Optical factors affecting the detective quantum efficiency of radiographic screens

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x-ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x-ray attenuation tables. Rather more substantial energy-dependent deviations of EA are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within +/- 10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.