Investigation of a direct conversion flat panel imager for portal imaging

Flat-panel based x-ray imaging is an emerging new technology that could be used to significantly improve the quality of on-line portal imagers. There are two types of flat panel imagers: direct and indirect conversion. Previous experimental work on flat panel detectors for portal imaging application used indirect-conversion imagers. In this paper, a direct-conversion amorphous-selenium flat panel imager is investigated for application in portal imaging. The imager has an active imaging area of 14 in.×17 in., i.e., 3072× 2560 pixels each with dimensions 139 µm×139 µm. The spatial frequency dependent detective quantum efficiency of the imager has been measured for a 6 MV beam and found to be amongst the best area detectors investigated for on-line portal imaging. Effects of changing pixel size, as well as possible improvements to both the image quality and convenience of operation are discussed. Comparison with an indirect conversion flat panel imager is also included.