Empirical and theoretical investigation of the noise performance of indirect detection, active matrix flat‐panel imagers (AMFPIs) for diagnostic radiology

Abstract

Noise properties of active matrix, flat‐panel imagers under conditions relevant to diagnostic radiology are investigated. These studies focus on imagers based upon arrays with pixels incorporating a discrete photodiode coupled to a thin‐film transistor, both fabricated from hydrogenated amorphous silicon. These optically sensitive arrays are operated with an overlying x‐ray converter to allow indirect detection of incident x rays. External electronics, including gate driver circuits and preamplification circuits, are also required to operate the arrays. A theoretical model describing the signal and noise transfer properties of the imagers under conditions relevant to diagnostic radiography, fluoroscopy, and mammography is developed. This frequency‐dependent model is based upon a cascaded systems analysis wherein the imager is conceptually divided into a series of stages having intrinsic gain and spreading properties. Predictions from the model are compared with x‐ray sensitivity and noise measurements obtained from individual pixels from an imager with a pixel format of 1536×1920 pixels at a pixel pitch of 127 μm. The model is shown to be in excellent agreement with measurements obtained with diagnostic x rays using various phosphor screens. The model is used to explore the potential performance of existing and hypothetical imagers for application in radiography, fluoroscopy, and mammography as a function of exposure, additive noise, and fill factor. These theoretical predictions suggest that imagers of this general design incorporating a CsI:Tl intensifying screen can be optimized to provide detective quantum efficiency (DQE) superior to existing screen‐film and storage phosphor systems for general radiography and mammography. For fluoroscopy, the model predicts that with further optimization of a ‐Si:H imagers, DQE performance approaching that of the best x‐ray image intensifier systems may be possible. The results of this analysis suggest strategies for future improvements of this imaging technology.