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Theoretical count rate capabilities of polycrystalline silicon photon counting imagers for CBCT applications

Photon counting detectors are of increasing interest for clinical imaging. By measuring and digitally recording the deposited energy of each detected x-ray photon, these detectors can decrease the effect of electronic readout noise and Swank noise, potentially leading to improved image quality at decreased dose. The energy information of each photon can also be used to perform advanced imaging techniques such as material separation and k-edge imaging. Photon counting detectors employing crystalline silicon for the detector backplane are currently used for mammographic imaging and are under active investigation for fan-beam breast computed tomography (BCT). Our group has been exploring the possibility of creating monolithic, large-area photon counting detectors in order to perform cone-beam CT (CBCT) for BCT and radiation therapy (kV CBCT) applications. The detectors employ polycrystalline silicon (poly-Si) – a thin-film material that can be used to economically manufacture monolithic, large-area backplanes. In addition, poly-Si transistors have demonstrated good radiation damage resistance. The introduction of poly-Si-based photon counting detectors to BCT and kV CBCT would combine the benefits of photon counting with those of CBCT. However, one major challenge is designing circuits that are capable of handling the x-ray fluxes associated with these applications. A previously developed simulation methodology has been employed to model various levels of input flux in order to evaluate the count rate capabilities of poly-Si circuit designs. In this paper, the count rate capabilities of a promising poly-Si amplifier design under BCT and kV CBCT conditions are reported.