Quantitative investigation of inter-crystal scatter and penetration in the GE Discovery RX PET/CT scanner using Monte Carlo simulations

As the dimensions of crystal elements in modern PET systems is getting smaller, the fraction of events undergoing inter-crystal scattering (ICS) and penetration increases. This has deteriorative impact on the spatial resolution of images. However, with the utilization of statistical image reconstruction methods in PET, compensation for ICS and penetration impacts is feasible by accurately modeling their effects in the projection space. We aim to perform such a work for the GE Discovery RX (DRX) PET/CT system. In this work, we investigated ICS and penetration in the DRX for acquiring a quantitative view of their respective contributions. We analyzed the events in the form of coincidences instead of single photons. Also as a novelty, we discriminated between the origins of the event mispositiong. For this, we applied the GATE (Geant4 Application for Tomographic Emission) Monte Carlo toolkit and used our previously validated model of DRX. Numerous points in different positions were analyzed inside the field of view (FOV) of DRX. Finally, using geometrical symmetries of the DRX and interpolation, the fraction of ICS and penetration could be determined for all possible positions inside the FOV. The results revealed that the notable variations in quantitative behavior of ICS and penetration occurred with varying radial positions; the fraction of ICS/penetration-induced mispositioned coincidences out of true coincidences ranged from 28.7% at the center to 57.8% at the edge of the transaxial FOV. This comprehensive quantification of ICS and penetration not only provides a deeper understanding of their respective contributions, but is also aimed to be utilized in refining the system matrix in the image reconstruction task to achieve resolution modeling in the scanner.