Development of a ring PET insert for MRI

Our research group constructed a 12-module PET detection ring composed of Hamamatsu multi-pixel photon counter (MMPC) silicon photomultiplier (SiPM) detectors placed in a ring that is fully MRI compatible. This brain imager can be placed around the head of a patient (clinical setting) or subject (research setting) and allow for comfortable upright imaging. However, an alternative way to use this device, as enabled by the technology, is to indeed scan individuals in the supine position in conjuncture with current MRI systems. This PET prototype is able to image simultaneously as the MRI scan is occurring, thus maximizing co-registration and the accuracy of the assignment of metabolically active voxels to their anatomically correct counterparts as identified by the MR image. In this study, we first conducted some basic instrumentation tests to ensure the device was functioning properly outside of the MRI, and additionally scan some phantoms (Derenzo; Hoffman Brain) to assess the quality in which our brain imager is able to produce adequate images. After these initial studies, we conducted multiple different experiments inside a 3 Tesla MRI in order to see how the magnetic field would influence the operation of the PET imager and vice versa. Through simultaneous PET/MRI scanning of a Hoffman brain phantom filled with F¹⁸ radioactivity and water, it was shown that the quality of the MR image was largely unaffected by the PET imager. Furthermore, although the quality of PET imaging was affected by the RF pulsing, an acceptable PET image was nevertheless produced. As we discuss following the results, the success of this study shows that our brain imager is indeed MR compatible, and that next generation devices based on its concepts will continue to improve combined PET/MRI functionality. As two of the major advantages of this imager are the potential for low-dose scanning and its adaptability to any MRI scanner, this study suggests that PET/MRI brain imaging w- th low dose is in principle possible using an insert which could be adapted to any MRI scanner, using standard RF coils for that scanner.