Accurate modeling of nuclear-medicine collimators in Monte Carlo simulation of high-energy photons

Monte Carlo (MC) simulation of nuclear medicine imaging is more difficult for procedures employing 131I than for those using 99mTc because of the higher probability of collimator penetration for the higher-energy 131I gamma. Additionally, an accurate model of the imaging camera must also simulate the scattering and penetration of photons emitted above the 364 keV photo peak (637 keV (6.5%) and 723 keV (1.7%)). Most MC codes used in nuclear medicine either employ severe approximations of the collimator interactions or have not been extended to higher energies. The MC program SKEPTIC, which has previously been used to simulate slit aperture penetration of 511 keV gammas, has been modified to permit modeling of parallel hole collimators. We present here a comparison of measured and simulated energy spectra and point spread functions for 131I.