SAFIR-II: Performance Evaluation of a High-Rate Preclinical PET-MR System

Abstract

SAFIR-II is a preclinical PET insert compatible with a Bruker BioSpec 70/30 magnetic resonance imaging (MRI) scanner. It was designed to acquire data at activities of up to 500 MBq, enabling truly simultaneous preclinical positron emission tomography magnetic resonance imaging for mice and rats using image acquisition times of as little as 5 s. We present a brief overview of the system’s design as well as the results of several performance evaluations. SAFIR-II features an axial field-of-view (FOV) of 145 mm, covered by lutetium-yttrium oxyorthosilicate crystals coupled to Hamamatsu silicon photomultiplier (SiPM) arrays. PETA8 application-specific integrated circuits are used to digitize the SiPM’s analog signals, and custom MR-compatible dc-dc converters condition the system’s internal voltages. The insert exhibits a coincidence timing resolution of 221-ps full width at half maximum (FWHM), a coincidence energy resolution of 12.1%, and a peak sensitivity of 3.89% observed following the NEMA-NU4 standard. It is capable of resolving 1.7-mm hot rods within a Derenzo phantom filled with $^{18}{\mathrm { F}}$ and features a peak noise-equivalent count rate of 1.12 Mcps observed at an activity of 451 MBq using the NEMA rat-like phantom. We furthermore present an evaluation of the system’s image quality determined using a NEMA image quality phantom, an evaluation of its MRI-compatibility, as well as images from an initial in vivo measurement using a Sprague-Dawley rat injected with 283-MBq fluordesoxyglucose.