Monte Carlo Simulation of a Preclinical CZT PET System to Investigate Multi-Interaction Events

Abstract

This work summarizes findings from GATE (GEANT4 Applications for Emission Tomography) simulations with the incorporation of certain experimentally-measured, ‘realistic' parameters in the data analysis. The simulation models a large volume cadmium zinc telluride (CZT) preclinical PET system with sub 9% energy resolution and sub-mm spatial resolution. 2 of the 4 heads of the final design have been constructed, each consisting of a 4x3 matrix of dual-CZT detector modules. Each detector module comprises two 40x40x5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> CZT crystals with 39 anode strips (100 um width, 1mm pitch each) on the top face and 8 cathode strips (4.9 mm width, 5 mm pitch each) on the bottom face. The crystals are stacked in an anode-cathode-cathode-anode configuration to form the detector module. Photons enter in an "edge-on" orientation, traversing a minimum of 40 mm of CZT which increases detection efficiency. A simulation of the system was constructed in GATE, and data were recorded as "hits," which are ideal, non-blurred interactions, to retain multiple interaction event information within the modeled detector modules. A preliminary energy system response function was generated using (a) photo-peak centroid vs FWHM for Ge-68, Ba-133, and Cs-137 point source data to form an energy-dependent normal distribution to randomly blur energy per simulation interaction, and (b) a Ge-68 point source-measured depth corrected energy vs cathode to anode (C/A) ratio energy plot which was used as a mask over a simulated C/A vs energy plot, isolating allowable C/A ratio-energy combinations. The energy resolution of the system measured with the Ge-68 point source was found to be 8.1%. After applying the measured energy system response function, the simulated energy resolution was found to be 6.3%. The extracted average number of interactions per 511 keV photon after application of the various experimental parameters was 1.05.