Se-based flat-panel detector for screening mammography

A selenium-based flat-panel direct converter detector suitable for digital mammography was developed. The detector is based on a TFT-array with a resolution of 2816 x 2048 pixels, and a pixel pitch of 85 mum. Although the geometric fill factor for each pixel is around 70%, the effective fill factor for the detectors closer to 88% due to internal electric field shaping within the selenium layer. A selenium multilayer p-i-n structure of 200 mum was deposited onto the array by selectively doping the regions near each contact to produce unipolar conducting blocking layers. This structure absorbs more than 95% of a typical mammography beam. The MTF of the device was measured by the slit technique and found to be around 40% at the Nyquist limit of 5.88 1p/mm. Quantum noise-limited performance was demonstrated over a range of exposures between 10 mR and 64 mR. Below 8 mR, fixed pattern noise dominates the SNR characteristics. Due to the high absorption characteristics and high effective fill factor, a DQE of 80% was measured over the exposure range between 10 and 64 mR. At an exposure of 1.7 mR, the DQE(0) drops to about 35%. Finally, the imaging performance of the device was compared to film/screen through a 6-observer scoring of the CDMAM contrast-detail phantom, and it was shown that the selenium detector outperformed film/screen at comparable exposure conditions.