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We are developing a novel detector concept for Positron Emission Tomography (PET) that will facilitate ultrahigh spatial resolution, high sensitivity and optimal light collection efficiency. Spatial resolution in PCT can be improved by using finer scintillation crystal array elements. Several groups are developing arrays with <2 mm crystal pixels for ultra-high resolution small animal PET systems. The challenge with finer crystals is extracting sufficient light to maintain a high detection signal-to-noise ratio (SNR), especially if the crystals are kept relatively long for good detection efficiency. Assuming the resulting light signals are sufficiently above noise, the finer crystals may be resolved in a flood histogram. However, the reduced detection SNR means lower count sensitivity since more pulses are now below a given threshold, and a loss in energy resolution, which can result in image contrast degradation. We propose scintillation detector concepts that use new avalanche photodiode (APD) arrays in novel readout configurations that promote very high light collection efficiency. These concepts exploit the high compactness of semiconductor photodetector arrays for tightly packed crystal readout schemes that are unavailable to PMTs. The schemes proposed have the potential to significantly improve detection SNR for ultra-high resolution PET. With a I mm wide LSO crystal coupled to a prototype APD array we obtained 13.7% energy resolution at 511 keV in a nonoptimized measurement. Flood irradiation measurements with a 1 mm thick LSO crystal sheet coupled to the array prototype indicate that ≤1 mm intrinsic spatial resolution is available to the design with high spatial linearity out to the crystal edge.