ultrasonography in ophthalmology

Ultrasonography in Ophthalmology

Sanket Parajuli

Introduction

Ultrasound :

• > 20 KHz sound waves – Ultrasound

• Technique that utilizes high-frequency sound waves to produce echoes as they strike interfaces between acoustically distinct structures

–Waves encounter tissues of different density and reflectivity

–Some deflected and refracted

–Others reflected back to the probe, where they are detected and displayed on an oscilloscope

History

• 1793: Lazzaro Spallanzani (Italy)

• Discovered that bats orient themselves with the help of sound whistles while flying in darkness

• The basis of modern ultrasound application

Bats use ultrasounds to navigate in the darkness

• First used in Ophthalmology by Mundt and Hughes

• Baum and Greenwood came up with two dimensional, immersion scan which was subsequently improved upon by Purnell and Coleman.

• Contact B scan was introduced by Bronson and it being portable, became part of everyday use in ophthalmology.

• Standardization of A scan was carried out by Ossoinig

• Lately Colour Doppler ultrasound has become a part of the present day's Standardized Echography.

Wavelength• As the frequency of USG increases, the wavelength decreases• Wavelength of an ultrasound determines its depth of tissue penetration and resolution Wavelength α Depth of penetration of the ultrasound • Larger the frequency of USG = shorter the wavelength = shallower is its

penetration = better is the resolution• Ophthalmic USG: 6-20 MHz

• High frequency : better resolution–A-scan : 8 MHz–B-scan : 10 MHz (best for posterior segment)

• Low frequency: 1-2 MHz (body scan)

• Reflections (echoes): produced at junction of tissues with different densities or impedance to sound

Impedance = sound velocity × density

• The greater the difference in density between the two tissues, the stronger is the reflection of echo

• Echoes are influenced by:– Absorption – Refraction– angle of incidence of ultrasound wave

(a) The difference in density is slight=little reflection. (b) Greater difference and the reflection is increased.

If returning echo to the probe is weaker Amplification

(a–c) Effect of probe positioning on the return of echo. The greater the obliquity of probe, less echo returns.

(d) The effect of an irregular interface on the echo.

• Amplification = Gain• Unit : decibels (dB).

Amplification

Angle of incidence

• Perpendicularity to the area of interest always should be maintained to achieve the strongest echo

• Perpendicularity = maximum reflection

• The more oblique the probe is held from the area of interest, the weaker the returning echo = compromised image

Ultrasonography is used for

• Biometry (A scan) for axial length measurement

• Standardized A scan (diagnostic) for the echo structure assessment.

• Diagnostic B scan (two dimensional) has to be coupled with the standardized A scan to arrive at a correct diagnosis

• Doppler ultrasonography is especially important in vascular lesions with different blood flow rates.

During examination, the following systematic approach is universally recommended:

1. Screening for lesion detection: A + B scan.

2. Topographic examination for shape, border, location and extension (if possible) of the lesion: B scan

3. Quantitative Echography to know the reflectivity, sound attenuation & internal structure of lesion : A scan.

4. Assessment for blood flow:: Colour Doppler

A scan probe

• small, pencil sized probe without a mark and easy to manoeuvre

• With this probe the ultrasound beam are parallel

• The probe should be placed at right angle to the area of interest in order to obtain appropriate spike height and thereby maximum information

• The probe can be kept directly over the globe after local anaesthesia or on the lid skin for which the overall gain of the machine has to be increased by 3-5db

B scan probes • Thick, with a mark (white dot)

• Emit focused sound beam at frequency 10mhz

• Mark on the B scan probe indicates beam orientation-area , towards which mark is directed appears at the top of the echogram on display screen

ELECTRIC CURRENT

TRANSDUCER

ULTRASOUND WAVES

SURFACE

AMPLIFIER

MONITOR

Transducer

Converts • Electrical to Sound energy [pulse ]• Sound to Electrical energy [Echo]

Basic Components –1. Piezoelectric plate2. Backing layer3. Acoustic Matching layer4. Acoustic lens

Basic Components

1. Piezoelectric Element :

• Generates ultrasonic waves• Coated on both sides with electrodes to which

a voltage is applied• Oscillation of element with repeat expanding

and contraction generates a sound wave

• Most common: piezoelectric ceramic ( lead zirconate titanate).

2. Backing layer :

• Location : behind piezoelectric element

• Function: dampens excessive vibrations from probe

• Helps improves image resolution

3. Acoustic matching layer :• Location: in front of piezoelectric element• Function: reduces reflections from acoustic

impedance between probe and object • Helps improves transmission.

4. Acoustic Lens : • Gray coloured rubber on tip • Helps in focussing the ultrasonic waves as a

slit beam.

Scanning techniques

• Patients’ explanation and consent

• Patient position: sitting (reclining chair)

• US machine position: examiner can see both the screen and the probe

• A foot pedal / switch on machine allows examiner to freeze image

• Modern machines - additional pedal to freeze video clips of a dynamic scan

Technique

The examination includes:

• Topographic echography : shape and location (B scan) • Quantitative echography : Reflectivity, attenuation and structure of any

abnormality (usually also requires an A-scan)

• Kinetic echography : mobility of normal and abnormal structures(spontaneous movement—indicate vessels, after movement—mobility)

Scanning Sequence

• Initially - high gain – visualize vitreous

• Later- lower gain- visualize retina, choroid and any solid lesions

• Probe Positioning

Probe Positioning

• Transocular Approach1. Axial2. Transverse3. Longitudinal

• The lowest possible decibel gain consistent with the maintenance of adequate intensity should be used to optimize the resolution of images.

• B scan pictures can be obtained by axial, transverse and longitudinal sections.

Transverse

Longitudional

Axial

Axial Scan

• Probe : centered on cornea

• White mark pointing nasally

• Nasal part of the retina appears superiorly on screen

• Optic nerve(reference) appears in the centre of the screen

• Probe can be rotated through 360º to examine any part of

posterior pole

• Documenting lesions & membranes in relation to optic

disc

Axial Scan

Transverse scan

• Probe site: opposite part of eye to be studied

• Patient is asked to look in position opposite to probe

• Scan passes through sclera and not the lens

• Movement of transducer: parallel to limbus

• Produces a circumferential slice through several meridians

• View: Lateral extent of a lesion

A transverse scan. Optic nerve appears in centre of picture.

Longitudinal scan

• Transducer –perpendicular to limbus• Probe marker -towards centre of cornea • Optic nerve appears at bottom of scan

View: • Peripheral retina• Antero posterior extent of the lesion• Best –demonstrating the insertion of membranes

to optic disc

A longitudinal scan:Shows posterior vitreous detachment.Optic nerve appears towards bottom of the scan

Indications

A. Opaque Media (Pathology of Posterior segment)

B. Transparent Media

Opaque Media (Anterior)

• Dense Cataract• Miosis• Hyphaema• Hypopyon• Corneal Opacity

• Vitreous Haemorrhage

• Vitritis/ Endophthalmitis

• Pupillary or Retrolenticular membrane

Opaque Media (Posterior)

Transparent Media

• Acquisition of Axial Length

• Inaccurate A – Scan data (Determination of dimensions of Eye ball)

• Proptosis (Poorly represented Orbital Apex)

• Orbital tumours

• Carotico- cavernous fistula(Dilated Ophthalmic vein)

• Suspected Intra Orbital Foreign Body

• Orbital Cellulitis

• Iris & Ciliary body anomalies

• Optic Disc anomaly

• Retinal Detachment (Rhegmatogenous /Exudative – shifting fluid)

• Choroidal Detachment.

Echo description of common intraocular conditions:

1. Vitreous floaters

B scan :: appear as one or more echo dots of less brightness in the mid /posterior vitreous cavity which show mobility with after movement display on

A scan :: these echo dots have extremely low to low reflectivity (2-20%) and to appreciate them better overall gain may be increased by 5- 6db

2. Vitreous haemorrhage

To pick up Fresh vitreous haemorrhage

B scan:: • the overall gain can be increased by 10 db• appear as multiple fine echo opacities dusting the vitreous

body which do not extend beyond the posterior vitreous border.

On A scan, these haemorrhagic spots (echo pulses) show low reflectivity (5-10%). (Figure 4)

3. In endophthalmitis/ vitritis the inflammatory cells which are seen dot like on B scan, are multiple, scattered diffusely or may be localised to the anterior, mid or the posterior one third of the vitreous cavity depending on the etiology. (Figure 7)

On A scan, these dot like opacities show low to medium reflectivity (10-60%).

4. Asteroid Hyalosis

• Is characterized clinically by presence of calcium crystals embedded in an amorphous matrix

• On B scan :: multiple, densely packed, homogeneously distributed echodense dots of medium to high reflectivity (50-100%) which are usually localized to the core of vitreous body

5. Posterior vitreous detachment (PVD):

In PVD ::one sees echogenic membrane concentric to the globe, infront of the retina with clear subvitreal space.

If lined with red blood cells its echo density increases.

On A scan, the reflectivity of this membrane is low if the PVD is thin but it may be high if it is thick and lined with red blood cells.

PVD usually does not show attachment to the optic nerve head. Vitreous haemorrhage

associated with a posterior vitreous detachment (PVD)

6. Retinal detachment

On B scan, it appears as echogenic dense membrane, biconvex or biconcave with 100% attachment at the optic nerve head (ONH) and 90- 100% reflectivity on A scan.

In PVR cases, vitreous body shows debris dots or membrane formation depending upon its grade and cystic degeneration may be present in an old RD.

After movement if present is suggestive of fresh RD.

• In rhegmatogenous RD, retinal tears especially operculated tears/ giant tears and even the trickle of vitreous haemorrhage from the break site into the vitreous cavity may be picked up.

• In tractional RD, fibrovascular frond within the vitreous cavity or along the vitreous face may be seen.

• It does not show after-movement and vitreous cavity may show evidence suggestive of old haemorrhage.

9. Choroidal detachment is usually in the periphery and may be localized or total. (Figure 17)

::seen as dome shaped elevation with clear sub choroidal space on B scan and 90-100% double peaked tall spike on A scan.

There is none or very little after movement on kinetic echography.

10. Intraocular tumors ::

Retinoblastoma is seen as a solid tumor arising from the retinal layer obliterating the vitreous cavity.

Calcification within the tumor mass is typical of retinoblastoma. (Figure 19)

There may be shadowing effect behind the lesion in the orbital mass.

On A scan, spikes with moderate internal reflectivity may be seen but in presence of necrosis and calcification, highly reflective, irregular spikes are observed.

Choroidal naevus/melanoma appears as a small dome shaped, localized, solid lesion, elevated from the ocular coats with low to medium reflective A scan spike (40-60%). Larger melanoma is seen typically as solid dome shaped mass, arising from the choroidal layer with strong border echoes, projecting into the vitreous cavity with solid RD and retromass shadowing effect

Collar stud pattern ( Mushroom shape )Regular internal structure , Acoustic shadowingLow to medium reflectivity , Internal vascularity suggestive of Choroidal Melanoma

Posterior scleritisFluid in Tenon’s layer

Optic nerve sheath is also involved, producing a T-sign.

11 Oculo-orbital trauma may have varied manifestation namely:

• Hyphaema with iridodialysis ·Soft globe with scleral rupture

• Crystalline lens/intraocular lens dislocation into the vitreous cavity

• Vitreous haemorrhage with or without RD

• Expulsive haemorrhage with orbital haemorrhage

• Intraocular foreign body (IOFB)/ orbital FB

• IOFB are seen as echodense spots with a 100% reflectivity on A scan spike irrespective of the nature of the FB and ultrasonography enables its exact sizing and localization.

• Shadowing effect is usually seen.

• Decreasing the gain on the machine by 10db helps in differentiating it from dense blood clot and lens fragment

• Also, shadowing is not seen with lens fragments.

• Foreign bodies less than 0.2mm in size and those in the orbit which are obscured by haemorrhage are best picked up by CT scan.

Posterior staphyloma is• a common finding observed in high myopes.

• It appears as a sudden bowing backward of the globe with thinning of the retinochoroidal layer.

• It is usually seen at the posterior pole and the axial length of the globe is increased, indicating axial myopia. There may be presence of vitreous debri.

Echography-advantages

–Easy to use–No ionizing radiation–Excellent tissue differentiation–Cost effective–Non invasive–High resolution echography provides reliable and accurate assessment– Ideal for follow up of lesion

Disadvantages

–Cannot image both orbits simultaneously–Difficult for a non specialist to interpret–High frequency sounds waves have limited penetration

Colour Doppler USG

• Use of B scan with colour Doppler• Non invasive approach to evaluate ocular blood flow• Useful to assess morphologic and velocitometric dara from ophthalmic artery,

central retinal artery, central retinal vein and posterior ciliary vessels• To evaluate ocular ischemia• To determine blood flow in large and medium sized vessels in

eye(carotid/ophthalmic)• Flow through small vessels—posterior ciliary artery can be visualized and

measured

• Recently power Doppler has 3 times the sensitivity of conventional colour Doppler in detecting blood flow and very useful in imaging of vascular lesions og the globe and orbit

• Another significant advance in recent years is the use of ultrasound biomicroscopy (UBM)

• Highly magnified clear near microscopic images of anterior segment is achieved

• Silverman and colleagues have developed a high frequency annular array transducer with higher depth of field

• Also ultrasonic tissue catheterization is the untangling of hidden patterns in pulse echo data to extract more information about tissue function and pathology than that seen in conventional images

References

1. Ophthalmology Investigation and Examination Techniques-Bruce James and Larry Banjamin

2. Yanoff & Duker Ophthalmology, 3rd ed3. Jack J kanski and brad bowling, 7th ed4. Internet sources