Monte Carlo Optimization of Crystal Configuration for Pixelated Molecular SPECT Scanners

Abstract

Resolution-sensitivity-PDA tradeoff is the most challenging problem in design and optimization of pixelated preclinical SPECT scanners. In this work, we addressed such a challenge from a crystal point-of-view by looking for an optimal pixelated scintillator using GATE Monte Carlo simulation. Various crystal configurations have been investigated and the influence of different pixel sizes, pixel gaps, and three scintillators on tomographic resolution, sensitivity, and PDA of the camera were evaluated. The crystal configuration was then optimized using two objective functions: the weighted-sum and the figure-of-merit methods. The CsI(Na) reveals the highest sensitivity of the order of 43.47cps/MBq in comparison to the NaI(Tl) and the YAP(Ce), for a 1.5×1.5mm2 pixel size and 0.1mm gap. The results show that the spatial resolution, in terms of FWHM, improves from 3.38 to 2.21mm while the sensitivity simultaneously deteriorates from 42.39cps/MBq to 27.81cps/MBq when pixel size varies from 2×2mm2 to 0.5×0.5mm2 for a 0.2mm gap, respectively. The PDA worsens from 0.91 to 0.42 when pixel size decreases from 0.5×0.5mm2 to 1×1mm2 for a 0.2mm gap at 15° incident-angle. The two objective functions agree that the 1.5×1.5mm2 pixel size and 0.1mm Epoxy gap CsI(Na) configuration provides the best compromise for small-animal imaging, using the HiReSPECT scanner. Our study highlights that crystal configuration can significantly affect the performance of the camera, and thereby Monte Carlo optimization of pixelated detectors is mandatory in order to achieve an optimal quality tomogram.