Performance of detector modules designed for the NeuroSphere brain PET system

Background The Human Dynamic NeuroChemical Connectome (HDNCC) scanner integrates a high spatio‐temporal resolution dedicated brain PET insert (NeuroSphere PET) with a commercial 7‐T MRI to enable studies of neurochemical dynamics simultaneously with functional MRI on comparable timescales. Purpose The NeuroSphere will achieve unprecedented sensitivity with a spherical geometry and novel detectors, which must provide time‐of‐flight (TOF) and depth‐of‐interaction (DOI) encoding without compromising crystal separation or energy resolution. We aim to develop a novel PET module for high‐resolution PET/MR applications with TOF and DOI capability. Methods The NeuroSphere PET detector is a 10 × 10 array of 26.0‐mm LSO crystals with 1.6‐mm pitch coupled to a 4 × 4 SiPM array with 4‐mm pitch, and a top light guide that provides DOI encoding via light sharing. Time‐to‐digital converter (TDC) performance and nonlinearity correction methods were evaluated with a pulsed laser. Crystal separation, energy resolution, and DOI resolution were evaluated, including a DOI‐based energy correction. The ASIC readout parameters were configured and timing corrections were implemented to optimize coincidence timing resolution (CTR), including a DOI‐based timing correction. Results The TDC achieved 55‐ps CTR between laser pulses after implementing nonlinearity correction, an improvement to the factory correction. The PET detector had good crystal identification with a peak‐to‐valley ratio of 11.43 5.87. DOI resolution was 7.45 2.02 mm full‐width‐half‐maximum (FWHM). A batch of 14 detector blocks had an energy resolution of 11.0% 1.8 % FWHM, improved from 12.6% 1.7 % FWHM by a DOI‐based correction. The best block‐to‐block CTR was found to be 510 ps FWHM, with 100 ps improvement after a DOI‐based correction, and a batch of 7 block pairs had a CTR of 526 15 ps FWHM. Conclusions This work has shown the capability of the NeuroSphere PET module to provide energy resolution, DOI encoding, and TOF PET competitive with other dedicated brain PET/MR systems. The CTR of 500 ps provides SNR gain for neuroimaging, and the energy resolution and CTR were improved by a DOI‐based correction using an easily scalable calibration procedure.