DEVELOPMENT AND VALIDATION OF A MONTE-CARLO SIMULATION OF PHOTON TRANSPORT IN AN ANGER CAMERA

The geometric component of the point spread function (PSF) of a gamma camera collimator can be determined analytically, and the penetration component can be calculated readily by numerical ray-tracing; however, a mathematical description of the collimator scatter contribution has eluded researchers. Collimator scatter is most important when imaging low or medium energy isotopes (e.g., In-111, I-123, and Ga-67) which emit additional high energy "contaminant" photons that can be detected in the desired lower energy window after scattering. To address this problem, we developed a Monte Carlo simulation of photon transport which includes collimator scatter. The simulation was implemented with an array processor which propagates up to 1024 photons in parallel, allowing accurate estimates of the total radial PSF in less than a day. The simulation was tested by imaging monoenergetic "point" sources of Tc-99m, Cr-51 and Sr-85 (140, 320, and 514 keV, respectively) on a General Electric Starcam(TM) with low energy, general purpose (LEGP) and medium energy (ME) collimators. Comparisons of measured and simulated PSF's demonstrate the validity of the model and the significance of collimator scatter in the degradation of image quality.