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.
For large cone angle multi-detector CT (MDCT), the scattered radiation remains a challenging problem as it is part of the physics process in X-ray interaction. For a photon counting CT system, the scattered radiation has more profound impact to the system performance, as the scattered photons dominate the low energy regime of the measurement. Without proper corrections, the scattered radiation could introduce significant errors in the material decomposition, and degrade the material characterization and quantification accuracy. To mitigate the scatter problem, typically, hardware rejection and software correction algorithms can be both employed. The anti-scatter grids (ASG) are commonly used to absorb the scattered photons and help generate cleaner measurements. For semiconductor based photon counting detectors (CdTe/CdZnTe), due to charge sharing and cross-talk effects (k-escape, scatter), different ASG designs also change the detector spectral response by masking different detector areas. In this study, we will evaluate a CdZnTe based photon counting CT performance with various ASG designs at the low flux condition through simulations. The detector spectral responses with 2 different detector pixel sizes (250 um and 500um anode size) are generated by our internal simulation tool, using no ASG, 1-D and 2-D ASGs respectively. The scattered radiation is generated by GATE, a Geant4 based Monte Carlo simulation tool, using a large (33 cm diameter) cylindrical water phantom with concentric iodine/calcium inserts, and then added to the simulated phantom energy bin count measurements. The impact of the residual scatter with 1-D and 2-D ASGs in the basis and mono-energetic images will be evaluated and compared.
Photon counting detectors are an appealing approach to spectral computed tomography for their theoretical benefits over conventional detectors. Detailed modeling and simulation is important for capturing the critical aspects of the counting and spectral performance of the detector. An approach to photon counting detector simulation is presented using a custom developed software program. The software consists of Monte-Carlo energy deposition, physics-based charge transport and current induction, and SPICE electronic simulation. It utilizes behind-the-scenes Gate for the photon interactions and energy deposition and ngspice for the SPICE electronic simulations. Various sensor geometries and definitions can be defined to simulation individual detector pixels or entire anode arrays for large-scale simulations. The simulation requires the specification of x-ray planar sources and can be specified on a per-channel basis with an energy distribution and flux. Given a sensor definition and a series of x-ray sources, the program calculates the energy-bin count read-out from each anode in the sensor array. The program can be used to study the detector response of various sensor and system geometries, including in the presence of anti-scatter grids, the performance of anti-charge sharing implementations, material decomposition algorithms, etc.
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