Semiconductor-based photon counting detectors (PCDs) measure each incident photon energy through direct conversion process. Comparing to the conventional scintillator-based energy-integrating detectors (EIDs), it can be made with smaller pixel sizes and hence improve the system spatial resolution. The actual performance of the PCDs in CT system is compromised from ideal mainly due to two factors: charge sharing effect and pulse pileup. In particular, the charge sharing effect introduces signal crosstalk between neighboring pixels and degrade the detection spatial resolution. In this study, we derive a rigorous charge sharing detection formalism based on a comprehensive detector response model. The simulated results are compared with a pixel-to-pixel covariance measurement from a CZT-based prototype photon counting system. The results suggests that while a large fraction of detected events is affected by charge sharing, the actual measured crosstalk between neighboring pixels is greatly suppressed by a proper detection threshold in the counting mode. To understand the practical impact in image quality and optimize system design, a simplified crosstalk model based on the estimated charge sharing event rate is integrated into a system level simulation framework with realistic system specifications. The simulated image MTFs are measured for systems with four different pixel sizes. Results indicate that with realistic charge sharing effect, in the pixel size range that we tested, the image MTF steadily increases as the detector pixel size decreases.
Accurate physics modeling of a photon counting detector is essential for detector design and performance evaluation, Computer Tomography (CT) system-level performance investigation, material decomposition, image reconstruction. The detector response is complicated because various effects involve, including fluorescence X-rays, primary electron path, charge diffusion, charge repulsion, and charge trapping. In this paper, we will present a comprehensive detector modeling approach, which incorporates all these effect into account.
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