Film Contrast and Film Latitude

Film contrast refers to the ability of radiographic film to provide a certain level of image contrast. High-contrast film accentuates more black and white areas, whereas low-contrast film primarily shows shades of gray. As discussed in Chapter 9, film latitude is closely related to film contrast. The latitude of film affects the range of radiation exposures that can provide diagnostic optical densities. Films manufactured to display higher contrast have a narrow exposure latitude compared with low-contrast films having a wider exposure latitude.

Spectral Sensitivity

Spectral sensitivity refers to the color of light to which a particular film is most sensitive. In radiography, there are generally two categories of spectral sensitivity films: blue-sensitive and green-sensitive (orthochromatic). When radiographic film is used with intensifying screens, it is important to match the spectral sensitivity of the film with the spectral emission of the screens. Spectral emission refers to the color of light produced by a particular intensifying screen. In radiography, two categories of spectral emission generally exist: blue light–emitting screens and green light–emitting screens. It is critical to use blue-sensitive film with blue light–emitting screens and green-sensitive film with green light–emitting screens. Spectral matching refers to correctly matching the color sensitivity of the film to the color emission of the intensifying screen. An incorrect match of film and screens based on spectral emission and sensitivity results in radiographs that display inappropriate levels of radiographic density.

Screen Speed

The purpose of intensifying screens is to decrease the radiation dose to the patient. Because screen phosphors can intensify the action of the x-rays by converting them to visible light, the use of screens allows the radiographer to use considerably lower mAs. The disadvantage of using screens is the reduction in recorded detail.

Screen manufacturers produce a variety of intensifying screens, which differ in how well they intensify the action of the x-rays and therefore differ in their capacity to produce accurate recorded detail.

The capability of a screen to produce visible light is called screen speed. A faster screen produces more light than a slower screen given the same exposure. Although very fast screens reduce patient exposure, they also degrade image resolution and increase quantum noise, so a balance must be chosen.

Several factors affect how fast or slow an intensifying screen is, including absorption efficiency, conversion efficiency, thickness of the phosphor layer, and size of the phosphor crystal (Table 12-1). The presence of a reflecting layer, an absorbing layer, or dye in the phosphor layer also affects screen speed.

Absorption efficiency refers to the screen’s ability to absorb the incident x-ray photons. A rare earth phosphor screen absorbs approximately 60% of the incident photons. Conversion efficiency describes how well the screen phosphor takes these x-ray photons and converts them to visible light. Increased absorption and conversion efficiency mean that rare earth phosphors have increased speed when compared with a previously used screen phosphor, calcium tungstate. This increased speed allows the radiographer to substantially reduce the x-ray exposure needed to produce images with the appropriate amount of density.

The thickness of the phosphor layer and the size of the crystal also have an effect on screen speed. A thicker phosphor layer contains more phosphor material than a thinner phosphor layer. The phosphor is the material that converts x-rays into light, so if more phosphor material is present in a screen, more light will be produced, increasing the screen speed. The size of the phosphor material crystals also affects screen speed. Larger phosphor crystals produce more light than smaller phosphor crystals. Again, more light being produced means that the screen is faster.

The final factors that affect screen speed are the presence or absence of a reflecting layer, a light-absorbing layer, or light-absorbing dyes in the phosphor layer. A reflecting layer is used to increase screen speed by reflecting light back toward the film (Figure 12-1). A light-absorbing layer or light-absorbing dyes present in the phosphor layer are used to decrease screen speed by absorbing light that would otherwise reach and expose the film.

The ability of the screen to produce visible light can also be described in terms of its relative speed. Relative speed results from comparing screen-film systems based on the amount of light produced for a given exposure. Most radiology departments that use film-screen technology have at least two different speeds of intensifying screen systems. A fast system usually is available with a relative speed of about 400. A 400-speed system is a good compromise between the beneficial effect of decreasing the patient dose and the detrimental effect of decreasing the recorded detail. A slower system is usually available, and it is sometimes labeled on the outside of the cassette as detail or extremity. The relative speed of this system typically is 100. Detail or extremity screen systems are relatively slow, and therefore require greater exposure and result in higher patient doses. However, the anatomic parts imaged with detail or extremity screen systems generally are small; therefore they do not require large exposures. Detail or extremity screen systems produce excellent recorded detail. The radiographer must be careful in selecting the appropriate screen system for the examination ordered. Cassettes with extremity and detail screens should be used only for tabletop examinations. They should never be used in the Bucky tray because of the excessive amount of exposure needed.

Screen Maintenance

The maintenance of intensifying screens is significant because radiographic quality depends in large part on how well the screens are continuously maintained. Two important maintenance procedures should be performed on intensifying screens. The first is regular cleaning. The outside surface of screens comes into contact with the environment and with the hands of those unloading and loading cassettes, which results in the natural oils on fingers and hands being deposited on the screen surface. These oils tend to attract dust and dirt, which can build up to the point at which they are actually imaged on radiographs as artifacts. Screen cleaning should be done routinely. The cleaning is accomplished with commercially available antistatic intensifying screen cleaner fluid and gauze pads.

The second important maintenance procedure is to check cassettes for film-screen contact. Good film-screen contact exists when the screen or screens are in direct contact with the film. Poor film-screen contact greatly degrades recorded detail and is usually seen as a localized area of unsharpness somewhere on the radiographic image. Rarely is film-screen contact so poor that unsharpness can be seen across the entire radiograph. A major part of testing for film-screen contact is identifying problem cassettes.

The film-screen contact test is easily accomplished, but it requires a special wire mesh test tool. The wire mesh tool is placed on the cassette in question and radiographed with an appropriate technique. The resultant radiograph (Figure 12-2) is then viewed from a distance of approximately 6 feet to determine any areas of unsharpness, which indicate poor recorded detail. Areas of poor contact appear darker than areas of good contact because of the increased spreading of the light photons.

The remaining component in the film-screen image receptor is the cassette. Serving as a container for both the intensifying screens and the film, the cassette must be light-proof, lightweight for portability, and rigid enough not to bend under a patient’s weight, all while allowing the maximum amount of radiation to pass through and reach the screens. Low x-ray–absorbing materials, such as thermoset plastic, magnesium, or even graphite carbon, can be found in the front of cassettes. Inside the back of cassettes may be a thin sheet of lead foil designed to absorb backscatter before it exposes the film.

Developing

The primary function of developing is to convert the latent image into a manifest, or visible, image. The purpose of the developing or reducing agents is to reduce exposed silver halide to metallic silver and to add electrons to exposed silver halide. Two chemicals are used to accomplish this purpose: phenidone and hydroquinone. Phenidone is said to be a fast reducer, producing gray (lower) densities. Hydroquinone is said to be a slow reducer, producing black (higher) densities.

Both phenidone and hydroquinone also act to soften and swell the emulsion layers. Phenidone and hydroquinone are said to be synergistic, or to have superadditivity. Superadditivity means that together these chemicals produce a greater effect on the film than they would individually. This is used to advantage by using both chemicals in combination to develop or reduce the exposed silver halide. The developer solution needs an alkaline pH environment for the chemicals to function properly.

During the development process, developer solution donates additional electrons to the sensitivity specks, or electron traps, in the emulsion layers of the film. These additional electrons attract more silver to these areas, thereby amplifying the amount of atomic silver at each latent image center. Exposed silver halide is reduced to metallic silver when bromide and iodide ions are removed from the emulsion. The atomic silver that was exposed to radiant energy (light and x-rays) is converted to metallic silver and presented as radiographic densities. Unexposed silver halide does not react immediately to developer because it has not been ionized and does not accept electrons from the developer. Given extended exposure to developing solution or exposure to excessively heated developing solution, however, even unexposed areas of film can react to developing solution. Exposed silver halide reacts to developer by accepting electrons because neutral atomic silver that was previously bonded to either bromide or iodide has room to accept electrons in its outermost electron shell (the O shell).

Fixing

The primary functions of the fixing stage are to remove unexposed silver halide from the film and to make the remaining image permanent. There are also two secondary functions of fixing. One is to stop the development process; the other is to further harden the emulsions. Fixing solution must function to remove all undeveloped silver halide while not affecting the metallic silver image.

The purpose of the fixing agent is to clear undeveloped silver halide from the film. A thiosulfate (sometimes also called hypo), such as ammonium thiosulfate, is the chemical used as this agent. The fixer solution needs an acidic pH environment for the chemicals to function properly.

Washing

The purpose of the washing process is to remove fixing solution from the surface of the film. This is a further step in making the manifest image permanent. If not properly washed, the resulting radiograph shows a brown staining of the image, resulting in image loss and a decrease in its diagnostic value. This staining is caused by thiosulfate (fixing agent) that remains in the emulsion layers. Some thiosulfate always remains within the film, but the goal of washing is to remove enough so that the radiograph can be used for an extended period.

The process by which washing works is referred to as diffusion. Diffusion exposes the film to water that contains less thiosulfate than the film. Because the film contains more fixing agent than the water, the fixing agent diffuses into the water.

Eventually, thiosulfate concentrations in the wash water can become greater than those in the films being processed; therefore the wash water must be replaced frequently. Water flows freely from the input water supply through the wash tank and down the drain while the roller transport system is operating. This type of system provides a constant supply of fresh wash water to aid in the diffusion process. The moving water also causes agitation and increases diffusion.

Drying

The final process in automatic processing is drying. The purpose of drying films is to remove 85% to 90% of the moisture from the film so that it can be handled easily and stored while maintaining the quality of the diagnostic image. As a result, finished radiographs should retain 10% to 15% of their moisture when processing is complete. If films are dried excessively, the emulsion layers can crack, which decreases the diagnostic quality of the radiograph.

Increased relative humidity decreases the efficiency of dryers in processors, so an increased drying temperature is necessary. Processors are equipped with thermostatic controls to allow selection of a wide range of dryer temperatures.

To chemically process a radiographic image, specialized equipment and systems must perform concurrently to move the film through the processing stages according to the manufacturer’s specifications.