruction(s) were

easily identified in both cases. Although the diagnostic capabilities of cbCT

sialography were not compared to any other form of imaging, radiation doses

delivered to the patients were addressed. Using rough estimates, the authors

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concluded that cbCT sialography delivered a radiation dose equal to fluoroscopic

sialography, but higher than plain image sialography.67 Our earlier work indicates

that this may not be the case. Indeed, the choice of using lower peak kilovolt and

milliampere settings may lower the patient radiation dose without compromising

image quality at all.6,7 Of note is that different iodine concentrations in the contrast

agent were used for the two cases (300 and 370 mg I/ml respectively) and by

observation, the authors concluded that a lower concentration of iodine (180 or 240

mg I/ml) might have been better.67 This conclusion is in general agreement with

our earlier in vitro work on image quality that demonstrated that the lowest

commercially available iodine concentration (140 mg I/ml) is adequate for cbCT

sialography.43

We achieved “moderate to very good” inter-observer agreement in

visualization of the normal gland structures and in identifying abnormal findings as

the three observers were all certified specialists in oral and maxillofacial radiology

with extensive training in sialography and advanced imaging interpretation. The

“fair to moderate” agreement for visualization of the parenchyma is not surprising

since the parenchymal appearance can vary depending on many factors such as

the degree of damage of the terminal acini, the amount of contrast injected, and

the amount of pressure used during injection. It is also encouraging that for

interpretation, the inter-observer agreement was greater for cbCT images than for

plain images.

In conclusion, our results which are based on image interpretation indicate

that CBCT sialography may be better than plain film sialography in visualizing the

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delicate structures of the parotid and submandibular salivary glands, identifying

sialoliths and single ductal strictures, as well as differentiating normal salivary

glands from those with secondary inflammatory changes.

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5 GENERAL DISCUSSION

In this series of studies, we have developed a novel technique for imaging

the parotid and submandibular salivary glands using sialography combined with

cone beam computed tomography (cbCT). This combination was chosen to make

the greatest use of the advantages of each examination, and to overcome many of

the shortcomings of other imaging modalities routinely used to image the major

salivary glands. These other imaging modalities are ultrasound, computed

tomography and magnetic resonance imaging. Ultrasound is a relatively

inexpensive and widely available imaging modality, however, it has limited

penetrability and therefore its application is limited to examining superficial

structures and guiding future imaging directions. Computed tomography and

magnetic resonance imaging are three dimensional (3D) imaging modalities with

high spacial and contrast resolution however their cost and accessibility have

limited their clinical applicability.

Ensuring that radiation doses are kept as low as reasonably achievable,

quantifying the radiation dose to patients is a crucial first step in the process of

developing a novel imaging technique. This is especially important in the head and

neck area because of the radiosensitive organs in the area being imaged.

Currently, plain imaging of the salivary glands following contrast introduction is the

standard of practice for sialography. Therefore, it was important early on to ensure

that the effective radiation dose (E) for cbCT sialography would be comparable to

doses that patients are currently receiving with plain imaging. Given that the

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multiple cbCT technical parameters of image acquisition can be varied and

controlled by the operator, dose calculations can also vary widely.

Our first study used thermoluminescent dosimeter (TLD) chips (Global

Dosimetry Solutions Inc., Irvine, CA, USA) placed in predefined strategic locations

throughout the head and neck of a RANDO®Man radiologic phantom (Alderson

Research Laboratories, Stanford, CT, USA). The imaging system we used, the

Hitachi CB MercuRay (Hitachi Medical Systems, Tokyo, Japan) allows the operator

to control the position and size of the field-of-view (FOV), x-ray tube voltage and x-

ray filament current. Centering the FOV on either the parotid or submandibular

salivary glands, we found significant dose reductions upward of 40% when the

FOV size decreased from 12” to 9”, and from 9” to 6”, with all other factors being

equal. This reduction is due, primarily, to some of the TL dosimeters no longer

being in the primary radiation field and a reduction of scatter radiation. A second

factor that we manipulated was x-ray tube peak kilovoltage (kVp). Reducing kVp

resulted in a decrease in E by approximately 30% and 60% when kVp was

reduced from 120 kVp to 100 kVp, and then from 100 kVp to 80 kVp, respectively.

With respect to x-ray filament current, when this setting was reduced from 15 mA

to 10 mA, all other factors being equal, we found a 37% reduction in E. These

findings are in general agreement with the results of other published studies and

demonstrate that altering the technical parameters can significantly reduce the

radiation dose. Indeed, altering these technical factors should be considered to

keep patient radiation doses as low as reasonably achievable.34,38

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Some of the differences in the reported E between our study and other

dosimetry studies are likely related to variations in the methodology of measuring

the absorbed dose and in the calculations of the effective dose. Two prominent

differences in the methodology relate to our decision to center the image FOV and

to expose TLD chips, one chip per exposure. In our study, the image FOV was

asymmetrically centered on the salivary gland of interest. In other studies, the

image FOV was centered in the anatomic midline of one or both jaws.34,38,39

Because this study was the first of its kind to calculate the effective dose for a

specific application of cbCT, imaging the major salivary glands, we believed it to be

more appropriate to localize the image field where we did, that is, asymmetrically.

We firmly believe that this technical modification has led to a more accurate

estimate of E for cbCT examinations of the parotid and submandibular glands. As

a result of asymmetric FOV positioning, the percentage contribution of each tissue

changed in each FOV. For example, the fraction of brain irradiated in the 12” FOV

centered on the left parotid gland was 100% but decreased to 50% in the 9” FOV

and decreased even further to 10% using the smallest 6” FOV. On the other hand,

it was estimated that only 5% of the brain was irradiated in the 6” FOV when the

FOV was centered on the submandibular gland. Tailoring the estimated fraction of

irradiated tissue according to FOV size and centering may have yielded more

meaningful dose results.

Regarding the number of TLD exposures, some investigators exposed

their TLD chips a minimum of three times, after which the measured absorbed

doses that were recorded were divided by the number of exposures.31,34,39 We

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believe that exposing the TLD chips only once allowed for a more realistic

simulation of the patient situation since patients will undergo only one cbCT scan

during the procedure. It also allowed us to truly examine the reproducibility of the

dose measurement, which was determined by calculating the coefficient of

variation, the mean of which was calculated to 9% for the 12” FOV, 80 kVp, and 10

mA cbCT protocol.

The calculated thyroid effective radiation dose was by far the largest

contributor to the overall E in all examinations centered on the submandibular

gland, whether cbCT or plain radiographs were employed as the imaging modality.

Because of the anatomic proximity of the two glands, it is inevitable that the thyroid

will receive more radiation dose during examinations of the submandibular gland.

This may be a source of concern because of the relative high radiosensitivity of the

thyroid, especially in children. Fortunately, obstructive conditions of the salivary

glands are very rare in children and adolescents,40 and it would be rare that an

individual from that age group will require a sialogram procedure. As for the risk of

thyroid cancer in adults from diagnostic radiation exposure, the data are still

inconclusive and no studies have been able to prove a causal relationship or even

a statistically significant association.41,42 Nevertheless, the inconclusive nature of

the data creates a dilemma for physicians and dentists interested in imaging the

submandibular gland. This dilemma is an excellent example where risk versus

benefit judgment comes into play; that is, the excess risk of imaging versus the

anticipated benefit to improved patient care and management.

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Radiation dose control is, however, just one factor to consider when

evaluating a new imaging technique. Equally important is a consideration of how

changing the technical factors that limit dose, affect the quality of the resulting

image. For the task of quantifying image quality, we began by calculating the

metric, signal-difference-to-noise ratio (SDNR), a relative measure of true image

signal to background noise. Not unexpectedly, we found that SDNR increased in

proportion to kVp when the mA was held constant because the number of x-ray

photons reaching the detector increased. With greater numbers of x-ray photons

per unit area arriving at the image receptor, the background image noise

decreased. However, in our experimental data, the positive relationship between

SDNR and kVp was exaggerated by another source of image noise, spatial non-

uniformity. An inverse cupping artifact was present in our images, which is

sometimes known as “capping”.26 This artifact occurs when there is

overcompensation for a cupping artifact; an image artifact caused by beam

hardening. The capping was more pronounced in the images acquired at lower

kVp, where the beam is less penetrating or “softer”. Therefore, the SDNR could be

improved at the lower kVp settings by appropriate corrections for spatial non-

uniformity (i.e. flat-fielding). A positive relationship was also noted between

increases in mA and SDNR, where, within the observed range of SDNR variability,

the SDNR increased by a factor of 5.1 when mA was increased by a factor of

1.5. This relationship indicates that quantum noise is likely the dominant factor

affecting image performance for each technique combination considered here.

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These quantitative results are consistent with the trends observed in the subjective

image quality ratings published by Kwong et al.48

Image artifacts in computed tomography (CT) represent discrepancies

between the attenuation coefficient of an object and its CT number in a

reconstructed image.26 There are four categories of artifacts: physics-based

(related to the physical process of data acquisition), patient-based (related to

patient movement or the presence of radiopaque dental materials), scanner-based

(which result from imperfections in scanner function), and reconstruction-based.27

As artifacts degrade image quality, every effort should be made to minimize or

eliminate them as part of the optimization process of any imaging technique or

protocol. Three artifacts that affected the quality of the cbCT images of our

sialography phantom were identified: dark streak, partial volume, and capping

artifacts. Dark streak artifacts occur when the x-ray beam is hardened to different

degrees as it passes through different parts of a heterogeneous object at various

tube positions. This type of artifact is common in bony regions of patients and

when contrast material is used as was the case in this study. Partial volume

artifacts are commonly seen in cbCT images. These artifacts occur when dense

parts of a heterogeneous object protrude partway into the FOV causing shading

artifacts to appear. Cupping artifacts result when the x-ray beam passing through

the thicker middle portion of an object becomes harder than the beam passing

through the thinner edges of the object. The harder beam is attenuated less and

the resultant image appears darker in the center. Beam hardening software is used

by manufacturers to correct for this artifact and sometimes overcorrection is

110

attempted to minimize blurring at the bone-soft tissue interface of an image. In

some instances, capping artifacts result and the reverse becomes true with the

edges of the image appearing darker than the center. Therefore, in some

instances, capping artifacts represent an overcompensation to correct for cupping

artifacts. The three types of artifacts identified in our images are from the physics-

based category, and two of these, dark streaks and capping artifacts, are related to

beam hardening. Understanding when and why artifacts occur in cbCT images will

allow us to better interpret anomalies identified on cbCT images.

The ability to detect the 3.5 mm diameter region-of-interest (ROI) we used

in this study was deemed adequate according to the measured SDNRRose and the

threshold specified by the Rose criterion.68 However, in sialography the structures

of interest are significantly smaller. The main duct of the parotid gland ranges in

diameter between 0.1 mm and 2.0 mm at different points along its course, and for

the submandibular gland, the main duct diameter ranges between 0.2 mm and 3.0

mm.69 We can estimate the ability to detect these smaller structures by calculating

the SDNRRose at these smaller feature sizes. As an example, SDNRRose for a duct

diameter of 0.2 mm imaged with a 6” FOV and using the technical parameters of

80 kVp and 10 mA and a pixel size of 0.2 mm means that a structure of 0.2 mm

diameter occupies just 1 pixel. Under these conditions, SDNRRose= pSDNRN =

SDNRp= 9 where N is the number of pixels in the region of interest, which is above

the Rose criterion cut-off point of five. However, using this same method of

analysis, we would find that a 0.2 mm diameter duct would not be reliably visible if

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the kVp was lowered to 60, while holding all of the other parameters constant,

since the SDNRRose= 2.3 in this case.

For the task of imaging technique optimization, we calculated a metric

called the figure-of-merit (FOM), which is the ratio of image SDNR to radiation

dose. Figure-of-merit values are influenced by kVp and mA. At lower kVp settings

of 60 kVp and 80 kVp, a direct relationship was noted between FOM and mA. At

higher kVp settings (100 kVp and 120 kVp), an inverse relation was demonstrated

because the dose increased significantly more than the SDNR. The only FOM

values that were significantly different than the others were those at 60 kVp. This

indicates that among the other examined techniques where image quality was

adequate, the one that should be adopted as a clinical protocol for sialography is

the one which imparts the lowest E.

Suomalainen et al. published an extensive study that evaluated the

radiation dose and image quality of four cbCT scanners and compared them to two

multislice CT (MSCT) scanners.70 Using appropriate phantoms, radiation doses

and contrast values were measured. The contrast-to-noise-ratio (CNR) was used

as an indicator of image quality, and the modulation transfer function (MTF)

quantity was used to determine the resolving power of each system.70 The authors

concluded that despite the large variation in results, cbCT scanners provide

adequate image quality with smaller effective doses as compared to MSCT.70

However, the authors were quick to emphasize the importance of optimizing

imaging factors for the different examinations.70 It is our opinion that this

recommendation can be accomplished only by optimizing these parameters for

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specific clinical applications of cbCT, and by objectively measuring the trade-off

between image quality and radiation dose as we have done in this study for cbCT

sialography.

Omnipaque® (Iohexil injection 39%, General Electric Healthcare Canada

Inc., Mississauga, ON) is a low osmolar, nonionic, iodinated contrast agent that is

produced in five concentrations: 140, 180, 240, 300, and 350 mg I/mL for different

indications of use. At our institution and for the indication of sialography, the

180mg/mL iodine concentration is used. In this study, we examined the effect of

the two smaller commercially available concentrations (140 and 180 mg I/mL) in

addition to two intermediate concentrations (120 and 160 mg I/mL) in order to fully

understand the impact of the iodine concentration on SDNR for cbCT sialography.

Greater concentrations of the contrast agent were deemed unnecessary because

the injection is locally confined to the ductal structures of the salivary glands as

opposed to vascular injection, where the agent quickly becomes diluted by

distribution throughout the whole body. An increase in SDNR was noted with the

increase in iodine concentration but there was no statistically significant difference

among the different concentrations for the technique using 80 kVp and 10 mA. This

relation is due to the substantial signal difference provided by the attenuation of

iodine in the salivary ducts.52

To our knowledge there are no other studies in the English literature that

examine the effect of varying the iodine concentration in contrast agents on the

quality of the resultant images for sialography studies. However, a similarity can be

drawn to studies of CT angiography. Unfortunately, these studies vary in their

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results from demonstrating no significant effect of the iodine concentration53 to

showing a statistically significant difference54,55 on vascular enhancement. The

results of these studies are complicated by many other variables such as imaging

techniques and the injection parameters.55 One particular study by Wang et al.

examined the effects of two iodine concentrations of the same contrast agent on

image quality and side effects for dynamic CT of the neck.56 They found that both

concentrations produced images of excellent quality but the lower concentration

resulted in fewer immediate minor complications.56 In the current study, we

demonstrated a positive linear trend between the iodine concentration and SDNRp,

as indicated by the linear fit to the data in Figure 18. This relationship is expected

because the signal difference will increase proportionally to the amount of iodine

present. However, no statistical differences in SDNRp were noted among the four

concentrations examined; the highest of which (180 mg I/mL) is the concentration

currently used at our institution for sialography. The likelihood of complications

from contrast media in relation to sialography procedures is minimal because of

the localization of the contrast material in the salivary ducts; nevertheless, it would

be more reasonable to use the commercially available smaller concentration of the

contrast agent, which is 140 mg I/ml.

Based on the findings of the dosimetry and image quality studies, the

following protocol for cbCT sialography was proposed. Following the introduction of

contrast material into the primary duct of the salivary gland of interest, a lateral

skull plain image is made to ensure adequate fill of the ducts with contrast

material. This is then followed by cbCT image acquisition of the gland centered in a

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6” FOV and using exposure factors of 80 kVp and 10 mA. After the completion of

the cbCT scan, another lateral skull plain radiograph is made five minutes after

removal of the catheter to assess contrast material clearance, an accepted indirect

indicator of gland function and saliva production. The two lateral skull radiographs

are made with the Kodak Directview CR850 PSP system (Kodak Medical Systems,

Rochester, NY) using 70 kVp, 15 mA, and 0.8 s exposure time. It is estimated that

a patient undergoing this cbCT sialography examination will receive an effective

radiation dose that ranges between 76 µSv for a parotid gland examination (16

µSv for the 2 lateral plain radiographs and 60 µSv for the cbCT examination) and

170 µSv for a submandibular gland examination (22µSv for the 2 lateral

conventional radiographs and 148 µSv for the cbCT scan). The estimated effective

radiation doses are comparable with the estimated E for a plain film sialography

(parotid 65 µSv and submandibular 156 µSv).

The final step in our project of developing a novel 3D sialography

technique for the parotid and submandibular salivary glands using cbCT was to

apply the protocol clinically, and to examine its diagnostic efficacy by comparing it

with two dimensional (2D) plain film sialography. Given that subject received both

2D and 3D imaging with sialography on the same day allowed us to perform these

comparisons with relative ease.

Three certified specialists in oral and maxillofacial radiology were used as

observers and asked to review the 2D and 3D image series separately, with a

wash out period of at least one week. The parotid and submandibular salivary

glands under examination were reviewed by the observers in three parts: the

115

primary duct, the secondary ducts and the parenchyma. Observers were able to

visualize the primary duct equally well with both modalities in 45 out of 47 cases.

For the more delicate secondary ducts, imaging agreement decreased to 39 of 47

cases. The observers reported a greater ability to visualize the delicate secondary

ducts and the parenchyma of the glands on cbCT images. This is likely largely due

to the image processing features permitted by cbCT image data that allowed multi-

planar viewing, 3D rendering, and elimination of superimposing structures, which is

a major limitation in the 2D plain images.

Interpretation was our primary outcome and we found the overall percent

agreement (“accuracy") between the two imaging modalities to be the same for

interpreting the examinations as normal or abnormal. We also noted that the

positive percent agreement (“sensitivity”) was higher for changes seen secondary

to inflammation and the negative percent agreement (“specificity”) was higher for

normal salivary glands. The high positive percent agreement for the identification

of abnormal glands, particularly those demonstrating changes secondary to

inflammation suggests that inflammatory changes can be confidently ruled out if

these changes are not seen on cbCT images. In contrast, the high negative

percent agreement for normal glands suggests that if an abnormal finding is

detected on cbCT images, then disease can be confidently ruled in.

Sialadenitis of the major salivary glands especially the chronic type is a

relatively common condition with approximately two-thirds of cases reportedly

being due to ductal obstruction.2,62-64 Ductal obstruction in turn, may have as

primary causes calculi, strictures and fibromucinous plugs, and as secondary

116

causes mass lesions that may impinge on the ductal structures and cause them to

occlude. In this study, sialoliths were the most common cause of obstruction

identified on both cbCT (46.8%) and plain (31.9%) images. These findings are

consistent with the work of Ngu et al (2007), who reported that the most common

cause of ductal obstruction (73.2%) was salivary calculi, followed by strictures

(22.6%) and mucous plugs (4.2%). Of note was our finding that more sialoliths

were identified on cbCT images than on plain images. Dreiseidler et al suggested

that 2D plain images have limited success in identifying sialoliths because of

overlapping anatomical structures.65 Moreover, Som and Brandwein-Genster,

estimate that approximately 20% of submandibular gland sialoliths and 40% of

parotid gland sialoliths are missed on plain images due to low calcium content.2

Solitary sialoliths are more common than multiple sialoliths, and these are

more commonly found in the primary ducts of glands.2 Our data are in agreement

with these findings. The high positive percent agreement between the cbCT and

plain imaging datasets suggests that if no sialoliths are detected on cbCT images,

then they can be confidently ruled out.

Strictures, like sialoliths, are most often single and more commonly found

in the primary duct.7 In the current study, the cbCT results support these findings.

However, more strictures were identified on plain images and they were more often

described as multiple on plain imaging. The high negative percent agreement of

these data suggests that if a stricture is identified on cbCT images, then an

obstruction can be confidently ruled in. Whereas the identification of strictures on

117

plain images may have been overestimated and confused with areas of duct

branching or bending.

Ductal obstruction, regardless of cause, results in a cascade of events that

starts with saliva build-up proximal to the duct opening. The build-up intensifies

during meal times when copious volumes of saliva are quickly produced, but meet

resistance at the obstruction. This results in the classic painful meal time swelling

that is frequently described in the literature and was the most common complaint of

subjects in this study.2,64,66 In most cases the obstruction rarely occludes the duct

completely, and saliva eventually finds its way around the obstruction and into the

mouth prompting the relief of the patients’ symptoms. If the obstruction is not

removed or dislodged, the process may result in chronic distention of the ducts

beyond the obstructive point. Many patients seek professional help at this point

and when imaged, sialectasia of the ductal structures is the most prominent feature

as is described in the literature and was demonstrated in this study.2,64 Within the

now dilated ducts, saliva may stagnate and become an inviting environment for

retrograde bacterial infections. Imaging of the gland at this point may demonstrate

globular collections of contrast material that represent contrast pooling. These

collections are characteristically heterogeneous in size and distribution, and may

appear not unlike the globular collections encountered in autoimmune diseases

which are comparatively more homogenous in size and distribution.2,8,64 Although

these changes can be seen in both minor and major salivary glands, approximately

83% of these appearances have been reported to involve the parotid and

submandibular glands, with the submandibular glands being the most commonly

118

affected.12,65 The reason for the higher prevalence of obstructions in the

submandibular gland ducts is believed to be related to the longer, more tortuous,

path of the major duct, and the nature and consistency of submandibular saliva

which has a higher mucous content making it thicker and more likely to form a

mucous plug. In this study, parotid examinations were more common than

submandibular ones. In another series, parotid examinations were performed more

frequently than submandibular ones.71 A possible explanation for parotid gland

examinations being more common than submandibular ones despite the higher

occurrence of obstructions in the submandibular gland may be related to the

presenting symptoms. Obstructive conditions commonly present as a facial

swelling, and the developing facial asymmetry may be more noticeable to patients,

making them more likely to seek professional help.

The imaging findings that have been discussed thus far influence not only

the interpretation of the images, but as well, management. Many management

options have become available, and several of these are minimally invasive and

radiologically guided such as basket retrieval for sialoliths and balloon ductoplasty

for ductal strictures.9,11 Their success, however, depends on having adequate and

accurate information provided by diagnostic imaging. Therefore, it is imperative

that imaging reports include a detailed description of any causes of obstruction,

their nature, number, location, and effect on the glandular structures. This

increased need for detailed diagnostic information highlights the importance that

diagnostic imaging plays not only in diagnosis, but in patient management, as well.

And indeed, the additional information provided by cbCT images, such as the type

119

of obstruction and its exact location in the three orthogonal planes, will no doubt

influence the management of patients.

Due to the novelty of cbCT sialography, only one similar publication was

found in the English literature; a report of 2 cases by Drage and Brown who used

sialography and performed their imaging using the i-CAT unit (Imaging Science

International, Hatfield, PA).67 Both cases were of females in their sixth decade of

life with classic symptoms of salivary obstruction. In the first case, a single sialolith,

which the authors indicated would have been very difficult to identify on plain

images, was identified in the right submandibular gland. The second case involved

a sialolith and a stricture in the right parotid gland, both of which were managed

with minimally invasive, radiographically-guided, interventional procedures.67 The

authors claim that the primary duct, secondary ducts, and obstruction(s) were

easily identified in both cases.67 Although the diagnostic capabilities of cbCT

sialography were not compared to any other form of imaging, radiation doses

delivered to the patients were addressed. Using rough estimates, but with no hard

data, the authors surmised that cbCT sialography delivered a radiation dose equal

to fluoroscopic sialography, and higher than plain image sialography.67 Of note is

that different iodine concentrations in the contrast agent were used for the two

cases (300 and 370 mg I/mL, respectively) and by observation, the authors

concluded that a lower concentration of iodine (180 or 240 mg I/mL) might have

been better.67 Although there is a lack of a reputable dataset from these 2 cases,

their supposition is in general agreement with our earlier work that demonstrates

120

that the lowest commercially available iodine concentration (140 mg I/ml) may be

adequate for cbCT sialography.43

We achieved “moderate to very good” inter-observer agreement in

visualization of the normal gland structures and in identifying abnormal findings as

the three observers were all certified specialists in oral and maxillofacial radiology

with extensive training in sialography and advanced imaging interpretation. The

“fair to moderate” agreement for visualization of the parenchyma is not surprising

since the parenchymal appearance can vary depending on many factors such as

the degree of damage of the terminal acini, the amount of contrast injected, and

the amount of pressure used during injection. It is also encouraging that for

interpretation, the inter-observer agreement was greater for cbCT images than for

plain images.

5.1 CONCLUSION

We have successfully developed a novel technique for imaging the parotid

and submandibular salivary glands using sialography combined with cone beam

computed tomography (cbCT) based on an extensive evaluation of the technical

factors that can be adjusted by the operator and their effect on both radiation

doses delivered to the patient and image quality. The protocol which was specific

for the Hitachi CB MercuRay unit (Hitachi Medical Systems, Tokyo, Japan)

recommends placing the major salivary gland of interest in the center of a 6” FOV

and imaging using 80 kVp and 10 mA. The diagnostic efficacy of the novel

technique was also examined clinically and compared to 2D plain film sialography

which is the standard of practice. The results were overwhelmingly promising as

121

they demonstrated that 3D cbCT sialography was superior to plain film sialography

in visualizing the delicate structures of the salivary gland such as the parenchyma.

Cone beam CT sialography may also be better at identifying sialoliths and single

ductal strictures (the two most common causes of obstruction), as well as

differentiating normal salivary glands from those with secondary inflammatory

changes.

The role of diagnostic imaging is no longer limited to providing an

interpretation which will contribute to the diagnosis. Diagnostic imaging now plays

a pivotal role in the overall management of patients. Therefore existing and new

imaging technologies and protocols must undergo an extensive evaluation of their

efficacy. In diagnostic imaging there is a well-established hierarchical model for

assessing efficacy that starts with technical efficacy (i.e. assessment of the

resultant image quality).72 Then the accuracy, sensitivity, and specificity of

interpreting the images is addressed in the second level which is the diagnostic

accuracy efficacy.72 The third level studies the diagnostic thinking efficacy, that is

the influence of the radiographic interpretation on the clinicians’ diagnosis.72 Level

four addresses the effect of the imaging information on the management plan

(therapeutic efficacy) and level five addresses the effect of the information on the

patient outcome (patient outcome efficacy).72 The final level is an analysis of the

societal costs and benefits of the diagnostic imaging technique (societal efficacy).72

The three studies presented in this thesis have successfully addressed the first two

levels of the diagnostic imaging hierarchy. Further studies are needed to address

the remaining four levels of diagnostic efficacy.

122

5.2 FUTURE DIRECTIONS

The cbCT sialography protocol recommended in this study is specific for

the Hitachi CB MercuRay cbCT scanner (Hitachi Medical Systems, Tokyo, Japan).

Future studies following the same approach used in this work could be applied to

other cbCT systems to address the applicability of this novel combination to other

cbCT machines and to optimize the protocol for those specific units.

One of the limitations of the current study was the lack of a gold standard

which restricted our ability to determine the diagnostic accuracy of cbCT

sialography. Ideally, future diagnostic efficacy studies should try to incorporate a

gold standard such as correlating the imaging findings with the surgical or

histopathological findings. Future studies should also address the remaining levels

of diagnostic efficacy mentioned earlier. “Diagnostic thinking efficacy” can be

determined by studying the influence of the imaging findings and interpretation on

the post-imaging diagnosis of the clinician. Determining whether the imaging

results helped chose the line of management or changed the management plan

will address the “therapeutic efficacy” of cbCT sialography. Correlating the imaging

results with the management outcome in terms of morbidity to the patient and

quality of life will ascertain the “patient outcome efficacy”. Finally, the “societal

efficacy” of cbCT sialography can be established by studying the critical balance

between the cost of the procedure and its benefits from a societal point of view.

In the present studies we examined the different commercially available

concentrations of iodine in one contrast agent which is manufactured primarily for

intravenous use. Our data demonstrated a trend that supports the adequacy of

123

lower concentrations of iodine in the contrast agent especially when used locally as

is the case for sialography. Future studies should examine a wider range of lower

iodine concentrations in contrast agents and their effect on image quality for

specific local indications such as sialography. This will hopefully prompt producing

companies to manufacture contrast agents with lower concentrations of iodine that

are more appropriate for local rather than intravenous use.

Cone bean CT sialography was superior to plain film sialography in

identifying sialoliths. However, the level of calcification of the sialolith was not

addressed in this study. An excellent future direction would be to conduct a study

that specifically answers the question of whether cbCT sialography is superior to

other imaging modalities in identifying non-calcified sialoliths which are usually

difficult to detect and diagnose using even the most sophisticated imaging

techniques.

Additionally, future studies could explore the possibility of canulating and

imaging not only the gland in question but also the contra-lateral gland in an

attempt to provide a comparative study. This can be easily achieved by increasing

the FOV size and centering the image field symmetrically on the patient’s midline

to include both glands. This of course will result in greater radiation exposure to the

patient but the diagnostic yield may outbalance this increase in risk.

124

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COPYRIGHT ACKNOWLEDGEMENT

From: Gould, Laura (ELS-OXF) [[email protected]] Sent: October-12-11 10:30 AM To: Fatima Jadu Subject: RE: Permission to use copyrighted material in my PhD thesis.

Dear Ms Jadu Ten Cate's Oral Histology: Development, Structure, and Function by Antonio Nanci; 3 figures only Head and Neck Imaging by Peter Som and Hugh Curtin; 2 figures only Oral Radiology: Principles and Interpretation by Stewart C. White and Michael J. Pharoah; 3 figures only As per your email below, we hereby grant you permission to reprint the below mentioned material at no charge in your thesis, in print and on the University of Toronto web site subject to the following conditions: 1. If any part of the material to be used (for example, figures) has appeared in our

publication with credit or acknowledgement to another source, permission must also be sought from that source. If such permission is not obtained then that material may not be included in your publication/copies.

2. Suitable acknowledgment to the source must be made, either as a footnote or in a

reference list at the end of your publication, as follows:

“This article was published in Publication title, Vol number, Author(s), Title of article, Page Nos, Copyright Elsevier (or appropriate Society name) (Year).”

3. Your thesis may be submitted to your institution in either print or electronic form. 4. Reproduction of this material is confined to the purpose for which permission is hereby given. 5. This permission is granted for non-exclusive world English rights only. For other

languages please reapply separately for each one required. Permission excludes use in an electronic form other than as specified above. Should you have a specific electronic project in mind please reapply for permission.

6. This includes permission for the Library and Archives of Canada to supply single

copies, on demand, of the complete thesis. Should your thesis be published commercially, please reapply for permission.

Yours sincerely Laura Gould Rights Associate

136

Appendix 1: Research ethics board approval letter

137

Appendix 2: Patient consent form

Title of Research Project:

Development and application of a technique for three-dimensional sialography using

cone beam computed tomography (cbCT).

Investigator(s):

1. Principal investigator

Dr. Fatima Jadu DDS, MSc

Dipl ABOMR

PhD candidate

Division of Oral and Maxillofacial Radiology

Faculty of Dentistry, University of Toronto

124 Edward Street, Division of Oral and Maxillofacial Radiology

Toronto, ON M5G 1G6

416-979-49-32 x 4040

2. Faculty Supervisor

Dr. Ernest Lam DMD, PhD, FRCD(C)

Associate Professor




Toronto, ON M5G 1G6

416-979-49-32 x 4385

Purpose of the Research:

The purpose of this study is to compare the diagnostic performance of cbCT

sialography with plain film sialography with respect to its ability to depict salivary gland

anatomy and function.

Description of the Research:

Subject population

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Adult patients referred to the Special Procedures Clinic in the Discipline of Oral and

Maxillofacial Radiology at the Faculty of Dentistry, University of Toronto for a

sialographic procedure.

Inclusion criteria: individuals with a suspected salivary gland obstructive condition as

determined by a pre-sialography clinical examination of the patient.

Exclusion criteria: subjects with acute inflammation of the salivary gland of interest,

subjects with known or suspected allergy to iodinated contrast agents, subjects with an

immediately anticipated thyroid function test and patients under the age of 18 years.

Collected data

The data that will be collected for each patient prior to the sialography procedure is

listed in the Materials and Methods section page 92. These data will be collected in the

clinic by the Principal Investigator at the time of clinical examination.

Sialography procedure

A thorough explanation of the sialographic procedure and its possible complications will

be given to each patient prior to consenting to the examination. Upon approval, the

procedure will be performed by a Resident in Oral and Maxillofacial Radiology at the

Faculty of Dentistry, University of Toronto. The resident will be closely supervised by the

Principal Investigator or a faculty member, all of whom are certified specialists in Oral

and Maxillofacial Radiology. Sialography procedures are routinely performed in the

Special Procedure clinic and the supervising faculty members are experienced in the

performance of this procedure.

The procedure will start by canulating the major duct of the salivary gland followed by

insertion of a 24G or 30G catheter (Pajunk, Medizintechnologie, Geisingen, Germany or

Cook, Bloomington, IN). Between 1 mL and 6 mL Omnipaque® (Iohexil injection 39%,

General Electric Healthcare Canada Inc., Mississauga, ON) will be injected slowly into

the primary duct of the gland until the patient reports maximum tolerance to the feeling

of pressure in the gland of interest. A lateral skull plain radiograph will be made using

the GE focus system (General Electric Corporation, Henry Schein Ash Arcona, Niagara-

on-the-Lake, ON) to confirm optimal fill of the gland being examined prior to the cbCT

scan. If fill of the gland is inadequate, additional contrast will be injected and another

lateral skull plain radiograph will be made. The patient will then be placed in the CB

MercuRay cone beam CT unit (Hitachi Medical Systems, Tokyo, Japan) with the

occlusal plane of the dentition parallel to the floor, the midsagittal plane perpendicular to

the floor, and the gland of interest centered in the image field. Imaging will be performed

using a 6” field-of-view, and x-ray tube factors of 80 kVp and 10 mA. Five minutes

following catheter removal a final lateral skull plain image will be made to evaluate

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contrast clearance from the gland. The lateral skull plain image demonstrating adequate

contrast fill and the postoperative lateral skull plain image will represent the

“conventional” part of the study and will be used for comparison with the cbCT images.

By following this protocol, it will be possible to compare the two imaging modalities

relative to the same patient and same salivary gland under investigation. Variables

related to the patient and the gland will be standardized without unnecessarily repeating

the procedure or exposing the patient to additional doses of radiation.

The results of the sialographic examination will be reported to the patient at the

completion of the examination. As well, a digital copy of the radiographic images that

were obtained for the patient will be couriered to the referring physician accompanied by

a radiographic report stating the findings of the examination.

Image analysis

The conventional plain images and cbCT images for each patient will be reviewed

independently by 3 certified oral and maxillofacial radiologists. As the images are all

digitally-acquired, they will be reviewed at one of the three computer workstations in the

oral radiology department where the observers will have the advantage of enhancing

the images by manipulating their brightness and contrast; as well as reviewing the 3D

renderings in their entirety. The reviewing radiologists will be blinded to the clinical data

and will be required to fill out the form outlines in Table 6 (in the Materials and Methods

section on page 96) for each image series. The presence or absence of a particular

radiographic feature will be based on agreement of 2 of the 3 radiologists, and there will

be no attempt to reconcile disagreement. As well, one of the radiologists will review the

series twice so that intra-observer reliability may be determined. Image analysis will be

done twice during the course of this project; half way through the study during year 1,

and again at the end of the study.

Potential Harm, Injuries, Discomforts or Inconvenience:

Sialographic procedures are minimally invasive and carry two possible risks:

hypersensitivity reaction to the contrast agent and infection. These adverse effects are

very rare especially when the appropriate precautions are taken. During the procedure,

every effort is made to minimize the risk of developing one or both of these potential

complications. A strict aseptic technique is always followed during the procedure as a

standard of practice. The risk of developing a hypersensitivity reaction is very low in

sialography. This risk is usually minimized further by using small amounts (2 to 6 mL) of

an ionic agent that is less likely to provoke an allergic reaction. Screening of patients

for a potential contrast agent-related hypersensitivity reaction is elicited during the initial

review of the patient’s medical history. In the event that a hypersensitivity reaction does

occur, the clinic is fully equipped with the proper medications and materials to deal with

medical emergencies, and the staff are trained in this aspect.

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With respect to radiation doses, we have recently completed a dosimetric study

comparing the effective radiation doses from a 2D plain radiography sialographic

examination with a 3D cbCT examination using an Alderson RANDO® phantom with

lithium fluoride (LiF) dosimeters placed in 25 locations in the phantom. A sialographic

examination of the parotid salivary gland includes a panoramic radiograph, 2 anterior-

posterior skull and 4 lateral skull views, while a submandibular salivary gland

examination includes a panoramic radiograph, a mandibular intra-oral occlusal

radiograph and 4 lateral skull radiographs. We compared the effective doses from

these 2 studies with cbCT studies where we varied field-of-view (FOV) size (6”, 9”, 12”),

killovoltage (kV) (80 kVp, 100 kVp, 120 kVp) and milliamperage (mA) (10 mA, 15 mA).

The results were presented recently at the 2008 meeting of the American Academy of

Oral and Maxillofacial Radiology in Pittsburgh, PA. Our results found that the effective

doses for plain radiograph parotid, plain film submandibular and cbCT examinations of

these glands were comparable when using the 6” FOV for the cbCT examination at 80

kVp and 10 mA:

Parotid gland (plain radiograph): 60 microSievert (µSv)

Parotid gland (cbCT): 65 µSv

Submandibular gland (plain radiograph): 156 µSv

Submandibular gland (cbCT): 148 µSv

Potential Benefits:

By agreeing to participate in this study, patients undergoing cbCT sialography will

greatly benefit from the 3D imaging of their gland. We hypothesize that 3D imaging of

the gland ductal structures will provide more detailed diagnostic information than

conventional 2D imaging techniques, by allowing multi-planar viewing of the gland and

by eliminating superimpositions of adjacent anatomic structures. We believe that the

additional information obtained by this technique will aid the treating physician or dentist

in choosing the most appropriate management option for the patient, and improve the

quality of patient care.

Alternatives:

If you elect at any time not to participate in this study, an alternative sialographic

examination using plain radiography will be offered.

Confidentiality:

Confidentiality will be respected and no information that discloses the identity of the

subject will be released or published without consent unless required by law. Patient

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data will be anonymized with the identities of the patients known only to the Principle

Investigator.

Participation:

Participation in research is voluntary. If you choose to participate in this study you can

withdraw at any time. Should you choose to withdraw, your data will be removed from

the database. There will be no consequences with respect to your future care in the

Faculty.

Contact:

If you have any questions about this study, please contact:

Dr. Fatima Jadu DDS, MSc

Dipl ABOMR

PhD candidate




Toronto, ON M5G 1G6

416-979-49-32 x 4040

If you have any complaints or concerns about how you have been treated as a research

participant, please contact:

Zaid Gabriel

Research Ethics Officer, Health Sciences

[email protected] or 416-946-5806

mailto:[email protected]

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CONSENT: By signing this form, I agree that:

The study has been explained to me. Yes No

All my questions were answered.

Possible harm and discomforts and possible benefits (if any) of this study have been explained to me.

I understand that I have the right not to participate and the right to stop at any time.

I understand that I may refuse to participate without consequence to continuing care at the Faculty.

I have a choice of not answering any specific questions

I am free now, and in the future, to ask any questions about the study

I have been told that my personal information will be kept confidential

I understand that no information that would identify me, will be released or printed without asking me first.

I understand that I will receive a signed copy of this consent form.

I hereby consent to participate. ______________________________ ________________ Signature Date Name of Participant and Age: ____________________________________ Telephone #: ____________________________________ Name of person who obtained consent: ___________________________ ____________________________ __________________ Signature Date

development and application of a technique for three ... · computed tomography (cbct) is a...

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