An Asynchronous Sampling-Based 128 128 Direct Photon-Counting X-Ray Image Detector with Multi-Energy Discrimination and High Spatial Resolution

Abstract—This paper presents a direct photon-counting X-ray image detector with a photoconductor for high-quality medical imaging applications. The proposed sampling-based charge preamplifier with asynchronous self-reset enables a pixel to detect single X-ray photon energy with higher sensitivity and faster processing rate. The use of the correlated double sampling enabled by the sampling-based architecture also reduces flicker noise and contributes to the achievement of high pixel-to-pixel uniformity. Discrimination of the energy level of the detected X-rays is performed by the proposed compact in-pixel ADC with low power consumption. Three 15-bit counters in each pixel count up energy-discriminated photons for the reconstruction of multi-spectral X-ray images. A 128 128 X-ray image detector with a pixel size of is implemented and measured using a 0.13- /0.35- standard CMOS process. It discriminates 3 energy levels of photon energy with a gain of 107 and a static power consumption of 4.6. The measured equivalent noise charge (ENC) and minimum detectable energy level of the detector pixel are 68 and 290, respectively. The measured maximum threshold dispersion in the pixel array is 164 without any calibration. The functionality of our chip is also successfully demonstrated using real X-ray images. Index Terms—ADC, arrays, CMOS, detectors, image sensors, photoconductor, photon counting, photonics, preamplifiers, sam-pling-based circuits, switches, x-ray imaging. I