Evaluation of a PET Insert for Trimodal Imaging: A Step Toward PET/MRI-Guided Focused Ultrasound

Combining positron emission tomography (PET) with magnetic resonance imaging (MRI) and focused ultrasound (FUS) has emerged as a promising hybrid technique in the medical imaging field. Despite the potential benefits of using simultaneous PET and MRI acquisitions to monitor therapeutic effects and ensure precision and safety during FUS applications, no commercial or academic PET/MRI-guided FUS system (referred in the following as trimodal PET-MRI-FUS) exists. This work presents the design and evaluation of a preclinical PET insert for simultaneous operation with MRI and FUS systems. The proposed PET insert is based on monolithic LYSO crystals arranged in two octagonal rings that define inner and outer diameters of 72 and 114 mm, respectively, with an axial length of 67 mm. The system performance was evaluated according to the NEMA NU 4 2008 protocol. It achieves a uniform spatial resolution of ~0.9 mm along the whole PET Field of View (FOV) when enabling Depth of Interaction (DOI) capabilities. Moreover, we found a sensitivity peak of 3.8% for an energy window of 255-766 keV and a peak noise equivalent count rate peak of 80 kcps at an activity of 19 MBq. The compatibility of the PET insert with both low- and high-field MRI systems, as well as with custom and commercially available FUS devices, was studied using phantoms and in vivo experiments, respectively. The results of the trimodal tests have proven that the PET insert works simultaneously with both MRI and FUS systems.