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A small animal PET scanner using 256-channel PS-PMTs has been designed, constructed, and evaluated. The scanner has twelve detector modules, each of which consists of a double-layer array of LYSO crystals and three PS-PMTs (Hamamatsu R8400-00-M256). In the LYSO crystal block, 32 x 53 crystal elements are optically coupled to 32 x 54 crystal elements with a shift of half the element pitch in the axial direction. The dimension of each crystal element is 1.275 x 2.675 [mm.sup.2] in cross section and 7 mm in depth. The twelve detector modules are positioned on a 182 mm diameter ring to form 107 detector rings with 1.4 nun pitch. The trausaxial FOV is 100 mm in diameter and the axial FOV is 151 mm, which is sufficient to cover the whole body of a mouse. In order to compensate for non-uniform outputs from the multi-anodes of the PS-PMTs, ASICs having 64-channel variable gain amplifiers and summing amplifiers are used in the front-end circuits. The preliminary experimental results are the transaxial resolution of 2.0 mm FWHM in the CFOV, and the axial resolution of 2.8 mm FWHM on the axis of the ring. The absolute coincidence sensitivity is 8.1% for a point source at the CFOV with setting an energy window of 350-750 keV and a timing window of 10 us. The applicable imaging capability of the scanner was demonstrated by animal studies with a rat. Index Terms--Depth of interaction (DOI), positron emission tomography (PET), position sensitive PMT, small animal imaging.

A new positron emission tomography (PET) scanner for whole-body studies has been developed. The scanner has 720 block detectors, each of which consists of a flat panel position-sensitive photomultiplier and a 16 x 8 BGO crystal array. The detector system is composed of 12 layers of block detector rings stacked axially, and each ring consists of a circular array of 60 block detectors. Since each block detector ring contains eight crystal rings, the total number of detector rings is 96 and the axial field of view (FOV) is 685 mm, which is sufficient to measure a whole-body with two steps of scan. The detector ring diameter is 1 020 mm and the transaxial FOV is 600 mm in diameter. Coarse slice septa are placed between the block detector rings, which are effective to reduce scattered coincidence events while keeping a high detection sensitivity. Parallel 16 personal computers are used for the data acquisition and processing to deal with a large amount of event data. The acquired data in 3-D manner are converted to 2-D data set with Fourier rebinning algorithm and reconstructed with a fast iterarive algorithm 'DRAMA'. The reconstructed images for the whole body are obtained within 5 min after the scan. By using the coarse septa, the scatter fraction measured with NEMA NU2-2001 standard was 31.4% whose value was significantly lower than that of 3-D PET without septa. The system sensitivity was 9.72 cps/kBq, and the peak counts of the noise equivalent count rate used with direct random-subtraction was 113.6 keps at 10.5 kBq/ml. Index Terms--High-throughput, position sensitive PMT, positron emission tomography (PET), whole-body scan.