Simultaneous PET/MR Images Acquired with an RF-Penetrable PET Insert

Simultaneous PET/MRI shows great promise for improved detection and diagnosis of disease. Commercially available whole body combined PET/MRI systems are currently prohibitively expensive, limiting access to the technology. MRI-compatible PET inserts present a lower-cost alternative, adding PET capabilities to existing MRI installations. A brain sized radio-frequency (RF)-penetrable PET insert has been designed for simultaneous operation within MRI systems. This insert takes advantage of electro-optical coupling and battery power to electrically float the PET insert relative to the MRI ground, permitting RF signals to be transmitted and received through small gaps between the PET modules. Non-magnetic silicon photomultipliers are used in conjunction with a compressed sensing signal multiplexing scheme, and optical fibers transmit analog PET detector signals out of the MRI room for decoding, processing, and image reconstruction. Results of the first simultaneous PET/MR imaging studies with the full RF-penetrable PET ring are presented in this work. The PET insert was placed within a 3T whole body MRI system, and tomographic images of a 3D-printed resolution phantom with hot and cold rods of varying sizes were acquired both with only the B-0 field present, and under continuous pulsing from three MR imaging sequences: gradient echo (GRE), fast spin echo (FSE), and echo planar imaging (EPI). The resulting PET images have comparable contrast-to-noise ratios (CNR) under all pulsing conditions, with an average (of all rod types) maximum CNR difference of 14.0% among images. MR images of the resolution phantom were successfully acquired through the RF-penetrable PET shielding using only the built in MR body coil, with mean CNR decreases of 19.5% and 29.8% for GRE and FSE, respectively. These results suggest that simultaneous imaging is possible with the RF-penetrable insert, and show promise for this technology as an alternative to costly integrated PET/MRI scanners.