A study of factors limiting spatial resolution using a 25 micron pixel pitch direct-detection amorphous selenium imaging system

An amorphous selenium direct detector with a 25 mu m pixel pitch is studied for mammography and fluoroscopy applications. In this paper we analyze spatial resolution by examining the main contributions to the inherent modulation transfer function (MTF) of amorphous selenium, such as primary photoelectron range and x-ray beam obliquity; and determine their significance relative to aperture MTF. Improvements in spatial resolution provided by a move to a small pixel size are complicated by deep carrier trapping. Trapped carriers cause residual charge induction across the pixel plane. We examine the significance of this effect on spatial resolution. The potential improvements of moving from a larger pixel pitch to a 25 mu m pixel pitch are studied, including all significant effects, at selected pixel pitches.