IMAGE QUALITY OF A DIGITAL CHEST RADIOGRAPHY SYSTEM BASED ON A SELENIUM DETECTOR

A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The sele nium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic c harge image which is formed on the selenium surface after x-ray exposu re is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristi c curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal-to-noise properties of t he detector in terms of detective quantum efficiency (DQE) and noise e quivalent quanta (NEQ) were derived. The results show that the seleniu m-based detector has a wide dynamic range and a significantly better D QE than screen-film and storage phosphor systems for spatial frequenci es below the Nyquist limit (2.7 lp/mm). As a consequence, the detectab ility of small, low-contrast details is considerably improved.