Detector evaluation of a prototype amorphous selenium-based full field digital mammography system

This study evaluated the physical performance of a selenium-based direct full-field digital mammography prototype detector (Siemens Mammomat Novation(DR)), including the pixel value vs. exposure linearity, the modulation transfer function (MTF), the normalized noise power spectrum (NNPS), and the detective quantum efficiency (DQE). The current detector is the same model which received an approvable letter from FDA for release to the US market. The results of the current prototype are compared to those of an earlier prototype. Two IEC standard beam qualities (RQA-M2: Mo/Mo, 28 kVp, 2 mm A1; RQA-M4: Mo/Mo, 35 kVp, 2 mm A1) and two additional beam qualities (MW2: W/Rh, 28 kVp, 2 mm A1; MW4: W/Rh, 35 kVp, 2 mm A1) were investigated. To calculate the modulation transfer function (MTF), a 0.1 mm Pt-Ir edge was imaged at each beam quality. Detector pixel values responded linearly against exposure values (R-2 0.999). As before, above 6 cycles/mm Mo/Mo MTF was slightly higher along the chest-nipple axis compared to the left-right axis. MTF was comparable to the previously reported prototype, with slightly reduced resolution. The DQE peaks ranged from 0.71 for 3.31 mu C/kg (12.83 mR) to 0.4 for 0.48 mu C/kg (1.86 mR) at 1.75 cycles/mm. for Mo/Mo at 28 kVp. The DQE range for W/Rh at 28 kVP was 0.81 at 2.03 mu C/kg (7.87 mR) to 0.50 at 0.50 mu C/kg (1.94 mR) at 1 cycle/mm. NNPS tended to increase with greater exposures, while all exposures had a significant low-frequency component. Bloom and detector edge artifacts observed previously were no longer present in this prototype. The new detector shows marked noise improvement, with slightly reduced resolution. There remain artifacts due to imperfect gain calibration, but at a reduced magnitude compared to a prototype detector.