Noise analysis of a novel hybrid active‐passive pixel sensor for medical X‐ray imaging

Abstract

Passive pixel sensor (PPS) is one of the most widely used architectures in large area amorphous silicon (a‐Si) flat panel imagers. It consists of a detector and a thin film transistor (TFT) acting as a readout switch. While the PPS is advantageous in terms of providing a simple and small architecture suitable for high‐resolution imaging, it directly exposes the signal to the noise of data line and external readout electronics, causing significant increase in the minimum readable sensor input signal. In this work we present the operation and noise performance of a hybrid 3‐TFT current programmed, current output active pixel sensor (APS) suitable for real‐time X‐ray imaging. The pixel circuit extends the application of a‐Si TFT from conventional switching element to on‐pixel amplifier for enhanced signal‐to‐noise ratio and higher imager dynamic range. The capability of operation in both passive and active modes as well as being able to compensate for inherent instabilities of the TFTs makes the architecture a good candidate for X‐ray imaging modalities with a wide range of incoming X‐ray intensities. Measurement and theoretical calculations reveal a value for input refferd noise below the 1000 electron noise limit for real‐time fluoroscopy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)