Determination of the point-of-first-interaction for an ultra-high-performance brain PET system with "onion ring" geometry.

We used the Monte Carlo GATE v9, (GEANT4) simulations to model annihilation photons interactions in our novel advanced brain-PET scanner in onion-ring geometry (US Patent 20210223414) with monolithic thin LYSO slab optically coupled to SiPM arrays to assess potential improvement of point-of-first-interaction (PFI) localization determination vs. conventional system. Three 250-mm-long cylindrical systems were evaluated: (i) 3-3-3 mm slab thicknesses concentric rings; (ii) 3-3-18 mm slab thicknesses concentric rings with 7-mm air-gaps and inner diameters of 250, 270 and 290 mm, respectively; and (iii) single-ring 24-mm-thick system with 250 mm inner diameter. The brain was simulated by a sphere of radioactive (F-18) water with 170-mm diameter confined by 8-mm thick spherical bone shell. The coincidence events studied via simulations involved photo-electric absorption (PA) and first-order Compton scatter (CS) interactions happening within predefined coincidence-time-window. If the distance between CS and PA event was shorter than the detector intrinsic spatial resolution, we considered it "pseudo single" event at energy-based averaged location (Pavg). We found that for the pseudo single events with energy deposited around 511 keV the median PFI-Pavg distance projected onto the detector plane was 0.5 mm for 3-mm ring, 0.9 mm for 18-mm ring, and 1.0 mm for 24-mm ring. We established that 3-3-3-mm system allowed obtaining more accurate depth-of-interaction (DOI) information with the mean true DOI for PFI of 1.4 mm and absolute uncertainty of 0.3 mm, as compared to the conventional 24-mm-thick single-ring system with the mean true DOI for PFI of 7.5 mm and with higher absolute uncertainty of 1.6 mm. The ratios of the number of intra-ring to inter-ring first-order Compton scattering events were lowest for the central rings (1.9 and 1.3 for 3-3-3-mm and 3-3-18-mm systems, respectively), higher for the distal and proximal rings, and strongly dependent on the ring thickness. The relative probability for forward vs. backward scattering was in the range of 60% to 89%, and 61% to 94% for 3-3-3-mm and 3-3-18-mm systems, respectively. We conclude that the spatial resolution provided by multi-ring geometries may offer substantial improvement when compared to conventional PET system designs with comparable sensitivity.