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We describe the Sherbrooke positron emission tomograph, a very high resolution device dedicated to dynamic imaging of small laboratory animals. Its distinctive features are: small discrete scintillation detectors based on avalanche photodiodes (APD) to achieve uniform, isotropic, very high spatial resolution; parallel processing for low deadtime and high count rate capability; multispectral data acquisition hardware to improve sensitivity and scatter correction; modularity to allow design flexibility and upgradability. The system implements the "clam-shell" sampling scheme and a rotating rod transmission source. All acquisition parameters can be adjusted under computer control. Temperature stability at the detector site is ensured by the use of thermoelectric modules. The initial system consists of one layer of 256 modules (two rings of detectors) defining 3 image slices in a 118 mm diameter by 10.5 mm thick field. The axial field can be extended to 50 mm using 4 layers of modules (8 rings of detectors). The design constraints and engineenng aspects of an APD-based PET scanner are reviewed and preliminary results are reported. * This work was supported in part by the Medical Research Council of Canada under Grant MA-8549. The first author is a senior scholar of Le Fonds de la Recherche en S a d did Qrribec.

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The prototype of a high resolution PET camera consisting of two opposite arrays of detectors with independent solid state readout was built and tested. The basic detector unit is the RCA C30994 detector module consisting of two 3 x 5~2 0 mm BGO scintillators, each coupled to one silicon avalanche photodiode. The two-dimensional stacking capability of the module allows a high resolution multi-ring detection system to be assembled without crystal coding. The prototype was used to simulate a 31 cm diameter dual ring tomograph suitable for animal studies. Coincident detector pair resolution was measured and the contributions t o resolution loss estimated using a platinum sheathed .75 mm 68Ge line source. The intrinsic resolution is 1.9 mm FWHM, 3.5 mm FWTM at the center of the field. The reconstructed point spread function resolution in a stationary mode is 2.3 mm FWHM at the center and 3.1 mm FWHM (tangential), 3.9 mm FWHM (radial) at 5 cm from the center. The axial resolution is less than 3.5 mm FWHM throughout the field. T h e ring sensitivity for an animal system is 67 Kcps/mCi in air for an axial line source and 3.3 Kcps/pCi/ml for a 10 cm diameter cylinder of 22Na solution.