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We propose to adopt the subsurface laser engraving (SSLE) technique for efficient and precise fabrication of finely pitched scintillation crystal arrays. However, its application to thicker crystals is still challenging. It is difficult to focus the laser beam tightly at a point far from the crystal’s surface because of the large refractive index of the scintillator. Therefore, a higher laser energy is needed to create microcracks at deep positions in the crystal. Because this would cause excessive damage to the scintillation crystal during laser scans, the process yield of SSLE is reduced. We found that this issue could be overcome by a novel SSLE technique using both picosecond (ps) and nanosecond (ns) pulse lasers. The experimental results indicated that the total laser energy required for creating microcrack walls in a LYSO crystal can be reduced compared to that of conventional SSLE using only a ns pulse laser. The SSLE technique using both ps and ns pulse lasers would enable the fabrication of finely pitched LYSO arrays with a higher process yield. [ABSTRACT FROM PUBLISHER]

We have developed a three-dimensional (3-D) position-sensitive radiation detector named X’tal cube, which can be applied to a PET detector. The X’tal cube is composed of a scintillation crystal block and multi-pixel photon counters (MPPCs). The crystal block is segmented three-dimensionally into small cubes by optical discontinuity (3-D segmentation) and no reflector is inserted inside. Scintillation light originating in a segment then spreads three-dimensionally so that the MPPCs are set on all six surfaces of the crystal block to detect the light. Regarding the 3-D segmentation of the crystal block, we have already succeeded in getting the segmentation inside of a monolithic scintillator block by a laser processing technique instead of the general way, arranging small cubic scintillator elements into a 3-D array. We have confirmed that utilizing the laser processing technique not only eliminates the difficulty of handling the small scintillator elements but also improves detector performance. As a new trial, we considered fabrication of the crystal block by stacking the scintillator plates which were segmented two-dimensionally by the laser processing technique (2-D segmentation). Plates are also easier to handle and for the laser processing, 2-D segmentation is simpler than 3-D segmentation. In this study, we prepared the X’tal cube with the scintillator plates (Plate-XC) and evaluated its performance to confirm its technical feasibility. For the Plate-XC, we segmented 18 mm \times 18 mm \times 2.0 mm LYSO plates two-dimensionally by laser processing so as to make a 9 \times 9 array of 2.0 mm \times 2.0 mm segments. The crystal block was composed of 9 stacked LYSO plates without using coupling material but with air gaps. The Plate-XC showed sufficient crystal identification performance when 662 keV gamma-rays were irradiated. Furthermore, to understand the characteristics of the Plate-XC, we also analyzed the scintillation light distribution in the crystal block. Results indicated that light spread from outer segments was influenced by the segment boundary conditions, air gaps or laser-processed gaps, while the spread from the center segment did not seem to have such an influence. Regarding energy performance, we obtained around 10% energy resolution for the outer segments as well as for the center segment. [ABSTRACT FROM PUBLISHER]

The X'tal cube is our original PET detector, which is being developed to achieve isotropic 3D positioning detectability. The X'tal cube is based on a 3D segmented crystal block for which all surfaces are covered with photo-detectors. Previously, instead of our initial approach of gluing segmented pieces of crystals, we successfully constructed a crystal block segmented by laser processing, and we developed the X'tal cube with the laser-processed 3D square grids of 2 mm length. In this paper, we extend the laser processing to 3D square grids of 1 mm length. The volume of a 1-mm crystal segment is 1/8 of that of a 2-mm crystal segment. We also evaluate imaging resolution performance with a newly developed one-pair prototype system to simulate a ring-type scanner, while our previous reports were limited to development and evaluation of a single detector. In particular, we compare 1-mm X'tal cube detectors to 2-mm X'tal cube detectors. The one-pair prototype system consisted of two X'tal cubes, two rotating stages, and a 192-channel data acquisition system. Each X'tal cube consisted of the LYSO cubic crystal block of 18\,\times\,18\,\times\,18 mm^3 in which the 3D grids of 1 mm pitch were fabricated by internal laser processing. The 4\,\times\,4 arrays of multi pixel photon counters were optically coupled to each surface of the crystal block. The detector positions were automatically controlled to simulate a ring-type PET with a 14.6 cm diameter. Data were collected for all assumed detector positions and then a sinogram was obtained. The data were reconstructed using filtered backprojection. The average spatial resolution of the 2-mm X'tal cube was 1.9 mm full width at half maximum (FWHM) over the field-of-view (FOV). On the other hand, the average spatial resolution of the 1-mm X'tal cube was 1.3 mm FWHM over the FOV. By applying deconvolution with the assumption that the point source was a Gaussian function of 1.0 mm FWHM, we estimated the average spatial resolution of the 2-mm X'tal cube and the 1-mm X'tal cube as 1.6 mm FWHM and 0.83 mm FWHM, respectively. The average spatial resolution of the 1-mm X'tal cube was improved approximately 48% compared to the 2-mm X'tal cube. In conclusion, we confirmed the potential of the X'tal cube for uniform and high resolution imaging. [ABSTRACT FROM AUTHOR]

The X'tal cube is a depth-of-interaction (DOI)-PET detector which is aimed at obtaining isotropic resolution by effective readout of scintillation photons from six sides of the crystal block. The X'tal cube is composed of a 3D crystal block with isotropic segments. Each face of the 3D crystal block is covered with a 4×4 array of multi-pixel photon counters (MPPCs). Previously, in order to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving technique to a monolithic crystal block instead of gluing segmented small crystals. A dense arrangement of multiple micro-cracks carved by the laser beam works efficiently as a scattering wall for the scintillation photons. The X'tal cube with the laser-processed block showed excellent performance with respect to crystal identification and energy resolution. In this work, for characteristics comparison between the laser-processed block and the conventional segmented array block, we made the laser-processed block and two types of segmented array blocks, one with air gaps and the other with glued segmented small crystals. All crystal blocks had 3D grids of 2mm pitch. The 4×4 MPPC arrays were optically coupled to each surface of the crystal block. When performance was evaluated using a uniform irradiation of 511keV, we found that the X'tal cubes with the laser-processed block could easily achieve 2mm3 uniform crystal identification. Also, the average energy resolution of each 3D grid was 11.1±0.7%. On the other hand, the glued segmented array block had a pinched distribution and crystals could not be separated clearly. The segmented array block with air gaps had satisfactory crystal identification performance; however, the laser-processed block had higher crystal identification performance. Also, the energy resolution of the laser-processed block was better than for the segmented array blocks. In summary, we found the laser-processed X'tal cube had excellent crystal identification performance and high energy performance due to its effective dense arrangement of micro-cracks. [ABSTRACT FROM AUTHOR]