Modelling of high-energy contamination in SPECT imaging using Monte Carlo simulation

¹²³I is a commonly used radioisotope employed in neurotransmitter SPECT studies. In addition to an intense line at 159 keV, the decay scheme of this radioisotope includes a low yield (~3%) of higher energy photons which have a non-negligible contribution to the final image when low-energy high-resolution (LEHR) collimators are used. This contribution of high-energy photons may achieve ~28% of the total counts in the projections. The aim of this work is to model each energy component of the high-energy Point Spread Function (hPSF) for fan-beam LEHR collimators in order to develop faster Monte Carlo (MC) simulations of high-energy ray contamination. The modelling of hPSF was based on the results of simulating photons through the collimator-detector system using the MC code PENELOPE. Since low-energy PSFs models for fan-beam collimators must tend to a Gaussian distribution, we use the same function for the hPSF modelling for high-energy photons. The parameters of these Gaussian functions were obtained by minimizing the root mean square (RMS) error between each simulated hPSF and the function g(x,y) using the efficiency of the simulated hPSFs as a constraint The RMS attained with fit of g(x,y) to the simulated hPSFs was always smaller than ~2% of the mean efficiency per pixel of the image. A very strong dependence of the efficiency on the type and thickness of the backscatter material behind the crystal was found. The hPSFs were parameterized for a wide range of energies, ranging from 350 keV to 538 keV. Our results indicate that Gaussian distributions approximate in a suitable way the hPSF responses for fan-beam collimators. This model will be an important tool to accelerate MC simulations of radiolabelled compounds which emit medium- or high-energy rays