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Fig. 4.13. DQE of a screen-film mammography system plotted versus spatial frequency and the x-ray exposure incident on the receptor (Bunch, 1999).
In general radiography, quantum mottle is usually the principal contributor to the optical density fluctuation seen in a uniformly exposed radiograph. Factors affecting the perception of quantum mottle include: (1) film speed and contrast, (2) screen absorption and conversion efficiency, (3) light diffusion, and (4) radiation quality. When speed is increased because of increased x-ray absorption (higher quantum efficiency) by the screen for a given film optical density, quantum mottle is not increased. When speed is increased because of increased light output of the screen per absorbed x ray or increased film speed (faster film or increased developer temperature), fewer x rays are used to form the image and, therefore, quantum mottle is increased. Spreading of light in the screen blurs the recording of quantum noise, so that it becomes less apparent, but also causes a decrease in spatial resolution. Higher energy x rays are more likely to be transmitted through the breast. More importantly, they produce more light per x ray, so that the required optical density can be achieved with fewer quanta, resulting in a higher degree of quantum mottle.
In screen-film mammography, quantum mottle may not be the limiting factor governing noise because of the high quantum efficiency (approximately 70 to 80 percent) of the screen, low average energy of the photons, and the relatively low light emission in the screen (Barnes, 1982; Nishikawa and Yaffe, 1985). In many cases, screen structural noise, variation in the amount of light produced per x-ray and film granularity, due to the random distribution of the finite number of developed silver halide grains, are major noise sources. Film granularity is generally the dominant noise source at spatial frequencies higher than a few cycles per millimeter (Bunch, 1997; Niskikawa and Yaffe, 1985). The graphs of NEQ (Figure 4.12) and DQE (Figure 4.13) for the screen-film system can be quite instructive in indicating where further improvements might be made. They indicate that the maximum DQE is only 35 to 40 percent, suggesting that there is room for at least a 2.5-fold increase in radiation detection efficiency of screen-film systems or an opportunity to produce images of higher information content without an increase in dose. This might be achieved with the use of finer grained film, screens of finer structure, and phosphors that produce a more constant amount of light for each absorbed x-ray quantum (Bunch, 1997). It is also seen that performance falls off in regions of the characteristic curve where the gradient is below its maximum value. This suggests that image quality might be improved by designing characteristic curves that provide a greater range over which the gradient is near maximal.
Higher speed mammographic screen-film combinations resulting in reduced radiation dose can be obtained by using a higher-speed screen or high-speed film. Assuming all other factors are optimized, high-speed systems are generally less sharp or present more noise than images produced using a conventional lower-speed screen-film emulsion combination (Table 4.4). A study (Haus et al., 2001) using data from the ACR-MAP on doses and phantom image evaluation showed that phantom failure rates depended on radiation dose (Figure 4.14). Possible reasons for failure at low doses include: (1) excessive noise from use of high-speed screens, high-speed films or overprocessing or (2) lack of contrast due to insufficient optical density. Reasons for failure at high doses include poor contrast due to excessive optical density, possibly
Table 4.4—Speed, dose, gradient and relative noise of various mammographic screen-film combinations (Haus, 1999b).
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