Resolution degradation due to K-characteristic photon absorption in dual phosphor image receptors

Image receptors based on the absorption of x rays by phosphor screens have a system resolution described predominantly by the modulation transfer function (MTF) of the phosphor screens utilized. This ideal MTF is modified by the presence of secondary effects within the imaging receptor such as scatter from the receptor cover, visible light crossover (in the case of screen film cassettes) and scatter generated within the phosphor itself, presumed to be almost entirely due to photoelectric interactions. In this paper the MTF characteristics resulting from the absorption of K-characteristic x rays emitted by one screen of a pair in an image receptor and absorbed by the adjacent screen (referred to as cross-fluorescence hereafter) are examined. This effect has been previously modeled [Med. Phys. 23, 1253 (1996)] and recently a computer program has been written to calculate intensity of the radial component of a point spread function (PSF) describing the above process. From this, the MTF of cross-fluorescence under varying conditions can be calculated. The model was applied to a dual screen experiment described in the literature [Med. Phys. 23, 871 (1996)], from which it was possible to show good agreement between calculated results and those derived from the measurement. The effect of screen thickness, screen separation, and phosphor material on the MTF of cross-fluorescence was investigated using simulated x-ray beam spectra in association with phosphor screen parameters. Results showed that the calculated MTF falls to below 0.1 at 1 cy/mm in all cases examined. This MTF was reduced further as the separation between the screens increased from that commonly used in a screen film cassette. Beam energy, phosphor thickness, and phosphor material had a minimal or no effect on MTF. The effect of cross-fluorescence on total image receptor MTF is to reduce the high-frequency components of that function by an amount that is in proportion to the scatter fraction of the effect.