Films

Most films used in mammography are single emulsion and are used in combination with a single back screen (AAPM, 1990; ACR, 1993; Haus, 1991; 1999b; Kimme-Smith, 1991; Yaffe, 1990). Some

Fig. 3.17. Relative emission spectrum of a Gd2O2S:Tb. The screen superimposed on a graph showing the spectral sensitivity of a commonly used mammographic film. The high spectral emission peak of the green emitting screen coincides with the high sensitivity of the film to green light (Haus, 1999b).

companies have introduced double-emulsion films used in combination with a single-intensifying screen for mammography. Singleemulsion films used for mammography are coated with larger amounts of silver halide and gelatin on a single side than are double-emulsion films used in conventional radiography. Three-dimensional silver halide grains have been widely used for mam-mography film emulsions. Recently, mammography films have been introduced with cubic grain emulsions. The uniform chemical and spectral sensitization of cubic grains result in high contrast, especially in the toe portion of the curve, which is very useful in mammography.

3.2.4 Film-Processing System

Film processing must be considered as part of a system, which includes the automatic film processor, film type, and chemicals (Batz and Haus, 1993; Haus, 1993; Haus and Jaskulski, 1997). These components must be considered together as a system and must be properly optimized, in order to obtain appropriate image quality in terms of proper optical density and film contrast of the processed radiograph. The resulting film speed affects the radiation dose to the patient. Automatic film-processor variables include: (1) processing cycle time, (2) temperature, (3) chemicals, (4) replenishment, (5) agitation, and (6) drying. Figure 3.18 illustrates the operation of a typical automatic film processor.

3.2.4.1 Processing Cycle Time. Processing cycle time is usually defined as the time it takes for: (1) the leading edge of the film to enter and exit the processor or (2) the leading edge of the film to enter and the trailing edge of the film to exit the processor. The latter definition will be used in this Section. Processing cycles range

Fig. 3.18. Operation of a typical automatic film processor. Typically, film is manually inserted into the processor transport system from the feed tray. The film is transported through (a) the developer rack, (b) the fixer rack, (c) the wash rack, (d) the dryer section, and (e) exits dry and ready to read. The film path is a "serpentine" route. This enables proper developer agitation, as well as maximum chemical-to-emulsion "coupling," which produces the optimum development for speed and contrast. Developer makes the latent image visible. Fixer essentially "stops" the development process and makes the resultant image "permanent" for archiving purposes. Washing removes chemicals to enable uniform drying and long-term, archival retention of the radiograph (Haus, 1999a).

Film Exit

Fig. 3.18. Operation of a typical automatic film processor. Typically, film is manually inserted into the processor transport system from the feed tray. The film is transported through (a) the developer rack, (b) the fixer rack, (c) the wash rack, (d) the dryer section, and (e) exits dry and ready to read. The film path is a "serpentine" route. This enables proper developer agitation, as well as maximum chemical-to-emulsion "coupling," which produces the optimum development for speed and contrast. Developer makes the latent image visible. Fixer essentially "stops" the development process and makes the resultant image "permanent" for archiving purposes. Washing removes chemicals to enable uniform drying and long-term, archival retention of the radiograph (Haus, 1999a).

from approximately 90 to 210 s depending on whether standard- or extended-cycle processing is used. Standard processing cycles are between 90 and 150 s. Developer temperature and replenishment rates are determined by the processing cycle in order to achieve the desired sensitometric characteristics (optical density contrast, speed, base-plus-fog values) for the type of film being used.

Extended-cycle processing has been used for some singleemulsion films (Kimme-Smith et al., 1989b; Tabar and Haus, 1989). In extended-cycle processing, the film remains in the developer longer and developer temperature is not altered significantly. For some single-emulsion films, the film contrast is higher and the film speed is increased resulting in a reduction of radiation dose when extended-cycle processing is used.

Recently introduced films for mammography with cubic grain emulsions, which are recommended for standard-cycle processing, provide film contrast comparable to or higher than films designed for extended-cycle processing. The cubic grain emulsions do not benefit from extended-cycle processing. Other benefits of standard-cycle processing over extended-cycle processing include improved productivity and reduced wet-pressure artifacts (Haus, 1999a).

3.2.4.2 Developer Temperature. Developer temperatures in automatic film processors range from 33 to 39 °C. The developer temperature depends on film type, chemicals, and transport speed. Figure 3.19 illustrates the effect of developer temperature differences on film speed, film contrast, and fog levels. These variables can be expected to change in similar fashion as a function of development time.

Note that when the developer temperature is lower than the manufacturer's recommendation, film speed is reduced. This may dictate an unnecessary increase in radiation dose to produce mam-mograms of proper optical density. Similarly, film contrast is reduced when developer temperature is lowered. Conversely, if the developer temperature is higher or development time longer (extended-cycle process), then the manufacturer's recommendation film speed is increased. This may permit a reduction in radiation dose and film contrast may also be increased. However, these changes can be expected to cause quantum mottle and, thus, radiographic noise to increase. In addition, film fog may increase with increased developer temperature. Developer stability may also be affected adversely when higher-than-recommended developer temperatures are used.

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