Scintillation event localization in hemi-ellipsoid detector for SPECT, a simulation study using Geant4 Monte-Carlo
Authors: Soysal H.R. , Dey J. , Donahue W.P. , Matthews K.L. 
Published: May 2022

Purpose: a high sensitivity Cardiac SPECT system
using curved crystals with pinhole collimation was proposed
previously (Dey,
IEEE
Trans. Nucl. Sci.
59 (2012) 334). Using
hemi-ellipsoid CsI crystals, in simulations, the system improved
sensitivity by a factor of 3 over newer generation SPECT systems
(DSPECT or GE Discovery) while keeping the resolution comparable
(Bhusal et al.,
Med. Phys.
46 (2018) 116). In this work, we hypothesize that the
high curvature detector results in measurable differences in light
distribution from events at different depths in the crystal. We
rigorously test this by analyzing the scintillation light using
Monte-Carlo (Geant4) and propose a statistical event localization
method in a hemi-ellipsoid detector. We evaluate the localization
error of the algorithm and the back-projected errors in object
space. Methods: to develop this localization capability for
the proposed design, 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 the
Geant4 simulation data. Thirteen zones were considered across the
crystal. In each zone, gamma-rays were simulated and the resulting
photon intensity on the surface was captured, serving as our
experimental interactions. An algorithm based on Poisson statistics
was developed to limit the search of the experimental gamma-ray
event locations into smaller regions of the LUT. The localization
was fine-tuned by comparing the light distribution of the gamma
interactions in selected patterns from the LUT points and then
recorded. The algorithm-localized gamma-ray events were also
individually back-projected to the object mid-plane, (expected
mid-plane of the heart), and the error at the plane was recorded as
well. Results: the light patterns of adjacent LUT points
showed visually discernible differences. 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. Moreover, when back-projected to the midplane of the region of
interest for Cardiac SPECT, the errors were <1 mm due to the high
system magnification afforded by the apex and other zones. The
average back-projection error at the mid-plane of the object was
0.4 mm±0.22 mm. Conclusion: we
modeled gamma-event interactions and scintillation light spread in
CsI hemi-ellipsoid detector and developed a robust statistical
algorithm that localized scintillation events to within 0.71 (±0.44) mm on the average within a hemi-ellipsoid CsI
detector. Moreover, due to the high system magnification afforded by
the crystal, the hemi-ellipsoid unit was capable of achieving
<1 mm average localization in the object space, assuming perfect
pinhole collimator resolution recovery. Thus, we show that this high
sensitivity system will be able to deliver images with high
resolution for Cardiac SPECT. In the future, the application of this
may be extended to Brain SPECT and small animal imaging.