RatCAP: A small, head-mounted PET tomograph for imaging the brain of an awake RAT

A small, head-mounted tomograph is being developed which will allow PET imaging of the brain of an awake rat. This device will permit neurophysiological studies to be carried out on small animals without the use of anaesthesia, which severely suppresses brain functions and behavior. The tomograph consists of a 4cm diameter ring consisting of 12 blocks of LSO crystals, each containing a 4×8 matrix of 2×2mm² pixels read out with a Hamamatsu S8550 avalanche photodiode array. The ring will be mounted to the head of the rat and supported by a tether that carries the weight and provides a pathway for electrical signals. Combined with additional mechanical components, it will allow nearly complete freedom of movement of the animal. In order to minimize the weight of the ring, and to keep all of the front end readout electronics as close as possible to the detector, a new ASIC is being developed in 0.18μm CMOS technology that will process the analog signals and provide digital readout of the pixel arrays and timing information. This paper will describe the novel features and challenges of this new detector, along with preliminary results obtained with a pair of block detectors used in a configuration similar to the final tomograph. Results are given on studies carried out to optimize the light output of the crystal arrays, measurements of the APDs, a preliminary design of the readout electronics chip, and reconstructed images of various types of phantoms in order to demonstrate the feasibility of the detector concept.