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Scattered radiation plays a significant role in mammographic imaging, with scatter fractions over 50% for larger, denser breasts. For screen-film systems, scatter primarily affects the image contrast, reducing the conspicuity of subtle lesions. While digital systems can overcome contrast degradation, they remain susceptible to scatter's impact on the image resolution and noise. To better understand this impact, we have created a Monte Carlo model of a mammographic imaging system adaptable for different imaging situations. This model flags primary and scatter photons and therefore can produce primary-only, scatter-only, or primary plus scatter images. Resolution was assessed using the edge technique to compute the Modulation Transfer Function (MTF). The MTF of a selenium detector imaged with a 28 kVp Mo/Mo beam filtered through a 6 cm heterogeneous breast was 0.81, 0.0002, and 0.65 at 5 mm-1 for the primary beam, scatter-only, and primary plus scatter beam, respectively. Noise was measured from flat-field images via the noise power spectrum (NNPS). The NNPS-exposure product using the same imaging conditions was 1.5 x 10-5 mm2x mR, 1.6 x 10-5 mm2x mR, and 1.9 x 10-5 mm2x mR at 5 mm-1 for the primary, scatter, and primary plus scatter beam, respectively. The results show that scatter led to a notable low-frequency drop in the MTF and an increased magnitude of the NNPS-exposure product. (This work was supported in part by USAMRMC W81XWH-04-1-0323.)
Saunders et al. (Thu,) studied this question.