MONTE-CARLO OPTIMIZATION OF METAL PHOSPHOR SCREENS AT MEGAVOLTAGE ENERGIES

The physics of imaging with metal/phosphor (Gd2O2S:Tb on brass) screen s at megavoltage energies has been investigated using Monte Carlo simu lation. It has been found that pair production is a significant contri butor to energy deposition for Bremsstrahlung beams with energies grea ter than 6 MV. The effects of different thicknesses of phosphor and me tal have been studied, and it is shown that the metal plays a signific ant role in establishing electronic equilibrium in the phosphor. The t ransport of optical photons through the phosphor has been modeled, and was found that only 10% to 20% of the light created in the phosphor e scapes from the surface, with much of the loss being due to total inte rnal reflection at the surface. Calculated results have been compared with experimental measurements of screen brightness for different phos phor and metal thicknesses. The SNR of a video electronic portal imagi ng device (VEPID) has been calculated as a function of x-ray and optic al photon detection efficiency. The non-Poisson distribution of energy deposition in the phosphor is an important contributor to the SNR. Th e results of this paper should serve as a useful guide to the engineer ing design of future electronic portal imaging systems.