Journal Article Development of a dual-ended readout detector with segmented crystal bars made using subsurface laser engraving technique

Mohammadi, Akram  ,  Yoshida, Eiji  ,  Nishikido, Fumihiko  ,  Nitta, Munetaka  ,  Shimizu, Kenji  ,  Sakai, Toshiaki  ,  Yamaya, Taiga

63 ( 2 ) 2017-12 , IOPscience
Depth of interaction (DOI) information is indispensable to improve sensitivity and spatial resolution of positron emission tomography (PET) systems especially for small field-of-view PET such as small animal PET and human brain PET. We have already developed a series of X’tal cube detectors for isotropic spatial resolution and we obtained the best isotropic resolution of 0.77 mm for the detectors with six-sided readout. However, it is still challenging to apply the detector for PET systems due to the high costs of six-sided readout electronics and carrying out segmentation of a monolithic cubic scintillator using the three-dimensional (3D) subsurface laser engraving (SSLE) technique. In this work, we proposed a more practical X’tal cube with the two-sided readout detector, which was made of crystal bars segmented by the one-dimensional (1D) SSLE technique. We developed two types of prototype detectors with a 3 mm cubic segment and a 1.5 mm cubic segment by using 3×3×20 mm3 and 1.5×1.5×20 mm3 crystal bars segmented into 7 and 13 DOI segments, respectively, using the 1D SSLE technique. First, performance of the detector composed of one crystal bar with different DOI segments and readout at both ends of two thorough silicon via (TSV) multi-pixel photon counters (MPPCs) were evaluated in order to demonstrate capability of the segmented crystal bars as a DOI detector. Then, performance evaluation was carried out for a 4×4 crystal array of 3×3×20 mm3 with 7 DOI segments and an 8×8 crystal array of 1.5×1.5×20 mm3 with 13 DOI segments. Each readout included a 4×4 channel of the 3×3 mm2 active area TSV MPPCs. The 3D position maps of the detectors were obtained by the Anger type calculation. All the segments in the 4×4 array were identified very clearly when there was air between the crystal bars as each crystal bar was coupled to one channel of the MPPCs; however, it was necessary to optimize optical conditions between crystal bars for the 8×8 array because of light sharing between crystal bars coupled to one channel of the MPPCs. The optimization was performed for the 8×8 array by inserting reflectors fully or partially between the crystal bars and the best crystal identification performance was obtained with the partial reflectors between the crystal bars. The mean energy resolutions at the 511 keV photo peak for the 4×4 array with air between the crystal bars and for the 8×8 array with partial reflectors between the crystal bars were 10.1% ± 0.3% and 10.8% ± 0.8%, respectively. Timing resolution of 783 ± 36 ps and 1.14 ± 0.22 ns were obtained for the detectors composed of the 4×4 array and the 8×8 array with partial reflectors, respectively. Practical X’tal cubes with 3 mm and 1.5 mm DOI resolutions and two-sided readout were developed.

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