Initial results of the Hyperion IIDPET insert for simultaneous PET-MRI applied to atherosclerotic plaque imaging in New-Zealand white rabbits.

Objective: In preclinical research, in vivo imaging of mice and rats is more common than any other animal species, since their physiopathology is very well- known and many genetically altered disease models exist. Animal studies based on small rodents are usually performed using dedicated preclinical imaging systems with high spatial resolution. For studies that require animal models such as mini- pigs or New-Zealand White (NZW) rabbits, imaging systems with larger bore sizes are required. In case of hybrid imaging using Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI), clinical systems have to be used, as these animal models do not typically fi t in preclinical simultaneous PET-MRI scanners. Approach. In this paper, we present initial imaging results obtained with the Hyperion IID PET insert which can accommodate NZW rabbits when combined with a large volume MRI RF coil. First, we developed a rabbit-sized image quality phantom of comparable size to a NZW rabbit in order to evaluate the PET imaging performance of the insert under high count rates. For this phantom, radioactive spheres with inner diameters between 3.95 and 7.86 mm were visible in a warm background with a tracer activity ratio of 4.1 to 1 and with a total 18-F activity in the phantom of 58MBq at measurement start. Second, we performed simultaneous PET-MR imaging of atherosclerotic plaques in a rabbit in vivo using a single injection containing 18-F-FDG for detection of infl ammatory activity, and Gd-ESMA for visualization of the aortic vessel wall and plaques with MRI. Main results. The fused PET-MR images reveal 18-F-FDG uptake within an active plaques with plaque thicknesses in the sub-millimeter range. Histology showed colocalization of 18-F-FDG uptake with macrophages in the aortic vessel wall lesions. Significance. Our initial results demonstrate that this PET insert is a promising system for simultaneous high-resolution PET-MR atherosclerotic plaque imaging studies in NZW rabbits.