Characterizing the TOF performance of the PennPET Explorer scanner (2018)

96 Objectives: UPenn is developing a long-axial field-of-view (LA-FOV) PET/CT system—referred to as the PennPET Explorer scanner—as part of the Explorer consortium with UC Davis. In addition to exploring the improvements in clinical imaging offered by high-sensitivity, total-body imaging with LA-FOV PET, a secondary goal of the project is to investigate improvements in time-of-flight (TOF) resolution and its benefit when combined with LA-FOV geometries. The PennPET Explorer scanner uses digital silicon photomultiplier (DSiPM) detector technology from Philips Healthcare, which we have reconfigured to provide additional improvements in TOF performance. The goal of this presentation is to characterize the temperature dependence of our DSiPM-based detector and optimize the achievable TOF resolution and photon sensitivity. We will also describe the design choices made for detector cooling and the system electronics architecture used in the PennPET scanner.

 Methods: Each ring of the PennPET Explorer is composed of 504 detector blocks. Each detector block is an 8×8 array of 3.86×3.86×19mm3 LYSO crystals coupled to a digital SiPM tile, in a 1-to-1 crystal to photosensor geometry. The performance of the detector block as a function of temperature was characterized using individual detector blocks in coincidence measured in a temperature controlled bench-top setup, where the detector separation and count rate was controlled. Several configurations of the DSiPM were tested, including the trigger level and number of inhibited microcells. For each configuration the photon sensitivity, timing resolution, and detector dead time were measured. Optimal settings determined from these measurements can then be translated to the full ring system, which can be operated between 3° to 20° C. A fully calibrated result from the system will be presented.

 Results: Results from individual detector blocks demonstrate excellent light collection, energy resolution, and crystal identification, as expected with a 1-to-1 coupled detector. A coincidence timing resolution (CTR) of 231 ps was measured using trigger level 1, and 307 ps when using trigger level 2. The CTR from all coincident crystal pairs was very uniform, range of 213 ps - 256 ps, which again can be attributed to the detector design and 1-to-1 coupling. The detector timing resolution at trigger 1 was found to be independent of temperature (239 - 231 ps, from a range of 18° to -5° C), however the photon sensitivity of the detector was highly dependent on temperature, and decreased from 96% at -5° C to 69% at 15° C. The detector count rate remained linear up to singles rates of ~140 kHz, suggesting that count losses are due to effects of thermal detector noise and a minimal influence of detector deadtime due to photon pile up. At a temperature of 5° C, the sensitivity of the detector at trigger 1 was matched to the sensitivity at trigger 2 at 15° C. Because of the fully digital detector architecture we expect no degradations as these measurements are replicated on the system level.
Conclusions: We have demonstrated that at lower temperatures better TOF is achieved while maintaining the system photon sensitivity when using advanced detector settings offered with DSiPM technology. This methodology will now be expanded to the full system and the imaging performance of both increased axial sensitivity as well as improved TOF will be explored for LA-FOV TOF-PET scanners.
Funding support: We acknowledge Philips Healthcare, NIH R01 CA206187 and NIH R01-CA113941.