The Monte Carlo evaluation of noise and resolution properties of granular phosphor screens

Abstract

The imaging performance of phosphor screens, used as x-ray detectors in diagnostic medical imaging systems, is affected by their both noise and resolution properties. Amplification and blurring processes are due to a sequence of conversion stages within the screen which contribute to fluctuations in the number and spatial distribution of the optical quanta recorded by the optical detector (e.g. film, television camera, CCD, etc). The purpose of this paper is to investigate the stochastic noise arising from granularity as well as the variation of spatial resolution of granular fluorescent screens in terms of the detector's structure. Using a custom-validated Monte Carlo model, the parameters of interest were evaluated for the widely used Gd2O2S:Tb phosphor material. We have studied the variations of (i) the modulation transfer function, (ii) the Swank factor and (iii) the zero-frequency detective quantum efficiency (DQE), under several conditions employed in conventional and digital mammography and radiology. Several evaluations are provided for the imaging metrics as a function of the x-ray energy (18 keV, 49 keV and 51 keV), phosphor coating weight (20 mg cm−2, 34 mg cm−2 and 60 mg cm−2), grain size (from 4 µm up to 13 µm) and packing density (from 50% up to 85%). It was found that screens of high packing density can combine high zero-frequency DQE with improved resolution properties. For a digital mammographic imaging system (34 mg cm−2, 18 keV), a packing density of 85% can improve the spatial resolution of the screen by 1.6 cycles mm−1 in comparison to that of 50% packing density. Similarly, for radiographic cases (60 mg cm−2, 49 keV), the spatial resolution can be improved by 1.7 cycles mm−1. The aforementioned findings provide the resolution benefits of using high packing density screens.