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A cascaded linear system model is developed for calculating the frequency-dependent detective quantum efficiency, DQE(f), of a direct conversion x-ray imaging detector by incorporating the effects of charge carrier trapping and reabsorption of K-fluorescent x-rays. The present model considers a combination of series and parallel processes and interactions between them. The modulation transfer function for K-fluorescent x-ray reabsorption is modelled by determining the line spread function and subsequent one-dimensional Fourier transform. The DQE model is applied to amorphous selenium (a-Se) and polycrystalline mercuric iodide (poly-HgI2) detectors. The charge carrier trapping has a significant effect on DQE in both a-Se and poly-HgI2 detectors. The charge carrier transport properties have higher influences on DQE performance in a-Se detectors than that in HgI2 detectors, because of relatively low conversion gain in a-Se detectors. High conversion gain can minimise the adverse effect of incomplete charge collection. A simplified model for the calculation of zero spatial frequency detective quantum efficiency, DQE(0), under parallel cascaded system is also proposed in this study. There exists an optimum photoconductor thickness, which maximises the DQE(0). The proposed model is compared with the published measured data and shows good agreement.