Analog Imaging I
Radiographic Film
By Professor Stelmark
The construction and characteristics of radiographic film are similar to those
of regular photographic film. Radiographic film is manufactured with
rigorous quality control and has a spectral response different from that of
photographic film; however, its mechanism of operation is much the same.
Radiographic film basically has two parts: the base and the emulsion. In most xray film, the emulsion is coated on both sides; therefore, it is called doubleemulsion film. Between the emulsion and the base is a thin coating of material
called the adhesive layer, which ensures uniform adhesion of the emulsion to the
base. This adhesive layer allows the emulsion and the base to maintain proper
contact and integrity during use and processing.
The emulsion is enclosed by a protective covering of gelatin called the overcoat.
This overcoat protects the emulsion from scratches, pressure, and contamination
during handling, processing, and storage and allows for relatively rough
manipulation of x-ray film before exposure. Processed film may be handled with
even less regard for damage.
Film Base
Conventional photographic film has a much thinner base than
radiographic film and therefore is not as rigid.
The base of radiographic film is semirigid, lucent, and made of polyester.
Emulsion
The emulsion is the heart of the x-ray film. It is the material with which x-rays
or light photons from radiographic intensifying screens interact and transfer
information. The emulsion consists of a homogeneous mixture
of gelatin and silver halide crystals.
The gelatin is similar to that used in salads and desserts but is of much higher
quality. It is clear, so it transmits light, and it is sufficiently porous for
processing chemicals to penetrate to the crystals of silver halide. Its principal
function is to provide mechanical support for silver halide crystals by holding
them uniformly dispersed in place.
The silver halide crystal is the active ingredient of the radiographic emulsion. In the typical
emulsion, 98% of the silver halide is silver bromide; the remainder is usually silver
iodide. These atoms have relatively high atomic numbers (ZBr =35, ZAg =47, ZI =53)
compared with the gelatin and the base (for both, Z ≈ 7). The interaction of x-ray and light
photons with these high-Z atoms ultimately results in the formation of a latent image on the
radiograph.
Depending on the intended imaging application, silver halide crystals may have tabular,
cubic, octahedral, polyhedral, or irregular shapes. Tabular grains are used in most
radiographic films.
Tabular silver halide crystals are flat and typically 0.1 μm thick, with a triangular, hexagonal,
or higher-order polygonal cross section
The shape and lattice structure of silver halide crystals are not perfect, and some
of the imperfections result in the imaging property of the crystals. The type of
imperfection thought to be responsible is a chemical contaminant, usually silver
sulfide, which is introduced by chemical sensitization into the crystal lattice, usually
at or near the surface.
This contaminant has been given the name sensitivity center. During exposure,
electrons and silver ions are attracted to these sensitivity centers, where they
combine to form a latent image center of metallic silver.
Differences in speed, contrast, and resolution among various radiographic
films are determined by the process by which silver halide crystals are
manufactured and by the mixture of these crystals into the gelatin. The
number of sensitivity centers per crystal, the concentration of crystals in the
emulsion, and the size and distribution of the crystals affect the performance
characteristics of radiographic film.
Direct-exposure film contains a thicker emulsion with more silver halide crystals
than screen-film. The size and concentration of silver halide crystals primarily
affect film speed. The composition of the radiographic emulsion is a proprietary
secret that is closely guarded by each manufacturer.
Radiographic film is manufactured in total darkness. From the moment the
emulsion ingredients are brought together until final packaging, no light is
present.
Immediately after exposure, no image can be observed on the film. An
invisible image is present, however, and is called a latent image. With proper
chemical processing, the latent image becomes a visible image or manifest
image.
The interaction between photons and silver halide crystals is fairly well
understood, as is the processing of the latent image into the visible image.
However, the formation of the latent image, sometimes called
the photographic effect, is not well understood and continues to be the
subject of considerable research.
The Gurney-Mott theory, the accepted, although incomplete, explanation of
latent image formation.
TYPES OF FILM
Medical imaging is becoming extremely technical and sophisticated, and this
is reflected in the number and variety of films that are now available. Each
major film manufacturer produces many different films for medical imaging.
When combined with the various film formats offered, more than 500
selections are possible.
Types of Film Used in Medical Imaging
Type
Emulsions
Characteristics
Applications
Intensifying screen
Two
Blue or green
sensitive
General
radiography
Laser printing
Single
Matches laser
used (about 630
nm)
Laser printers
attached to CT,
MRI, ultrasound,
etc.
Copy or duplicating
Single
Pre-exposed to
Dmax
Duplicating
radiographs
Dental
Two packed in
sealed envelope
Has lead foil to
reduce back
scatter
Dentistry
Standard Film Sizes
English Units
SI Units
7×7 in
18×18 cm
8×10 in
20×25 cm
10×12 in
24×30 cm
14×14 in
35×35 cm
14×17 in
35×43 cm
When light is emitted by a radiographic intensifying screen, it not only exposes
the adjacent emulsion, it can also expose the emulsion on the other side of the
base. When light crosses over the base, it causes increased blurring of the
image
The addition of a light-absorbing dye in a crossover control layer reduces
crossover to near zero.
Spectral Matching
Perhaps the most important consideration in the selection of modern screenfilm is its spectral absorption characteristics. Since the introduction of rare
Earth screens in the early 1970s, radiologic technologists must be particularly
careful to use a film whose sensitivity to various colors of light—its spectral
response—is properly matched to the spectrum of light emitted by the screen.
Improper handling or processing can cause artifacts, the marks or spurious
images that sometimes appear on the processed radiograph. Artifacts also can
be generated by the useful x-ray beam. Radiographic film is pressure sensitive,
so rough handling or the imprint of any sharp object, such as a fingernail, is
reproduced as an artifact on the processed radiograph.
Calcium tungstate screens, which emit blue and blue-violet light, have been
largely replaced with rare Earth screens, which are faster. Now, many rare Earth
phosphors emit ultraviolet, blue, green.
If green-emitting screens are used, they should be matched with a film that is
sensitive not only to blue light but also to green light. Such film
is orthochromatic and is called green-sensitive film.
This is distinct from panchromatic film, which is used in photography and is
sensitive to the entire visible light spectrum.
Radiographic films are blue-sensitive or green-sensitive, and
they require amber- and red-filtered safelights, respectively.
Safelights
The use of radiographic film requires certain precautions in the darkroom.
Most safelights are incandescent lamps with a color filter; safelights provide
enough light to illuminate the darkroom while ensuring that the film remains
unexposed.
Proper darkroom illumination depends not only on the color of the filter but
also on the wattage of the bulb and the distance between the lamp and the
work surface.
HANDLING AND STORAGE OF FILM
Radiographic film is a sensitive radiation detector and must be handled
accordingly. Improper handling and storage result in poor radiographs with
artifacts that interfere with diagnosis. For this reason, it is essential that anyone
who handles radiographic film should be careful not to bend, crease, or
otherwise subject it to rough handling. Clean hands are a must, and hand
lotions should be avoided.
Creasing of the film before processing produces a line artifact. Dirt on the
hands or on radiographic intensifying screens produces specular artifacts. In a
dry environment, static electricity can cause characteristic artifacts.
During automatic processing, a worn or dirty transport system can cause
artifacts that are usually identifiable by their repetition.
Heat and Humidity
Radiographic film is sensitive to the effects of elevated temperature and humidity,
especially for long periods. Heat increases the fog of a radiograph and therefore
reduces contrast. Consequently, radiographic film should be stored at temperatures
lower than approximately 20 °C (68 °F). With higher storage temperatures, the
longer the time of storage, the more severe is the loss of contrast that results from
the increase in fog.
Ideally, radiographic films should be kept in refrigerated storage. Storage for a year
or longer is acceptable if the film is maintained at 10 °C (50 °F). Film should never
be stored near steam pipes or other sources of heat.
Storage under conditions of elevated humidity (e.g., over 60%) also reduces
contrast because of increased fog. Consequently, before use, radiographic film
should be stored in a cool, dry place, ideally in a climate-controlled environment.
Storage in an area that is too dry can be equally objectionable. Static artifacts are
possible when the relative humidity dips to below about 40%.
Light
Radiographic film must be stored and handled in the dark. Any light at all can
expose the emulsion before processing. If low-level, diffuse light exposes the
film, fog is increased. If bright light exposes or partially exposes the film, a
gross, obvious artifact is produced.
Control of light is ensured by a well-sealed darkroom and a light-proof storage
bin for film that has been opened but not clinically exposed. The storage bin
should have an electrical interlock that prevents it from being opened while the
door to the darkroom is ajar or open
Radiation
Ionizing radiation, other than the useful beam, creates an image artifact by
increasing fog and reducing contrast. Film fog is the dull, uniform optical
density that appears if the film has been inadvertently exposed to light, x-rays,
heat, or humidity.
Darkrooms usually are located next to x-ray rooms and are lined with lead.
However, this is not always necessary. It is usually acceptable to lead-line only
the storage shelf and the film bin.
Radiographic film is more sensitive after an exposure
than before. This is called sensitization.
It is bad practice to store film and boxes of chemistry in the same cupboard.
Most hospitals receive film each month and purchase enough film for 5
weeks of use. The extra few days beyond monthly use are necessary to
cover civil emergencies that require an unexpectedly large number of x-ray
examinations. Given a 5-week supply schedule and the first-in, first-out
rule, 30 days is a reasonable maximum storage time for radiographic
film.
Film Storage
Following are some important characteristics of radiographic film:
•Contrast. High-contrast film produces black-and-white images. Low-contrast film
produces images with shades of gray.
•Latitude. Latitude is the range of exposure techniques (kVp and mAs) that
produce an acceptable image.
•Speed. Speed is the sensitivity of the screen-film combination to x-rays and light.
Fast screen-film combinations need fewer x-rays to produce a diagnostic image.
•Crossover. When light is emitted from a radiographic intensifying screen, it
exposes not only the adjacent film emulsion but also the emulsion on the other
side of the base. The light crosses over the base and blurs the radiographic
image.
•Spectral Matching. The x-ray beam does not directly expose the x-ray film.
Radiographic intensifying screens emit light when exposed to x-rays and the
emitted light then exposes the radiographic film. The color of light emitted must
match the response of the film.
Static Discharge
Finger Marks
Backscatter
Light Leak
Radiation Fog
Water Stain