Fast Monte Carlo Simulation Method for Medium- and High-Energy SPECT

Monte Carlo (MC) simulation of SPECT imaging using medium-energy (ME) and high-energy (HE) photons involves intense calculations of photon transport within both the phantom modeling the patient and the collimator where the penetration (P) and scatter (S) components of the total response are non-negligible as compared to the geometric (G) component. The purpose of this study is to develop fast MC simulation methods that substantially reduce computation time while maintaining high accuracy. The SIMSET MC code was used to simulate transport of photons from a point source or source distribution from the 3D NCAT phantom. The energy, position, and direction of travel of each photon were stored in a listmode data file. The 2D collimator point source response function (PSRF), including the G, P and S components, was determined at a series of source distances uing the MCNP MC code. Lookup table of the 2D PSRFs as a function of source distance for a specific collimator design and photon energy was generated by interpolating the series of MC simulated PSRFs. The G component at each source distance was normalized to one and the P and S components were scaled accordingly. An algorithm was developed to read the listmode data, select photons within a given energy window, calculate the distances between the photons and the collimator, determine the PSRFs from the appropriate lookup table, and add them to the projection data. The PSRFs at different source distances obtained from using the fast MC method showed excellent agreement with direct MC simulation. We conclude that by using lookup tables obtained from previously defined MC simulated PSRFs of the collimator, accurate simulation of SPECT imaging using ME and HE photons can be obtained at substantially reduced computational time.