Characterization of three layer depth of interaction PET detectors for small animal imaging

High sensitivity and spatial resolution are two main parameters of each small animal imaging system. Multi-layers phoswich detectors have been developed to measure the depth of interaction and to improve system spatial resolution. In the past, efforts have been made to develop these kinds of detectors with different crystal materials and lengths. In this work, three layers phoswich detectors based on GE Healthcare eXplore VISTA PET scanner geometry composing LYSO, GSO and BGO scintillators with different crystal orders and lengths were investigated to find the optimum case with the highest sensitivity and uniform spatial resolution. All simulations were performed using the Monte Carlo simulation tool, the Geant4 Application for Tomographic Emission (GATE). In order to validate GATE simulation package, the GE eXplore VISTA small animal PET system was modeled and output results were compared with the experimental data. The length of each of the three layers varied, while the total length (LYSO GSO BGO) was fixed at 15 mm. The order of these crystal layers was also changed, so that totally we have considered 55 x 6 = 330 different configurations. Using three layer phoswich detector, a 25%-68% improvement in the sensitivity at central slice was found compared to the dual layer VISTA PET scanner, dependent on different detector configurations. In this study, among all posible configurations, detector length permutations with higher efficiency values (12 x 6 = 72 cases) were choosen to evaluate spatial resolution. The radial and tangential spatial resolutions were markedly improved for all studied different detector configurations compared to the VISTA PET scanner. Among all the possible selected detector configurations, LYSO (4 mm) GSO (4 mm) BGO (7 mm) and BGO (5 mm) LYSO (5 mm) GSO (5 mm) cases gave the best DOI radial and tangential resolutions for an energy threshold of 250 keV, respectively. In the LYSO (4 mm) GSO (4 mm) BGO (7 mm) scanner configuration, the radial resolution was kept below 1.154 mm, over 25 mm field ofview (FOV). The tangential resolution variations were minimized to less than 1.029 mm, over 25 mm FOV, using the BGO (5 mm) LYSO (5 mm) GSO (5 mm) detector type. As a result, our new designed three layers phoswich detectors with excellent DOI resolution will lead to small animal PET scanners with higher sensitivity and uniform spatial resolution across the FOV.