Alternate pixel architectures for large-area medical imaging

Abstract

Amorphous silicon active matrix flat-panel imagers have gained considerable significant in digital diagnostic medical imaging applications in view of their large area readout capability. The pixel, forming the fundamental unit of the active matrix, consists of a detector and readout circuit. The most widely used architecture is a passive pixel sensor (PPS) where the pixel consists of a detector and an a-Si:H thin-film transistor readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, reading the low PPS output signal require external circuitry such as column charge amplifiers. More importantly, these amplifiers add a large noise component to the PPS that reduces the minimum readable sensor input signal. This work presents an alternate pixel architecture that can perform on-pixel input signal amplification, i.e. an active pixel sensor (APS). Two operating modes of the APS, voltage output (V-APS) and current output (C-APS) are introduced but the focus is on C- APS. Theoretical calculations indicating the feasibility of the C-APS for low-noise, real time imaging applications (e.g. fluoroscopy) are presented. Specifically, signal gain, dynamic range, readout rate and noise of the C-APS are examined. Lastly, initial experimental results of C-APS linearity and gain are presented in addition to a discussion on APS threshold voltage stability.