Simulating the emission efficiency and resolution properties of fluorescent screens by Monte Carlo methods

The aim of this study was to predict quantum detection efficiency (QDE) and modulation transfer function (MTF) of granular phosphor screens, used in detectors of medical imaging systems, using Monte Carlo methods. The transfer of energy through the screen was modeled as a series of energy converting stages, assuming that the screen was divided into a number of elementary thin layers of specific thickness. Screen characteristics were predicted considering either polychromatic or monochromatic X-ray beam in the energy range used in mammographic applications. The screen coating weight was selected to be 31.7 mg/cm² which corresponds to a screen thickness of 85 μm. X-ray photon interaction site was determined by calculating the probability mass function for each layer, separately. Light quanta were considered to be isotropically created within phosphor´s mass. The number of light quanta generated per absorbed X-ray photon was calculated using the intrinsic X-ray to light conversion efficiency. Light absorption and light scattering were assumed to be the possible mechanisms for light attenuation. Results were verified using previous data for Gd₂O₂S:Tb phosphor material. QDE and MTF were found in close agreement to analytically calculated data published in two previous studies. The agreement between Monte Carlo and theoretical values was better than 0.2% and 5% respectively.