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

This paper presents a direct photon-counting X-ray image detector with a
${\rm HgI}_{2}$
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 multispectral X-ray images. A 128
$\times$
128 X-ray image detector with a pixel size of
$60\times 60\ \mu{\rm m}^{2}$
is implemented and measured using a 0.13-
$\mu{\rm m}$
/0.35-
$\mu{\rm m}$
standard CMOS process. It discriminates 3 energy levels of photon energy with a gain of 107
${\rm mV/ke}^{-}$
and a static power consumption of 4.6
$\mu{\rm W/pixel}$
. The measured equivalent noise charge (ENC) and minimum detectable energy level of the detector pixel are 68
${\rm e}^{-}\ {\rm rms}$
and 290
${\rm e}^{-}$
, respectively. The measured maximum threshold dispersion in the pixel array is 164
${\rm e}^{-}\ {\rm rms}$
without any calibration. The functionality of our chip is also successfully demonstrated using real X-ray images.