Monte Carlo simulation of scintillation photons for the design of a high-resolution SPECT detector dedicated to human brain

Abstract

In a typical single photon emission computed tomography (SPECT) system, intrinsic spatial resolution depends on the accuracy of the identification of an interacting point, which is dominated by propagation of the scintillation photons in the detector block. This study was intended to establish a Monte Carlo simulation-based evaluation tool taking into account the propagation of scintillation photons to estimate the intrinsic spatial and energy resolutions of the position-sensitive scintillator block in a SPECT detector. We employed Geant4 Monte Carlo simulation library which incorporated the optical photon processes for two different designs of the position-sensitive scintillator blocks. The validation of the simulation code was performed for a monolithic NaI(Tl) scintillator (251×147×6.4mm(3)) coupled to 15 flat-panel type multi-anode photo multiplier tubes (PMT) (H8500: Hamamatsu) and results were compared with those obtained experimentally. The code was then applied to a LaBr(3)(Ce) scintillator of 120mm square with varied thicknesses for designing high-resolution detector. The simulation resulted in 2.6mm full width at half maximum (FWHM) of spatial resolution and 9.0% FWHM of energy resolution for the NaI(Tl)-based detector, which were in a good agreement of the experimental results, i.e., 2.7mm and 10%, respectively. These findings suggest that Geant4 simulation including optical photon processes enables to predict the spatial and energy resolutions of a SPECT detector block accurately. The simulation also demonstrated that 2mm spatial resolution can be obtained for a 6mm thickness of the LaBr(3)(Ce), which is a significant improvement in performance as compared to existing gamma camera system that employs the scintillation detector fitted with PMTs. The Monte Carlo simulation-based evaluation tool was established to estimate the intrinsic spatial and energy resolutions of SPECT detector with position sensitive PMTs. This simulation may be useful to provide an optimal design of a SPECT detector without physical experiments.