Optimization of operational conditions for direct digital mammography detectors for digital tomosynthesis

Amorphous selenium (a-Se) flat-panel digital mammography detectors are being investigated for tomosynthesis, which poses tremendously challenges on the detector temporal and low dose performance. Our previous investigation has demonstrated that a-Se detectors provide adequate temporal performance (lag and ghosting) for tomosynthesis, however its detective quantum efficiency (DQE) at 1 mR (1/10 of average exposure in screening mammography) was only ~1/3 of the value at 10 mR due to electronic noise. Before engineering methods can be developed to overcome this problem, optimization of imaging parameters, such as x-ray spectrum and anti-scatter grid, can greatly improve the detector performance at the low dose used in tomosynthesis. The purpose of this paper is to determine the optimal x-ray spectrum and whether an anti-scatter grid is beneficial in tomosynthesis. The SNR of a 200 μm microcalcification within the breast was calculated as a function of x-ray spectra. Two target materials (Mo and W) were used. The density and thickness of the breast were varied. The scatter to primary ratio behind the breast with and without grid was calculated. The detector performance of a state-of-the-art a-Se digital mammography detector with 85 micron pixel size was incorporated in the calculation of SNR. The total breast dose was kept constant at 1.6 mGy. Our results showed that for tomosynthesis with 11 acquisition views, the optimal kVp is at least 2-3 kVp higher than the optimum for screening mammography. In the extreme case of an 8 cm dense breast, the optimal spectrum was 39 kVp (W/Rh), which was 9 kVp higher than the optimal kVp when detector noise is negligible. W/Rh was found to be the optimal target filter combination for all breast thicknesses (2-8 cm). Our results also showed that grid has no clear advantage even for breast thickness of 8 cm.