  A high sensitivity Cardiac SPECT system using curved crystals with pinhole
collimation was proposed previously (Dey, IEEE TNS 2012, Bhusal et al, Med.
Phys. 2019). Here, we hypothesize that a high curvature hemi-ellipsoid detector
results in measurable differences in light distribution from events at
different depths in the crystal. This was tested by analyzing the scintillation
light in hemi-ellipsoid detector using Monte-Carlo (Geant4) and evaluation of
both the localization error at detector and the back-projected errors in
object-space. We used Geant4 to simulate the propagation of scintillation light
in a monolithic hemi-ellipsoidal CsI crystal. A look-up table (LUT) was created
to map the points inside the crystal to the expected light pattern on the
crystal surface using Geant4. In thirteen zones across the crystal, gamma-rays
were simulated and the resulting scintillator light intensity on the surface
was captured, serving as our experimental interactions. A
Poisson-statistics-based algorithm was developed to limit the search of the
gamma-ray event locations into small regions of the LUT and fine-tuned by
interpolating between selected LUT points by comparing the light distribution
of the gamma interactions and LUT light patterns. The localized events were
individually back-projected to the object mid-plane, and the errors recorded.
Excluding some outliers (up to 2%), the localized errors averaged over all the
zones 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 errors
were <1mm and average error was 0.4(+/-0.22) mm, due to the high system
magnification afforded by the detector. Thus, for our high sensitivity system,
we were also able to achieve high resolution, assuming perfect pinhole
collimator resolution recovery for Cardiac SPECT application.
