Monitoring of Quantum Efficiency of Digital X-Ray Detectors Based on Photostimulated Luminophores

Abstract

Complexes based on photostimulated luminophores (CR systems) are now widely used in medical organiza tions of the Russian Federation as digital X ray systems. CR systems are rather simple and provide relatively inex pensive transition from X ray film diagnosis to digital technology: the entire X ray apparatus need not be replaced, but only reequipped with X ray cassettes of standard typical size, sampling and reading unit for mem ory screens, and a computer based automatic workplace (AWP) [1]. Imaging quality control in such systems differs from imaging quality control in digital detectors based on flat panels (amorphous silicon and amorphous selenium), CCD matrices, XRII, and scanners (solid state and gas). The goal of this work was to analyze this difference.Construction of CR systems has some specific fea tures:-transition characteristic (dependence of image brightness on detector input density of quantum flux) of some CR systems is nonlinear; -X ray detector (cassette with memory screen and sampling and reading unit for memory screens) is not controlled by the X ray radiation source. Combined cali bration of X ray detector and X ray radiation source is difficult to be implemented in practice;-interaction of IR laser beam with memory screen (during information reading) induces optical signal, scat tering resulting in signal correlation at detector output channels. Experiments demonstrated that this correlation increased upon increasing the spatial resolution of the detector.These specific features have an effect on estimation of general characteristic of image quality in a digital X ray detector (detection quantum efficiency at zero spatial fre quency DQE and Nyquist spatial frequency DQE(u,v)). Estimate of Transition Characteristic NonlinearityNonlinear signal transformation in detector chan nels should be avoided during open field image formation and detection for DQE and DQE(u,v) estimation [1 3]. It was noted above that the transition characteristic of some CR systems was nonlinear at detector output chan nels (logarithmic transition characteristics of channel amplifiers). In this case, data linearization is required. In other words, the data should be transformed to the initial form (i.e., the form prior to nonlinear transformation).Let signals before X ini and after X res nonlinear trans formation be correlated using the following expression:whereThe linearization procedure starts with dependence of mean brightness of open field image on detector input signal amplitude. Open field images detected at different detector input plane dose are used (standard [2] requires clinical use of the apparatus at dose range from 0 to four fold mean dose level). Discrete dose variation step should provide logarithm of variation range 0.1. This dependence is approximated by the following function X res = a + b ln(X ini ).Equation (1) can be recast as the sought function:Equation (2) can be used for transformation of initial data for detector plane dose providing the following cal culations of DQE and DQE(u,v). It should ...