Imaging performance of amorphous selenium based flat-panel detectors for digital mammography: Characterization of a small area prototype detector

Abstract

Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a- Se ) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [ DQE (f)] of a-Se flat-panel detectors for digital mammography. Since the K edge of a-Se is 12.66 keV and within the energy range of a mammographicspectrum, a theoretical model was developed based on cascaded linear system analysis with parallel processes to take into account the effect of Kfluorescence on the modulation transfer function(MTF), noise power spectrum (NPS), and DQE (f) of the detector. This model was used to understand the performance of a small-area prototype detector with 85 μm pixel size. The presampling MTF, NPS, and DQE (f) of the prototype were measured, and compared to the theoretical calculation of the model. The calculation showed that Kfluorescence accounted for a 15% reduction in the MTF at the Nyquist frequency (f Ny ) of the prototype detector, and the NPS at f Ny was reduced to 89% of that at zero spatial frequency. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to charge trapping in the blocking layer at the interface between a-Se and the active matrix. This introduced a drop in both presampling MTF and NPS at high spatial frequency, and reduced aliasing in the NPS. As a result, the DQE (f) of the prototype detector at f Ny approached 40% of that at zero spatial frequency. The measured and calculated DQE (f) using the linear system model have reasonable agreement, indicating that the factors controlling image quality in a-Se based mammographicdetectors are fully understood, and the model can be used to further optimize detectorimaging performance.