A CMOS readout pixel circuitry for spectral-CT applications

Spectral Computed Tomography (CT) based on Photon-Counting Detectors (PCDs) is an emerging technology that can provide 3D images in several energy bands of the X-ray spectrum. PCDs are composed of a pixelated photodetector, typically a Cadmium Telluride (CdTe) crystal, hybridized to a CMOS readout circuit capable of detecting each signal pulse induced by X-rays and classifying them into coarse energy bins. The main challenge for PCDs is to achieve a good spectral accuracy while operating at very high count rates. In this work, we develop a new CMOS readout circuit design of a Photon Counting Detector with advanced correction techniques inside the pixel to improve its performance. In this paper, we describe the front-end analog circuit based on a Capacitive Transimpedance Amplifier (CTIA) with a "hard-reset" feedback circuit that achieves a fast pulse response, while ensuring a nonparalyzable behavior. Simulations show that this amplifier stage delivers fast pulses with 10 ns width. A Baseline Holder (BLH) is able to compensate the detector leakage current at any photon period. Finally, the consumption of the analog stage is less than 150 mu W/pixel.