Optical blur and collection efficiency in columnar phosphors for X-ray imaging

Columnar phosphors are common in modern indirect X-ray digital imaging systems. However, a comprehensive description of the light scattering processes within such structures has not yet been reported. We present a Monte Carlo analysis of the depth-dependent optical blur and collection efficiency of columnar phosphor screens. We present line-spread (LSF) and modulation transfer (MTF) functions, and optical collection efficiencies for screens with transparent phosphors and different back-layer reflectance. Our phosphor model consists of a uniformly spaced columnar array (9 μm diameter with 1 μm gaps). We present data for single X-ray interactions occurring at varying depths (99, 90, 40, 30, 20, 10, and 1 μm) for a given screen thickness of 100 μm, and for screens of varying thicknesses (10, 20, 40, 100, 200, and 300 μm) with exponentially absorbed X-ray beams corresponding to typical mammography spectra. We show quantitatively that depth of interaction and phosphor thickness have a strong effect on LSF and MTF. Our Monte Carlo results are in agreement with published experimentally measured MTF results. The depth-dependent collection efficiency is affected by optical absorption, while being independent of interaction depth and thickness for transparent phosphors.