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A high sensitivity Cardiac SPECT system using curved crystals with pinholecollimation was proposed previously (Dey, IEEE TNS 2012, Bhusal et al, Med.Phys. 2019). Here, we hypothesize that a high curvature hemi-ellipsoid detectorresults in measurable differences in light distribution from events atdifferent depths in the crystal. This was tested by analyzing the scintillationlight in hemi-ellipsoid detector using Monte-Carlo (Geant4) and evaluation ofboth the localization error at detector and the back-projected errors inobject-space. We used Geant4 to simulate the propagation of scintillation lightin a monolithic hemi-ellipsoidal CsI crystal. A look-up table (LUT) was createdto map the points inside the crystal to the expected light pattern on thecrystal surface using Geant4. In thirteen zones across the crystal, gamma-rayswere simulated and the resulting scintillator light intensity on the surfacewas captured, serving as our experimental interactions. APoisson-statistics-based algorithm was developed to limit the search of thegamma-ray event locations into small regions of the LUT and fine-tuned byinterpolating between selected LUT points by comparing the light distributionof the gamma interactions and LUT light patterns. The localized events wereindividually back-projected to the object mid-plane, and the errors recorded.Excluding some outliers (up to 2%), the localized errors averaged over all thezones was 0.71 (+/-0.44) mm with a worse case of 1.36 (+/-0.67) mm at the apex.When back-projected to the midplane of the object for Cardiac SPECT, the errorswere <1mm and average error was 0.4(+/-0.22) mm, due to the high systemmagnification afforded by the detector. Thus, for our high sensitivity system,we were also able to achieve high resolution, assuming perfect pinholecollimator resolution recovery for Cardiac SPECT application.

Scintillation Event Localization in Hemi-Ellipsoid Detector for SPECT, a simulation study using Geant4 Monte-Carlo