Cascaded systems analysis of photon-counting field-shaping multi-well avalanche detectors (SWADs)

Single-photon-counting (SPC) x-ray detectors are expected to play a key role in the next generation of medical x-ray imaging. The spatial resolution of SPC x-ray detectors is an important design criterion, in particular for mammography in which one of the primary aims is to detect and differentiate micro-calcifications. The purpose of this abstract is to extend the cascaded systems approach to investigate the influence of reabsorption of characteristic x rays on SPC spatial resolution. A parallel-cascaded model is used to describe reabsorption of characteristic x rays following photoelectric interactions. We use our model to calculate the large-area gain and modulation transfer function (MTF) of amorphous selenium (a-Se) SPC detectors that use a field-Shaping multi-Well Avalanche Detector (SWAD) structure to overcome the low conversion gain of a-Se. Our model accounts for emission and reabsorption of characteristic x rays, x-ray conversion to electron-hole pairs, avalanche gain and gain variance, integration of secondary quanta in detector elements, electronic noise, and energy threshold. Theoretical predictions are compared with the results of Monte Carlo simulations. Our analysis shows that under mammographic imaging conditions, the a-Se/SWAD structure with an avalanche gain of 10 or greater results in minimal loss of photon counts below the electronic noise floor for electronic noise levels ∼500 - 700 e-h pairs. Double counting of characteristic x-rays inflates the large-area gain by ∼20% relative to the quantum efficiency, and results in modest MTF degradation relative to energy-integrating systems. Excellent agreement between theoretical and Monte Carlo analyses was observed. This approach provides a theoretical framework for understanding SPC detector performance and for system optimization.