Depth-dependent phosphor blur in indirect x-ray imaging sensors

The influence of phosphor screens on the digital system image quality has been studied in a number of papers. However, there has been no detailed description of the effect of depth of x-ray interaction on the blur characteristics of the phosphor and on optical collection efficiency for both powder and structured screens. We present an analysis based on optical Monte Carlo simulations of the depth-dependent phosphor blur of two classes of single-layer phosphor screens: homogeneous and columnar. The spectral sensitivity of the optical sensor is modeled according to a typical a-Si:H photodiode absorption profile. We used Gd2O2S:Tb and CsI:Tl emission spectra respectively for the powder and columnar phosphor models. We present line-spread (LSF) and modulation transfer (MTF) functions associated with the spread of signal in the phosphor, and optical collection efficiencies. We find good agreement between the Monte Carlo estimates of the MTF and the analytical solutions available in the literature. Our optical collection efficiency results show depth dependence only for the screens with highly scattering and absorptive phosphor with reflective backing, and for the case of scattering phosphor with absorptive backing.