A staggered three-layer DOI PET detector with a 1 mm crystal pitch for a high resolution small animal PET imaging

Objectives: Development of a staggered three-layer DOI PET detector with a 1 mm crystal pitch for realizing a 0.7 mm resolution small animal PET imager of 52 mm ring diameter.Methods: The proposed small animal DOI PET detector consists of three-layer staggered LYSO crystal arrays, an acrylic light guide with a thickness of 1 mm, and 4×4 SiPM (Hamamatsu, S13360-3050NE-04, Japan) array with a pixel pitch of 3.2 mm. The LYSO crystal had a dimension of 0.9 × 0.9 × 5.0 mm3 with a pitch of 1 mm. The 1st (10 × 9), the 2nd (10 × 10), and the 3rd (11 × 11) LYSO layers were stacked with a staggered configuration to provide DOI information in the 2D crystal map. In order to investigate the optimal reflector and light guide thickness, the performance of the PET detector module was evaluated with different reflectors (ESR and BaSO4), and different light guide thicknesses (0.5, 1.0, and 2.0 mm). A 22Na source was used to irradiate 511 keV photon to the detector module. The sixteen SiPM output signals were multiplexed into four position signals by using the Anger logic circuit, and a trigger signal was generated by using the common cathode signals. The SiPM signals were digitized by CAMAC DAQ.Results: The BaSO4 reflector resulted in better crystal identification for the three-layer staggered LYSO crystal arrays as compared to the ESR. However, the energy resolution with BaSO4 (38.8%) was worse than that of the ESR (20.3%). The acrylic light guide thickness of 1 mm resulted in the best crystal identification for both BaSO4 and ESR reflector cases. Conclusions: A staggered three-layer DOI PET detector with a 1 mm crystal pitch was developed. All the LYSO crystals could be identified clearly on the 2D flood histogram by using the BaSO4 reflector. In future, a prototype high resolution small animal DOI PET scanner will be developed by using the proposed three-layer staggered DOI PET detector module.2019 Annual Meeting of Society of Nuclear Medicine and Moldecular Imagin

Radiation hardness of organic photodiode detectors for carbon beam irradiation

Recently, many groups have been developing organic devices such as photodiodes, electroluminescence cells, photovoltaic cells, and transistors as various detection devices. These organic devices are thin, flexible, printable and inexpensive to manufacture. If they can be used in radiation measurements, it is expected that many innovative radiation detectors can be fabricated because of these desirable characteristics. Especially interesting are organic photodiodes (OPDs) that work like conventional Si semiconductor photodiodes. Based on these points, the OPDs are suitable for application as carbon therapy dosimeters. Therefore, we are investigating the OPDs as radiation detectors for heavy ion beam irradiation. In this presentation, we evaluate the radiation hardness of the OPD detectors to the carbon beam irradiation and report our results.\nThe OPD detector structure consisted of layers of IZO/ PEDOT: PSS / PCBM: P3HT / Al. The OPDs were fabricated on 10 mm × 10 mm black ABS boards by spin coating; fabrication was done 3 days before an experiment. Various sizes of the OPD were used in the experiment. We performed the experiment in the PH2 course of the HIMAC at NIRS. The energy of the 12C beam was 290 MeV/u which is typically used in actual carbon therapy. The OPDs were irradiated by the 12C beam that passed through an ion chamber to monitor the number of irradiated particles. The beam intensity was 1.8×109 particles per second. The diameter of the 12C beam was 1 cm. The induced charges of the OPDs by the carbon beam irradiation and dark current components were measured in the experiment. Experimental results showed that the performance degradations of the OPDs by carbon beam irradiation were negligible for irradiation times in this experiment. Finally, we concluded that the radiation hardness of the OPD detector is sufficient for application as dosimeters for carbon beams.IEEE NSS-MIC201

Suitability of a 280 ps-CRT non-DOI detector for the helmet-neck PET

We are developing a compact and high-sensitivity brain-dedicated PET scanner, which consists of a hemispherically arranged detector unit and an add-on detector unit. Following our first idea of using a chin position as the location of the add-on detector (helmet-chin PET), a neck position has been selected as the location (helmet-neck PET). We have developed prototypes of both geometries using the 4-layered depth-ofinteraction (DOI) detector (2.8 mm sized GSOZ scintillators and a 64 ch PMT) which does not have time-of-flight (TOF) capability. On the other hand, a new TOF-PET detector module which consists of 4.1 mm sized LFS scintillators and multi pixel photon counters (MPPCs) is now commercially available (C135004075LC-12, Hamamatsu Photonics K.K.). Using one-to-one crystal-photodetector coupling, it is possible to achieve a 280 ps coincidence resolving time (CRT), although the Anger-type calculation, which is essential for the 4-layered DOI detector, cannot be applied. In this paper, therefore, we investigated the suitability of this TOF-PET detector module for the next helmetneck PET. At first, the helmet-neck PET with 20 mm crystal length, which is a standard parameter of the module, was modeled by GEANT4, and performance results were compared with those of the current helmet-neck PET prototype. We observed a compromised image quality; the non-DOI capability counteracted the TOF gain. On the other hand, the helmet-neck PET with shortened crystal length (10 mm) outperformed the current helmet-neck PET prototype; the shortened crystal length reduced the parallax error, and TOF information compensated for the loss of sensitivity. Based on the GEANT4 simulation results, we fabricated a one-pair prototype of the TOF-PET module with 10 mm length, which yielded 11.6 % energy resolution at 511 keV and 281.8\pm 9.6 ps CRT at FWHM. In conclusion, we confirmed suitability of the 280 ps-CRT non-DOI detector for the next helmet-neck PET

Development of a 4-Layer DOI TOF-PET detector module with a 6 mm-pitch MPPC array

PET detectors with both time-of-flight (TOF) and depth-of-interaction (DOI) capabilities are ideal, but DOI-TOF PET detectors have not been studied well. In this paper, we develop a 4-layer DOI TOF detector. The key item is a recently commercialized module of multi pixel photon counters (MPPCs) (C13500 series, Hamamatsu Photonics K. K., Japan). The MPPC array is the through silicon via (TSV) type with an effective area of 6 x 6 mm 3 (0.075 mm pitch sub pixels), which is arranged as an 8 x 8 array. At first, a pair of lutetium fine silicate (LFS) crystals each sized in 4 x 4 x 10 mm 3 was placed on the center of each one MPPC channel. The CRT value was 252 ps. Afterward one of those detectors was used as the reference detector. Then cerium doped lutetium yttrium orthosilicate (LYSO) crystals sized in 2.8 x 2.8 x 5 mm 3 were arranged in 8 x 8 x 4-layer based on our special reflector arrangement. The 4-layer LYSO array were coupled to the MPPC modules via a light guide. In the position map of the 4-layer LYSO crystal block, most of crystals could be identified. The averaged CRT value for crystals of the central part in the position map for the reference detector was 486 ± 106 ps. In timing spectrum of the third layer and the fourth layer, other components were observed. This was presumed to be due to contamination caused by incomplete crystal identification. Individual specificity for each MPPC channel was required to be calibrated for complete crystal identification. In conclusion, we developed a 4-layer DOI TOF detector and obtained around 500 ps averaged CRT value for the reference detector

Four-layer DOI PET detector with 1 mm crystal pitch

Depth of interaction (DOI) information can minimize the parallax error at the peripheral field of view of a positron emission tomography (PET). In the previous study, we developed a four-layer DOI PET detector with 1.5 mm crystal pitch. The aim of this study is to develop a four-layer DOI PET detector with 1 mm crystal pitch to improve the spatial resolution. The proposed four-layer DOI PET detector consists of an 8 × 8 LYSO crystal array (1 × 1 × 5 mm3) stacked in four-layer which yielded a total thickness of 20 mm, an acrylic light guide (13 × 13 × 1 mm3), and 4 × 4 SiPM array (Hamamatsu, S13361-3050NE-04, Japan) with a pixel pitch of 3.2 mm. In order to encode the DOI information into 2D flood histogram, a combination of enhanced specular reflector (ESR) and room temperature vulcanizing (RTV) rubber was used (Fig. 1). The sixteen SiPM output signals were multiplexed into four positional signals (A, B, C, and D) using charge division circuit, and the multiplexed signals were digitized using CAMAC DAQ (Fig. 2). A Na-22 point source was used to irradiate 511 keV gamma photon to the LYSO crystal array. The energy resolution was measured to be 30.8% (Fig. 3). All the LYSO crystals could be identified in 2D flood histogram except at the corner region (Fig. 4). In conclusion, a single-ended readout four-layer DOI PET detector with 1.1 mm LYSO crystal pitch was developed. In future, the corner crystal identification will be optimized.第66回応用物理学会春季学術講演