Evaluation of dual-energy subtraction of digital mammography images under conditions found in a commercial unit

Radiological contrast-to-noise ratio (CNR) is evaluated in subtracted images of microcalcifications in breast tissue. CNR is calculated for dual-kVp subtraction combining beams available in a Senographe 2000D, assuming single breast compression. Spectra were obtained from Boone et al (1997 Med. Phys. 24 1863-73), and the study was limited to lowest 25 kV Mo/Mo and highest 40 kV Rh/Rh beams, for 2.58 x 10-4 C kg-1 (1R) total exposure. For a standard case combining 25 kVp Mo/Mo and 40 kVp Rh/Rh beams, predicted maximum CNR for 300 μm calcification in 5 cm thick, 50% glandular, breast is about 1.2, below Rose's criterion for visualization. Total mean glandular doses are about 2.5 cGy for a standard case. The effect that input factors might have on predictions has been evaluated. Choice between alternative spectra can affect CNR by 50%. Assumed calcification composition leads to differences of 67% in calculated CNR, and assumed breast tissue composition can alter CNR by 45%; these results are weakly dependent on calcification or breast thickness, or on the assumed fraction of glandular tissue. CNR values are related to detected spectra effective energy. Calculations predict that above 37 kVp Mo/Mo beams are more energetic than Rh/Rh at the same kVp, due to beam hardening