Geometry optimization of dual-layer offset detectors for compact ring diameter PET systems

Compact ring diameter PET systems suffer degradation in spatial resolution with increasing radial offset due to the parallax error caused by depth of interaction (DOI) effects. In this work, we study the problem of optimizing layer thicknesses for a detector that uses a dual-layer offset (DLO) scintillator array. In this design, a half-crystal offset exists between the crystals in the front and back layer. The goal of this optimization is to minimize parallax related event mispositioning for several fixed total detector thicknesses. GATE Monte Carlo simulations were used to investigate detector geometries for a brain dedicated PET insert that fits into the Siemens Magnetom 7 T MR scanner. The bore diameter of the MR scanner is 360 mm. LYSO is the choice of scintillation material. Total crystal thickness ranges from 10 to 30 mm with front-to-back layer ratios of 10/90 to 100/0 (i.e. single layer). A crystal pitch of 2 mm was assumed in this study. Collectively, a total of 50 detector geometries were considered in this work. For each total thickness, the optimum thickness ratios were found such that the average radial displacement between the first point of interaction in the scintillator array and the assigned position of the event is minimized. Results show that the radial blurring is minimized when the front layer is 40-60% of the total scintillator thickness for 10 mm thick arrays, 40-50% for 15 and 20 mm arrays, 30-40% for 25 mm arrays, and 20-40% for 30 mm arrays. For total detector thicknesses greater than 10 mm, the optimal front layer thickness was always equal to or thinner than the back layer thickness.