Selenium Detectors

x-Ray images have traditionally been acquired by two general methods: those using indirect conversion via a phosphor or scintillator layer, and those using direct conversion with a semiconductor. Although many materials have been employed for experimental direct conversion systems, currently large area medical systems use amorphous selenium (a-Se). The history of this material (first used commercially for Xeroradiography) and the reasons for its emergence is the initial part of this chapter. Next its current medical applications are discussed with reference to the fundamental physical processes involved. In addition the image quality parameters possible with direct conversion methods are critically compared with phosphor based methods. Finally the prospects for future improvements are critically analyzed.