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New monolithic scintillation detectors for PET have been developed, where the crystals are processed using an internally focused laser processing technique called sub-surface laser engraving (SSLE). When high intensity light pulses of short duration from a laser are focused into a scintillation crystal, they induce multi-photon absorption at the focal point and result in refractive index changes or micro-cracks inside the crystal. By applying the SSLE technique to a monolithic scintillation block, fine segmentation in the crystal can be formed without inter-pixel gaps. We have fabricated 2D segmented arrays by using a Nd:YAG laser to engrave various patterns of micro-cracks inside monolithic LYSO crystal blocks. The processed crystal array, segmented to 12 × 12 with 1.67 mm pitch, has been evaluated by coupling to a position-sensitive photomultiplier tube (PS-PMT). The 2D position histograms were measured for uniform irradiation of gamma rays, and each crystal segment was clearly separated. The average energy resolution was 9.7%, similar to that of the conventional arrays, so the laser-processed LYSO crystals have kept their primary scintillation properties. We have also evaluated the laser-processed crystals by using multi-pixel photon counters (MPPCs) to investigate their possibility as a future PET detector. The results suggest that it is possible to fabricate high performance PET detectors using the SSLE technique.