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Monte Carlo calculations were performed to generate the point spread functions of x-ray photons absorbed in a CsI:Tl x-ray detector at the x-ray energies normally used in mammography (i.e., 20 keV to 50 keV). The corresponding modulation transfer functions [MTF(f)] for the CsI:Tl screen were also computed, taking into account the optical spread of light within the CsI:Tl crystals. The computed MTF(f)s were dominated by scintillation light lateral dispersion within the CsI:Tl screen. For the photon energy range encountered in digital mammography, the MTF(f) was a minimum at an x-ray photon energy just above the k-edge of Iodine (33 keV). Noise propagation theory for a cascaded imaging system was subsequently used to derive a theoretical expression for the detector DQE(f), including the dependence of DQE(f) on the spatial distribution of x-ray photon energy deposition. Detector performance was investigated as a function of x-ray exposure, CCD electronic noise, coupling efficiency of the fiber optical coupler, and the CCD quantum efficiency. Although most of the x-rays are absorbed via the photoelectric effect, the deposited x-ray energy spread within the CsI:Tl screen from the emission of characteristic x-rays can have a marked effect on detector performance, and the DQE(f) was found to decrease rapidly with photon energy just above the Iodine K-edge. X-ray exposure levels to the detector should be greater than or equal to 5 mR with a CCD electronic noise of approximately 20 electrons rms to ensure that DQE(f) performance is not significantly degraded at the spatial frequencies important in digital mammography (i.e., 0 to 10 lp/mm). Light dispersion within the CsI:Tl crystals was the major factor degrading imaging system DQE(f) at higher spatial frequencies. Optical coupling efficiency and CCD quantum efficiency are important system design parameters, which need to be maintained at a relatively high value. An optical coupling efficiency of approximately 0.7, and a CCD quantum efficiency of approximately 0.4, would still permit system DQE values greater than 60% at a spatial frequency of 5 lp/mm.
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