From (0.8 mm)3 to (0.77 mm)3: Improved X'tal cube PET detector for better crystal identification

Depth-of-interaction (DOI) measurement is essential for PET to reduce the parallax error, which results in improved spatial resolution while maintaining the sensitivity. One of the ultimate goals for improvement of DOI detectors is achieving their isotopic spatial resolution. For this purpose, we have developed an isotropic 3D position-sensitive PET detector named the X'tal cube detector. The X'tal cube detector is composed of a scintillation crystal block segmented into cubes and arrays of multi pixel photon detectors (MPPCs) which cover all surfaces of the crystal block. Segment responses are made as result of the 3D Anger-type calculation and are drawn in a 3D position histogram. In our previous study, although we achieved (0.8 mm)
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isotropic spatial resolution in most of the detector, we were not able to discriminate the outermost responses and the second outermost responses. This was because of a small mismatch of the size between the MPPC array (13 × 13 mm
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) and crystal block (13.6 × 13.6 × 13.6 mm
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), a dead space, 0.5 mm width gap, existed at the edge. In this study, we improved both the crystal identification performance at the edge and got 10% higher volume resolution than that of the previous detector without any degradation. To reduce the gap between the MPPC array and the scintillator crystal block, the scintillation crystal block used was a 13.1 × 13.1 × 13.1 mm
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monolithic LYSO having 3D segmentation in a 17 × 17 × 17 array of 0.77 × 0.77 × 0.77 mm
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grids fabricated by laser processing. Each face of the crystal block was coupled to a monolithic 4 × 4 array of MPPCs of 13 × 13 mm
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. We irradiated 511 keV annihilation gamma rays from two
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Na sources to the detector and acquired the data with NIM and CAMAC. According to the profile of the response, 17 segments in a row were clearly identified. The peak-to-valley ratio was 4.4 ± 0.6 at the central.