First images from a combined PET and field-cycled MRI system

Objectives Recent efforts to combine PET and MRI have either modified PET detectors to make them compatible with magnetic fields or changed the MR system to enable the use of conventional PMT-based PET detectors. The authors present the first hybrid images from a system that combines conventional PMT-based PET with field-cycled MRI (FCMRI).

Methods The FCMRI system was operated by sequentially pulsing a 0.3-T polarizing magnet and a 94-mT readout magnet. During the cooling period of the resistive magnets, all magnetic fields were turned off to permit the operation of PMTs for PET imaging. Two PET detectors, each containing a BGO scintillator cut into 8x8 crystals with 6.5x5.5x30-mm³ pitch, were coupled to four PMTs and inserted into a 9-cm opening in the FCMRI system. A phantom consisting of a triangular puck containing three Na-22 point sources embedded into an onion was imaged. Since only two PET detectors were available, the phantom was rotated in a step and shot method and PET data were recombined post-acquisition.

Results PET and MR images were superimposed. The layers of the onion are visible in the MR image and the positions of the three point sources are visible in the PET image. No major streak artifacts appear in the PET image and the MR image has no visible phase encode or ghosting artifacts.

Conclusions The authors have demonstrated that dual modality PET/FCMRI imaging can be achieved without significant interference between modalities. PET performance was limited by the use of clinical detectors with coarse crystal pitch. Designs for a future PET/FCMRI system, currently in development, are based on the integration of a commercial small-animal PET system and a higher field-strength FCMRI system.