Design of a small animal MR compatible PET scanner

Using a combination of Monte-Carlo simulations and experimental measurement s we have designed a small animal MR compatible PET (McPET) scanner for sim ultaneous PET and MR imaging of mice and rats in vivo. The scanner consists of one ring of 480 LSO crystals arranged in 3 layers with 160 crystals per layer. The crystal dimensions are 2 x 3 x 7.5 mm(3). This was based on a t arget resolution of 2.5 mm and simulations showing that a depth of 7.5 mm a voided significant depth of interaction effects across the desired field of view. The system diameter of 11.2 cm is large enough to accommodate the an imal positioned inside a stereotactic frame. Each crystal will be coupled t hrough 2 mm diameter optical fibers to multi-channel PMT's which reside out side the main magnetic field. Through 50 cm of optical fiber, a photopeak i s clearly seen and the measured energy resolution is 25%. Prototype optical fiber connectors have been tested to increase the flexibility of the syste m and result in a light loss of only 6%. The proposed system will have adeq uate resolution and sensitivity for a number of applications in small anima ls and will be the first practical device for simultaneous in vivo imaging with PET and MR.