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 x-ray film, the emulsion is coated on both sides; therefore, it is called double-emulsion 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.

Type Emulsions Characteristics Applications

Intensifying screen TwoBlue or green

sensitiveGeneral

radiography

Laser printing SingleMatches laser

used (about 630 nm)

Laser printers attached to CT, MRI, ultrasound,

etc.

Copy or duplicating SinglePre-exposed to

DmaxDuplicating radiographs

DentalTwo packed in

sealed envelope

Has lead foil to reduce back

scatterDentistry

Types of Film Used in Medical Imaging

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

Standard Film Sizes

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 screen-film 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.

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.

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.

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