Theoretical investigation of advanced design active matrix flat-panel imagers for mammography

Abstract

A theoretical investigation of the system performance of active matrix, flat-panel imagers (AMFPIs) under mammographic imaging conditions is reported. These imagers employ either indirect or direct detection of the incident radiation. The x- ray converter materials assumed in these studies were Gd 2 O 2 S:Tb and CsI:Tl scintillators for indirect detection and a-Se and PbI 2 photoconductors for direct detection. A model based on cascaded systems formalism was used to predict the detective quantum efficiency (DQE) of various AMFPI designs incorporating these converters. The functional dependence of DQE performance on pixel-to-pixel pitch, collection fill factor and additive electronics noise was investigated under mammographic conditions. Incorporation of a continuous photodiode structure in indirect detection AMFPI arrays is necessary in order to achieve reasonable DQE performance for devices with pixel pitches significantly below 100 micrometer. In the case of CsI:Tl, a-Se, and PbI 2 , through optimization of the converter thickness, the DQE performance of these advanced AMFPI designs is predicted to exceed that of an AMFPI system incorporating a conventional Gd 2 O 2 S:Tb mammographic screen. Finally, the model predicts that the incorporation of a high gain converter such as PbI 2 leads to a high value of DQE which is independent of exposure over the mammographic exposure range at all spatial frequencies, even at 50 micrometer pitch.