Screen design for flat-panel imagers in diagnostic radiology

The image quality of Gd2O2S:Tb phosphor screens used in a flat-panel photodiode army system is examined. The presampled Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum surface (NNPS) and the resulting Detective Quantum Efficiency (DQE) are discussed for a variety of screens used in such a system. A technique for extracting the limiting DQE for a system with significant electronic noise is described. This allows for the examination of the imaging performance of the X-ray converter (or phosphor screen) and removes issues of photodiode array and readout electronics performance. It is shown that depending on the metric being used to judge the imaging performance (i.e., MTF or DQE), it is possible to design a more optimal screen than those currently available for use in screen-film imaging. Evidence is also presented for a significant degradation of the DQE at higher spatial frequencies due to the variation in the light spread MTF through the depth of the screen.