Design of a small animal MR compatible PET scanner

Using a combination of Monte Carlo simulations and experimental measurements we have designed a small-animal MRI-compatible PET (McPET) scanner for simultaneous PET imaging and MRI of mice and rats in vivo. The scanner consists of one ring of 480 LSO (Lu₂SiO₅ ) crystals arranged in three layers, with 160 crystals per layer. The crystal dimensions are 2×3×7.5 mm³. This was based on a target resolution of 215 mm and simulations showing that a depth of 7.5 mm avoided significant depth-of-interaction effects across the desired field of view. The system diameter of 11.2 cm is large enough to accommodate the animal positioned inside a stereotactic frame. Each crystal is coupled through 2-mm diameter optical fibers to multi-channel photomultiplier tubes which reside outside the main magnetic field. Through 50 cm of optical fiber, a photopeak is clearly seen and the measured energy resolution is 25%. Prototype optical fiber connectors have been tested to increase the flexibility of the system and result in a light loss of only 6%. The proposed system has adequate resolution and sensitivity for a number of applications with small animals and is the first practical device for simultaneous in-vivo imaging with PET and MRI