Flat panel detector for digital radiology using active matrix readout of amorphous selenium

A large area, flat panel solid state detector is being investigated for both digital radiography and fluoroscopy. The detector employs amorphous selenium (a-Se) to detect x- rays. The charge image formed on the surface of the a-Se is read out in situ using an active matrix array. A theoretical analysis of the spatial frequency dependent detective quantum efficiency (DQE) is performed. Because of the very high intrinsic resolution of a-Se, the detector is inherently undersampled and aliasing is always present. An interpretation of DQE(f) for the undersampled a-Se detector will be given. The analysis shows that the main factors, besides the quantum efficiency of the a-Se layer, affecting DQE(f) are: (1) aliasing; (2) gain fluctuation noise of a- Se, i.e., the Swank factor of a-Se; (3) electronic noise which prevents quantum noise limited operation at low exposure levels such as those used in fluoroscopy and (4) temporal response which causes a reduction in noise by averaging. The validity of the theoretical model was confirmed experimentally using our prototype detector with the Swank factor being established using pulse height spectroscopy. The model was then applied to three important x-ray imaging applications: mammography, chest radiography and fluoroscopy. The results show that the most important strategy for maximizing DQE(f) is to increase the pixel fill factor which can be unity using specialized techniques Methods for reducing aliasing in the detector will be described.