Performance evaluation and MR compatibility assessment of an optical fibre based pre-clinical MR-compatible PET scanner

We have designed and constructed a small animal MR-compatible PET system for fully simultaneous MR/PET acquisitions. The scanner uses long optical fibres to distance the field sensitive PET PMTs from the high magnetic field at the centre of an MR scanner. The system has been designed to operate inside a number of different high field MRI scanners. We present a detailed performance evaluation and high field MR compatibility assessment of the PET system. The PET-only performance results are encouraging. We report a reconstructed transaxial resolution of 1.49mm at the centre of the field of view which falls off gradually to 2.52mm at 26.5mm from the centre. The average axial resolution is 2.7mm. The slice sensitivity to a point source is 0.95%, when corrected for both scatter and randoms. The count rate is linear up to an activity of 6 MBq (∼ 5kcps). The scatter fraction was 42% which could be reduced to 26% using MR compatible gamma shields. We have produced images of various hotspot phantoms that demonstrate good image quality. We assessed the MR compatibility of the PET system in a Philips Achieva human 3T scanner. We found there were no artefacts or distortions imposed on either the PET 2D flood position histograms of the PET PMTs or the PET images when the PET system was operated inside the MRI scanner. We have demonstrated that the MRI scanner did not pick up any significant levels of RF interference from the PET electronics by acquiring data across the entire frequency range of the RF head coil whilst the PET was inside the MRI scanner acquiring data. We have characterised the effects of the PET scanner materials on the main magnetic field of the MR scanner using a field map of a large uniform phantom with the PET scanner in the MRI scanner. We found that there were no significant distortions seen in MR images when the PET was located in the MR FOV. We acquired simultaneous MR and PET images of a mouse brain using a dedicated small animal coil in the 3T - - human system, which demonstrated good image quality. We now plan to use the system to demonstrate novel pre-clinical applications of simultaneous PET/MR.