Development of PET/MRI with insertable PET for simultaneous imaging of human brain

The purpose of this study was to develop a dual-modality PET/MRI with insertable PET for simultaneous imaging of human brain. The PET insert consisted of 18 detector blocks arranged in a ring of 390 mm diameter with 60 mm axial FOV. Each detector block was composed of 4 × 4 matrix of detector modules, each of which consisted of a 4 × 4 array LYSO coupled to a 4 × 4 GAPD array. The PET gantry was shielded with gold-plated conductive fabric tapes with thickness of 0.1 mm. The charge signals of PET detector transferred via 4 m long flat cables were fed into the position decoder circuit. The flat cables were shielded with a mesh-type aluminum sheet with 0.24 mm thickness. The position decoder circuit and FPGA-embedded DAQ modules were enclosed in an aluminum box with 10 mm thickness and located at the rear of the MR bore inside MRI room. A 3-T human MRI system with a Larmor frequency of 123.7 MHz and bore inner diameter of 60 cm was used for PET/MRI hybrid system. A custom-made radio frequency (RF) coil with inner diameter of 25 cm was fabricated. The PET was positioned between gradient and the RF coils. PET performance was measured outside and inside MRI scanner with spin echo, turbo spin echo and gradient echo sequences. SNR and of MR phantom image were also measured with and without PET insert. The stability of developed PET insert was evaluated and simultaneous PET and MR images of brain phantom were acquired. No significant degradation of the PET performance caused by MR was observed when the PET was operated with various MR imaging sequences. Uniformity and SNR of MR phantom image were degraded about 1% and 7% by the PET insert, respectively. The change of gain of PET detector was <;3% (n=256) for 60 minutes and simultaneous PET and MR images of brain phantom were successfully acquired. Experimental results indicate that the simultaneous PET and MR imaging is feasible using MR compatible PET insert proposed in this study without significant interference.