Imaging the unanesthetized rat brain with PET: a feasibility study

The current requirement of anesthesia in PET imaging of animals has undesirable effects on the study of neurological systems and limits the types of studies that can be performed. To enable functional studies of the unanesthetized rat brain, the feasibility of a miniaturized PET tomograph that is secured directly to the head of the rat while allowing collective motion of the head and scanner, has been investigated. The proposed geometry is a septa-less, 4 cm-diameter ring tomograph with an axial extent of 2 cm, that fits between the eyes and ears of a typical laboratory rat, positioning nearly the entire brain within the field of view. This geometry allows a normal posture and can be affixed with techniques currently used to secure microdialysis probes in the rat brain. The proposed detectors are state-of-the-art, but commercially available arrays of 2×2×10 mm³ LSO crystals and matching avalanche photodiode arrays. A realistic mockup of the scanner weighing 125 g was tested on a rat and tolerated well. Studies have been carried out to estimate the performance characteristics of the proposed scanner, which are compared to those of the rodent microPET systems (UCLA prototype and commercial Concorde scanners). The parallax contribution to the spatial resolution at the edge of the brain can be reduced to microPET levels by using 2 layers of 5 mm-long crystals. Sensitivity, deadtime/countrate capability, scatter, and randoms from activity outside the field of view are estimated to be comparable to the microPET systems as well.