Abstract
The effects of charge carrier trapping on image resolution in multilayer photoconductive imaging detectors was analyzed by developing an analytical model for calculating the modulation transfer function (MTF) of the imaging detectors. The MTF model was developed by considering the charge carrier trapping/recombination in the blocking layer and the distributed trapping in the bulk photoconductor layer of multilayer photoconductive imaging detectors. The relative importance of the bulk and blocking layer trapping, as well as the K-fluorescence reabsorption on the resolution, was also analyzed. The model was applied to amorphous selenium- (a-Se) based multilayer detectors for both chest radiographic and mammographic applications. The theoretical model agreed well with experimental results. The charge carrier trapping/recombination in the blocking layer and K-fluorescence reabsorption mainly controlled the resolution of the a-Se mammographic detectors. In addition to these two effects, the bulk trapping also had a significant effect on the resolution of chest radiographic detectors. The blocking layer thickness had more of a profound effect on the resolution than the amount of trap centers in the blocking layer. The resolution deteriorated drastically with increased blocking layer thickness.
Highlights
The effects of charge carrier trapping on image resolution in multilayer photoconductive imaging detectors was analyzed by developing an analytical model for calculating the modulation transfer function (MTF) of the imaging detectors
Amorphous selenium- (a-Se) based direct-conversion X-ray detectors are widely used in commercial digital mammography because they offer much better spatial resolution when compared to the scintillator-based (e.g., CsI) indirect-conversion X-ray detectors [1–3]
A straightforward analytical expression for the modulation transfer function (MTF) due to the distributed carrier trapping in the bulk of the photoconductor in metal/amorphous selenium- (a-Se)/metal type single layer detector structure has been described in Ref. [7]
Summary
Amorphous selenium- (a-Se) based direct-conversion X-ray detectors are widely used in commercial digital mammography because they offer much better spatial resolution when compared to the scintillator-based (e.g., CsI) indirect-conversion X-ray detectors [1–3]. The blocking layers are mainly used to reduce the dark current, but these layers have adverse effects on the charge carrier transport and imaging performances, such as the X-ray sensitivity and the image resolution. If all of the charge carriers are trapped right at the interface between the photoconductor and blocking layer only, its corresponding MTF can be calculated by a simple analytical expression [8,9]. An analytical model considering the charge carrier trapping/recombination in the blocking layer and the distributed trapping in the bulk of the photoconductor has been developed. The relative importance of the bulk and the blocking layer trappings, as well as the K-fluorescence reabsorption on the resolution, has been analyzed
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