chapter 5Film Imaging

X-RAY FILM

COMPOSITION• (1) emulsion and • (2) base

Emulsion• The two principal components of • 1)silver halide grains,

• which are sensitive to x radiation and visible light • composed primarily of

crystals of silver bromide

•are flat, tabular crystals •are oriented parallel with

the film surface to offer a large cross-sectional area to the x-ray beam

• • INSIGHT film has about twice the number of silver grains so that it requires only half the exposure of Ultra-speed film.

• 2)matrix »in which the crystals are suspended. »absorbs processing solutions, allowing the chemicals to reach

and react with the silver halide grains .

overcoatAn additional layer of vehicle is added to the film emulsion

helps protect the film from damage by scratching, contamination , or pressure from rollers when an automatic processor

Base• function:

» support the emulsion. • Composition:

» polyester polyethylene terephthalate

•provides the proper degree of flexibility

•withstand exposure to processing solutions without becoming distorted

•uniformly translucent and casts no pattern on the resultant radiograph

INTRAORAL X-RAY FILM

dot• One corner of each dental film has a

small, raised dot that is used for film orientation.

• After the film has been exposed and processed, the dot is used to orient the patient’s right and left side images properly.

lead foil• A thin lead foil backing with an

embossed pattern is between the wrappers in the film packet.

• It shields the film from backscatter (secondary) radiation, which fogs the film and reduces subject contrast (image quality).

• It also reduces patient exposure by absorbing some of the residual x-ray beam.

• Most importantly, if the film packet is placed backward in the patient’s mouth so that the tube side of the film is facing away from the x-ray machine, the lead foil will be positioned between the subject and the film.

• In this circumstance, most of the radiation is absorbed by the lead foil, and the resulting radiograph is light and shows the embossed pattern in the lead foil

Periapical View• are used to record the crowns, roots,

and surrounding bone. • three sizes: • (1) size 0 for small children • (2) size 1, which is relatively narrow

and used for views of the anterior teeth

• (3) size 2, the standard film size used for adults

  various sizes of Dental x-ray film

Bitewing View

• Bitewings are useful for detecting interproximal caries and evaluating the height of alveolar bone.

• Size 2 film is normally used in adults; the smaller size 1 is preferred in children. In small children, size 0 may be used. A relatively long size 3 is also available.

Occlusal View• Occlusal film, size 4, is more than

three times larger than size 2

• It is used to• 1) show larger areas of the maxilla or

mandible than may be seen on a periapical film.

• 2) obtain right-angle views to the usual periapical view.

SCREEN FILM• film is used with intensifying screens

to reduce patient exposure• The intensifying screens absorb x

rays and emit visible light, which exposes the film.

• film. It is designed to be sensitive to visible light b

• Crystal type: tabular-shaped (flat) grains of silver halide to capture the image. flat surfaces facing the radiation source, providing a larger cross section (target) and resulting in increased speed without loss of sharpness.

INTENSIFYING SCREENS

FUNCTION• The presence of intensifying screens

creates an image receptor system that is 10 to 60 times more sensitive to x rays than the film alone.

• are used with films for extraoral radiography, including panoramic, Cephalometric, and skull projections.

• are not used intraorally with periapical or occlusal films

• because their use would reduce the resolution of the resulting image below that necessary for diagnosis of much dental disease.

• resolving power of screens is related to their speed:

• the slower the speed of a screen, the greater its resolving power, and vice versa.

COMPOSITION

1) Base

2 )Phosphor Layer

3 )Protective Coat

FORMATION OF THE LATENT IMAGE

• Before exposure, film emulsion consists of photosensitive crystals containing primarily silver bromide

• When the silver halide crystals are irradiated, x-ray photons release electrons from the bromide ions

• The negatively charged sensitivity site

• attracts positively charged free interstitial silver ions

• When a silver ion reaches the negatively charged sensitivity site, it is reduced and forms a neutral atom of metallic silver

PROCESSING SOLUTIONS• Film processing involves the following

procedures:• 1. Immerse exposed film in

developer.• 2. Rinse developer off film in water

bath.• 3. Immerse film in fixer.• 4. Wash film in water bath to remove

fixer.

DEVELOPING SOLUTION• Before exposure, many silver 

bromide crystals (gray) are present  in  the emulsion.

•  After exposure,  the exposed crystals 

• containing neutral silver atoms at latent image sites (orange dots within some crystals) constitute the latent image. 

•  The developer converts the exposed crystals containing neutral silver atoms at the latent image sites into solid grains of metallic silver (black).  

• The fixer dissolves the unexposed, undeveloped silver bromide crystals, leaving only the solid silver grains that form the radiographic image.

DEVELOPING SOLUTION• The developing solution contains four

components, all dissolved in water: (1) developer, (2) activator, (3) preservative, and (4) restrainer.

Developer

ActivatorHydroquinone

provides an electron to reduce the oxidized phenidone back to its original active state so that it can continue to reduce silver halide grains to metallic silver.

Preservative• phenidone and hydroquinone• Phenidone the first electron donor converts silver ions to metallic silver

DEVELOPER REPLENISHER• 8 ounces of fresh developer

(replenisher) per gallon of developing solution.

• This assumes the development of an average of 30 periapical or 5 panoramic films per day

RINSING• the films are rinsed in water for 30

seconds

• Rinsing dilutes the developer, slowing the development process. It also removes the alkali activator, preventing neutralization of the acid fixer.

FIXING SOLUTION• fixer has removed the unexposed

silver bromide crystals.

• Fixer also hardens and shrinks the film emulsion.

• As with developer, fixer should be replenished daily at the rate of 8 ounces per gallon.

• Fixing solution also contains four components, all dissolved in water:

• (1) clearing agent, (2) acidifier, (3) preservative, and (4) hardener.

Clearing Agent• ammonium thiosulfate (“hypo”) • dissolves the unexposed silver halide

grains. • Excessive fixation (hours) results in a

gradual loss of film density because the grains of silver slowly dissolve in the acetic acid of the fixing solution.

Acidifier• contains an acetic acid buffer

system (pH 4 to 4.5) to keep the fixer pH constant.

• The acidic pH is required to promote good diffusion of thiosulfate into the emulsion and of silver thiosulfate complex out of the emulsion. The a

Preservative• Ammonium sulfite is the preservative

in the fixing solution, as it is in the developer.

• It prevents oxidation of the thiosulfate clearing agent, which is unstable in the acid environment of the fixing solution.

Hardener usually aluminum sulfatecomplexes with the gelatin during fixing and prevents damage to the gelatin during subsequent handling. This reduction of swelling lessens mechanical damage to the emulsion and shortens drying time.

WASHING• to remove all thiosulfate ions and

silver thiosulfate complexes. • Any silver compound or thiosulfate

that remains because of improper washing discolors and causes stains, which are most apparent in the radiopaque (light) areas.

IMAGE CHARACTERISTICS

1 -RADIOGRAPHIC DENSITY• The degree of darkening or opacity of

an exposed film is referred to as optical density.

• The optical density of an area of an x-ray film can be measured as follows:

characteristic curve

characteristic curve• A plot of the relationship between film

optical density and exposure• As exposure of the film increases, its

optical density increases.• A film is of greatest diagnostic value

when the structures of interest are imaged on the relatively straight portion of the graph, between 0.6 and 3.0 optical density units.

base plus fog• An unexposed film, when processed,

shows some density. This appearance is caused by the inherent density of the base and added tint and the development of a few unexposed silver halide crystals.

• typically is 0.2 to 0.3.

a- Exposure• Increasing the milliamperage (mA),

peak kilovoltage (kVp), or exposure time increases the number of photons reaching the film and thus increases the density of the radiograph. Reducing the distance between the focal spot and film also increases film density.

b- Subject Thickness• The thicker the subject, the more the

beam is attenuated, and the lighter the resultant image.

• If exposure factors intended for adults are used on children or edentulous patients, the resultant films are dark because a smaller amount of absorbing tissue is in

• the path of the x-ray beam. The dentist should vary exposure time

Aluminum step wedge .

c- Subject Density• The greater the density of a structure

within the subject, the greater the attenuation of the x-ray beam directed through that subject or area

• radiopaque object• radiolucent object.

2- RADIOGRAPHIC CONTRAST• the range of densities on a

radiograph.

• An image that shows both light areas and dark areas has high contrast; this also is referred to as a short gray scale of contrast because few shades

low contrast high contrast

a-subject contrast• Subject contrast is the range of

characteristics of the subject that influences radiographic contrast.

• It is influenced largely by the subject’s thickness, density, and atomic number. The subject con-

The energy of the x-ray beam, selected by the kVp, influences image contrast.

• Changing the time or mA of the exposure (and holding the kVp constant) also influences subject contrast. If the film is excessively light or dark, contrast of anatomic structures is diminished.

b- film contrast• Film contrast describes the inherent

capacity of radiographic films to display differences in subject contrast—that is, variations in the intensity of the remnant beam.

• A high-contrast film reveals areas of small difference in subject contrast more clearly than a low-contrast film.

Film contrast usually is measured as the average slope of the diagnostically useful portion of the characteristic curve.

C) scattered radiation• results from photons that have

interacted with the subject by Compton or coherent interactions.

• This scattered radiation causes fogging of a radiograph—an overall darkening of the image—and results in loss of radiographic contrast.

• In most dental applications, • the best means of reducing scattered

radiation are to use a relatively • low kVp and to collimate the beam to

the size of the film to • prevent scatter from an area outside

the region of the image.

3- RADIOGRAPHIC SPEED• Radiographic speed refers to the

amount of radiation required to produce an image of a standard density.

• Film speed frequently is expressed as the reciprocal of the exposure (in roentgens) required to produce an optical density of 1 above base plus fog

• A fast film requires a relatively low exposure to produce a density of 1, whereas a slower film requires a longer exposure for the processed film to have the same density.

• Film speed is controlled largely by

the size of the silver halide grains and their silver content.

Characteristic curves for two films demonstrating greater inherent latitude of film B compared with film A.

4- FILM LATITUDE• Film latitude is a measure of the

range of exposures that can be recorded as distinguishable densities on a film.

• A film optimized to display wide latitude can record a subject with a wide range of subject contrast.

A film with a characteristic curve that has a long straight-line portion and a shallow slope has wide latitude

5- RADIOGRAPHIC NOISE• On intraoral dental film, mottle may

be seen as film graininess, which is caused by the visibility of silver grains in the film emulsion, especially when magnification is used to examine an image. Film graininess is most evident when high-temperature processing is used.

• Radiographic mottle is also evident when the film is used with fast intensifying screens.

• Two important causes of the phenomenon are quantum mottle and screen structure mottle.

• Quantum mottle is caused by a fluctuation in the number of photons per unit of the beam cross-sectional area absorbed by the intensifying screen.

• Screen structure mottle is graininess caused by screen phosphors.

• Quantum mottle and screen structure mottle are each most evident when fast film-screen combinations are used.

6- RADIOGRAPHIC SHARPNESS AND RESOLUTION• Sharpness is the ability of a

radiograph to define an edge precisely(e.g., the dentin-enamel junction, or a thin trabecular plate).

• Resolution, or resolving power, is the ability of a radiograph to record separate structures that are close together.

• The groups of lines and spaces are arranged in the test target in order of increasing numbers of lines and spaces per millimeter.

a) Image Receptor Blurring• With intraoral dental x-ray film, the

size and number of the silver grains in the film emulsion determines image sharpness:

• the finer the grain size, the finer the sharpness.

• In general, slow-speed films have fine grains, and faster films have larger grains.

•

Use of intensifying screens in extraoral radiography has an adverse effect on image sharpness. The spreading light causes a blurring of fine detail on the radiograph.

Intensifying screens with large crystals are relatively fast, but image sharpness is diminished.

• The presence of an image on each side of a double-emulsion film also causes a loss of image sharpness through parallax.

• When intensifying screens are used, parallax distortion contributes to image unsharpness because light from one screen may cross the film base and reach the emulsion on the opposite side.

Parallax unsharpness  results when double-emulsion film  is used

b) Motion Blurring• Image sharpness also can be lost

through movement of the film, subject, or x-ray source during exposure.

• Movement of the x-ray source in effect enlarges the focal spot and diminishes image sharpness. Patient movement can be minimized by stabilizing the

c) Geometric Blurring• The larger the focal spot, the greater

the loss of image sharpness.

• Image sharpness is improved by increasing the distance between the focal spot and the object and reducing the distance between the object and the image receptor.

7- IMAGE QUALITY• It combines the features of density,

contrast, latitude, sharpness, resolution, and perhaps other parameters.

• Often a system can be optimized for one of these parameters, but this usually is achieved at the expense of others.

COMMON CAUSES OF FAULTY RADIOGRAPHS

LIGHT RADIOGRAPHS

• Processing Errors• Underdevelopment (temperature too low; 

time too short; thermometer inaccurate)• Depleted developer solution• Diluted or contaminated developer• Excessive fixation• Underexposure• Insufficient mA• Insufficient kVp• Insufficient time• Film-source distance too great• Film packet reversed in mouth 

DARK RADIOGRAPHS

• Processing Errors• Overdevelopment (temperature too 

high; time too long)• Developer concentration too high• Inadequate time in fixer• Accidental exposure to light• Improper safelighting• Storage of films without shielding, at 

too high a temperature, or past expiration date

• Overexposure• Excessive mA• Excessive kVp• Excessive time• Film-source distance too short

INSUFFICIENT CONTRAST

• Underdevelopment• Underexposure• Excessive kVp• Excessive film fog

FILM FOG

• Improper safelighting (improper filter; excessive bulb wattage; inadequate distance between safelight and work surface; prolonged exposure to safelight)

• Light leaks (cracked safelight filter; light from doors, vents, or other sources)

• Overdevelopment• Contaminated solutions• Deteriorated film (stored at high 

temperature; stored at high humidity; exposed to radiation; outdated)

DARK SPOTS OR LINES

• Fingerprint contamination• Protective wrapping paper sticking to film 

surface• Film in contact with tank or another film 

during fixation• Film contaminated with developer before 

processing• Excessive bending of film• Static discharge to film before processing• Excessive roller pressure during automatic 

processing• Dirty rollers in automatic processing

LIGHT SPOTS

• Film contaminated with fixer before processing

• Film in contact with tank or another film during development

• Excessive bending of film

YELLOW OR BROWN STAINS

• Depleted developer• Depleted fixer• Insufficient washing• Contaminated solutions

BLURRING

• Movement of patient• Movement of x-ray tube head• Double exposure

PARTIAL IMAGES

• Top of film not immersed in developing solution

• Misalignment of x-ray tube head (“cone cut”)

EMULSION PEEL• Abrasion of image during 

processing• Excessive time in wash water

thanks