Screen Film Combination

The goal in screen-film mammography for mass screening and diagnosis is to produce consistently high-contrast, high-resolution, low-noise images at the lowest radiation dose consistent with these image-quality requirements. In recent years, there have been many significant technologic improvements in mammographic screen-film combinations (AAPM, 1990; Haus, 1991; 1999b; KimmeSmith, 1991; Rothenberg and Haus, 1995; Yaffe, 1990). Until the early 1970s, direct-exposure (industrial type) x-ray films were used which often required long exposure times (causing blur due to motion) and resulted in high radiation exposure (Bassett et al., 1992; Egan, 1976; Gold et al., 1990; Haus and Cullinan, 1989). Films were processed manually in tanks or in film processors with long processing times. In the early 1970s, screen-film combinations for mammography became commercially available (Bassett et al., 1992; Haus and Cullinan, 1989; Ostrum et al, 1973; Wayrynen, 1979).

Today, mammography is performed with screen-film combinations having significantly improved imaging characteristics designed specifically for mammography. Film processing has also improved significantly over the years. Figure 3.15 shows characteristic curves of (1) a typical direct-exposure mammographic film

Single Emulsion Film
Fig. 3.15. Characteristics curves for (a) a direct-exposed film, (b) a single-screen, single-emulsion film used in the 1970s and early 1980s, and (c) a single-screen, single-emulsion film combination used for mammogra-phy today (Haus, 1999b).

widely used approximately 40 y ago; (2) a typical single-screen, single-emulsion film combination commonly used in the 1970s and early 1980s; and (3) a typical single-screen, single-emulsion film combination used today. These curves illustrate relative speed and contrast differences.

3.2.2 Screens

The great majority of mammographic images are produced with a single-intensifying screen used as a back screen in combination with a single-emulsion film (Figure 3.16). Many mammographic screens incorporate phosphors containing metals from the lanthanide series of elements such as terbium-activated gadolinium oxysulfide (Gd2O2S:Tb). Screens may incorporate light absorbers in the phosphor that increase sharpness. Intensifying screens have a

Radiology Intensifying Screens

Fig. 3.16. Diagrams comparing physical configurations for a single-emulsion film in contact with a single back-intensifying screen (as used for mammography) and double-emulsion film sandwiched between two intensifying screens (used for other radiologic procedures). Note that the x-ray source would be above in both cases (Haus, 1999b).

Fig. 3.16. Diagrams comparing physical configurations for a single-emulsion film in contact with a single back-intensifying screen (as used for mammography) and double-emulsion film sandwiched between two intensifying screens (used for other radiologic procedures). Note that the x-ray source would be above in both cases (Haus, 1999b).

protective overcoat to resist surface abrasion and are edge-sealed to minimize edge wear and moisture absorption. The screen base includes a backing layer to eliminate screen curl. Mammographic screens consisting of Gd2O2S:Tb material have their primary emission peak emit in the green spectral region (545 nm) and they also emit in other regions of the visible spectrum from 382 to 622 nm (Figure 3.17) (Haus, 1999b).

In the future, intensifying screens with other phosphors may become available that may offer benefits such as increased spatial resolution and reduced radiographic noise, without increasing radiation dose (Kitts, 1997).

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    Why single emulsion film is used in mammography?
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