Extended dynamic range CMOS active pixel architecture for X-ray detectors

Abstract

X-ray detectors increasingly utilize active pixel CMOS instead of amorphous silicon technology because of its superior noise, pixel lag, readout speed and offset stability. We already demonstrated [1] that adding an a additional pixel capacitance to a standard 3T CMOS pixel architecture allows operating the detector in either high sensitivity (HS) or high saturation dose (HD) mode. Since the charge capacity is limited in HS mode, a large signal will saturate the pixel, causing a loss of information. In HD mode, a very low exposure will lead to a loss of contrast-to-noise ratio (CNR) due to the inherently higher noise floor in this mode. Using the same 3T pixel architecture, we propose a dual readout method to combine the benefits of HS and HD modes. After a single exposure the pixel signal is read twice, respectively in HS and HD mode. The two linear signal values are then combined to create the pixel value of the final image. In this paper proof of concept is demonstrated using images acquired separately and combined offline. The benefits of this method are demonstrated for different x-ray imaging modalities such as mammography, extra-oral dental, interventional and non-destructive testing. Using different detector models, results show that extended dynamic range combined with low noise leads to better image quality without introducing artifacts. It is expected that implementing the fast CMOS-sensor dual readout and image synthesis inside the detector will preserve important application requirements such as frame rate, data bandwidth and power consumption.