Development of the 2nd prototype of add-on PET: a head coil with DOI-PET detectors for PET/MRI

We are developing a brain dedicated PET-MRI system in which our original, four-layer DOI-PET detectors are integrated with a birdcage RF-coil. The main point is making the PET ring diameter as small as possible while avoiding interference with the RF-coil, in order to realize a highly sensitive PET system with high spatial resolution by making full use of DOI measurements. Previously, we reported 1.6 mm spatial resolution with prototype PET detectors integrated with the RF-coil. However the PET system had only a 12 mm axial FOV and a large electrical circuit was needed. We developed a new four-layer DOI-PET detector for the full system of the proposed PET-MRI; it has an extended axial FOV (96 mm) compared to the previous system which is achieved by arranging three 16 × 16 × 4-layer crystal blocks along the axial direction. The LYSO crystal dimensions are 2.0 mm × 2.0 mm × 4.0 mm. The single-channel multi-pixel photon counters (MPPCs, S12641PA-050) are arranged in an 8 × 8 array with 4.1 mm pitch and 1 mm dead spaces between the MPPCs which have a 3 × 3 mm2 sensitive area. The 64 channel signals are read out through an ASIC functioning as an amplifier and sum circuit to reduce the circuit board size. We evaluated performance of the new PET detector and influence of the detector on MRI images during simultaneous measurements. As a result, for PET performance, we found the averaged peak-to-valley ratios were 2.60 (without MRI measurement), 2.60 (fast spin echo, FSE) and 2.63 (echo planar imaging, EPI). Energy resolutions of 14.0 % (without MRI), 14.6 % (GRE) and 14.7 % (SE) were obtained for single crystal elements. For the MRI, the B0 drop was smaller than 5.0 ppm and the SNR degradation was lower than 7% by the PET measurement.2015IEEE NSS&MI