A laboratory PET scanner with depth-of-interaction detectors for mouse studies and molecular imaging

We have developed a compact laboratory PET scanner specific for mouse imaging. This simple, low-cost device is designed for measuring the pharmacokinetics of new tracers and drugs. The small cross-sectional profile of a mouse allows high sensitivity to be reached using only a small number of detectors close to the animal. However, a depth-of-interaction (DOI) encoding detector is required to correct the severe parallax error resulting from the 2 cm thick scintillation detectors and the small detector separation. Several DOI detector designs have been investigated. One approach was to place WLS fibers on the end of the arrays for both crystal identification and depth-decoding. Optimization work was done to maximize the light collection from the fiber ends and to minimize the optical cross-talk between fibers. Assembling LSO arrays with WLS fibers, a best DOI FWHM resolution of 5 mm FWHM was achieved. However due to the large light loss through fiber read-out, the flood histograms using fiber signals were not very good and the crystal identification was challenging. This idea was then modified by placing one end of the array directly to a MC-PMT with the other end read out by WLS sheet for DOI. Such a detector also had about 5 mm DOI FWHM and the flood histograms were greatly improved. The modified design was applied to build two detector modules for a prototype scanner. Preliminary experiments showed that the prototype scanner had 1.9-2.0 mm spatial resolution, 3.5 ns timing resolution and 6.6% system sensitivity at the center of the field-of-view (FOV).;A novel idea for depth-encoding has been investigated by coating the scintillator with a layer of phosphor to convert a fraction of the scintillation photons into phosphor photons. The coating changed the pulse shape in a depth-dependent manner. Up to 20 ns decay time changes were observed when coating half sides and the end of a LSO array with YGG phosphor. Using same technique to build a LSO-YGG array, about 8 mm DOI resolution has been achieved. This approach will be further investigated in the future to improve the depth-encoding properties.