PET-Compton hybrid imaging simulation of a small animal whole-gamma-imaging system

Whole gamma imaging (WGI) is a new nuclear medicine imaging method that combines PET and Compton imaging. By inserting a scatter detector ring inside a PET detector ring, it is possible to combine two different imaging modalities into one system.
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Zr is one of the PET tracers that is suitable for imaging based on an antigen-antibody reaction. However, the 909 keV single gamma ray emission, which occurs four times more frequently than positron emission, has been a factor to degrade image quality in conventional
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Zr-PET. Since WGI can utilize the single gamma rays for imaging by Compton imaging, WGI is expected to improve the image quality by using image reconstruction from combined PET events and Compton events. On the other hand, it has not been clear how much improvement is expected in image quality by combining data with different characteristics. Therefore, in this work, we modeled a small animal WGI system currently under development and assessed image quality to be achieved by the PET-Compton hybrid imaging. Using GEANT4, we modeled the small animal WGI system employing HR-GAGG crystals as the scatter and LGSO crystals as the absorber. The scatter crystal was 15 mm long, but three-layer depth-of-interaction discrimination was enabled. In a contrast phantom, at the same contrast recovery, the hybrid reconstruction offered 4% lower noise than the PET image reconstruction did. In a Derenzo-like phantom, there was no significant difference observed between the PET reconstruction and the hybrid reconstruction. We concluded the hybrid image reconstruction can improve image quality, but the improvement is limited in the simulated small animal WGI system. We consider this is mainly due to limited energy resolution of the scatterer detector; Compton imaging suffered from degraded spatial resolution, and therefore the effect of an increased number of counts may have been cancelled by the degraded spatial resolution in the Compton events.