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The development of a high-resolution Positron Emission Tomography (PET) technique with sub-millimeter spatial resolution, which utilizes newly designed reverse-type APD-arrays, is uderway. All the detector blocks are modularized with the overall dimension of each module, including the APD array, LYSO scintillator matrix and Front-End Circuits (FECs), which are only 30\times 30\times 80\ mm^3. Each APD device also has a monolithic 16 \times 16 pixel structure with an active area of 1.0 mm^2 per pixel. The FEC includes two identical analog ASICs specifically designed for APDs with a noise characteristic of 560+30\ e^-/pF and a timing resolution of 460 ps (rms), respectively. An energy resolution of 13.7 \pm 1.1% (FWHM) with 662 keV gamma-rays was measured using the 16 \times 16 arrays. At this stage a pair of module and coincidence circuits has been assembled into an experimental prototype gantry. Spatial resolutions of 0.9, 1.4, and 1.3 mm (FWHM) were obtained from FBP reconstructed images in preliminary experiments with a point source positioned centrally, and 1 and 5 mm off-center, respectively. Comparison with a Monte-Carlo simulation of a fully-designed gantry over a wider range of field-of-view showed good correlation with the experimental data. A simple but conceptual design of a DOI configuration is also proposed as a test example of a future APD-PET scanner. [ABSTRACT FROM PUBLISHER]