Positron emission tomography (PET) plays important roles in cancer diagnosis, neuroimaging and molecular imaging research. The National Institute of Radiological Sciences, QST (NIRS) has been focusing on PET research since 1979 when NIRS developed the first PET scanner in Japan. Now NIRS is recognized as one of the world’s leading institutions in the field of molecular imaging research.

Regarding instrumentations, potential points remain for which big improvements could be made, including spatial resolution, sensitivity and manufacturing costs. Therefore, research on next generation PET technologies remains a hot topic worldwide.

• Recognized as a Person of Collaboration Merits by the Minister of Health, Labour and Welfare, for the realization of a depth-of-interaction (DOI) detector

  

• Awarded the Science and Technology Prize by the Minister of Education, Culture, Sports, Science and Technology, for research on OpenPET

  

• First prototyping of the novel concept “whole gamma imaging”

  

Imaging Physics Group (IPG) succeeded in developing a novel 4-layered DOI detector, which is a key technology to get any significant improvement in sensitivity while maintaining high spatial resolution. IPT expanded PET application fields by making full use of DOI detectors. IPT invented the world’s first, open-type PET geometry "OpenPET", which is expected to lead to PET imaging during treatment. In addition to continuing work for on-going projects (OpenPET, helmet-type PET and add-on PET for MRI), IPT has started a new development project for a whole gamma imager (WGI), which is a new concept to combine PET and Compton imaging. Both DOI and OpenPET are long-term research projects which were started in 2000 and 2007, respectively. These awards, which celebrate the fruits of our labor, remind us to plant new seeds for the next generation research.

Over view

The first midterm plan of NIRS (2001-2005)

IPT succeeded in developing a novel 4-layered DOI detector, which is a key technology to get any significant improvement in sensitivity while maintaining high spatial resolution. This DOI detector is the base for Shimadzu’s new line of positron emission mammography products.

The second midterm plan of NIRS (2006-2010)

IPT expanded PET application fields by making full use of DOI detectors. IPT invented the world’s first, open-type PET geometry OpenPET, which is expected to lead to PET imaging during treatment. The DOI detector itself evolved through application of recently developed semiconductor photodetectors, i.e., silicon photomultipliers (SiPMs). We developed a SiPM-based DOI detector X’tal cube to achieve the theoretical limitation of PET imaging resolution.

The third midterm plan of NIRS (2011-2015)

IPT made big progress with these technologies. In the OpenPET project, which received the German Innovation Award in 2012, IPT finally developed a full-scale OpenPET prototype. In addition, the flexible detector system of the OpenPET prototype enabled realization of an innovative brain scanner; this is the helmet-chin PET, which is now being commercialized in collaboration with ATOX Co., Ltd. On the other hand, technologies developed for the X’tal cube enabled a new idea of add-on PET, which can be applied to any existing MR systems in theory.

Since April 2016

NIRS has reorganized as part of the new organization, the National Institutes for Quantum and Radiological Science and Technology (QST). In addition to continuing work for these three on-going projects (OpenPET, helmet-chin PET and add-on PET), IPT has started a new development project for a whole gamma imager (WGI), which is a new concept to combine PET and Compton imaging.

• OpenPET

  

  OpenPET is our original idea to realize the world’s first open-type 3D PET scanner for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. The principal of dose verification for particle therapy is based on the measurement of positron emitters which are produced through fragmentation reactions caused by proton or 12C ion irradiation. Even with a full-ring geometry, the OpenPET has an open gap through which the treatment beam passes. In 2016, IPT showed that the difference between a PET peak position and a dose peak position could be smaller than 1 mm with the newly established 15O beam irradiation.

  In 2017, IPT continued its international collaboration. Outstanding HIMAC experiments which made full use of the potential of the OpenPET system were done. A PET-dose conversion method not only for RI beams but also for stable (12C etc.) beams was studied in collaboration with Ludwig-Maximilians-Universität München. Validation of Monte Carlo codes was investigated in collaboration with Australian Nuclear Science and Technology Organisation (ANSTO) / University of Wollongong. These results were presented at IEEE MIC 2017 as oral presentations, which were demonstrating the high quality and impact of these works.

  Brain dedicated helmet-type PET

  

  Life extension is now causing another issue of rising numbers of dementia cases. To satisfy the potential demand for brain imaging, prototypes of brain PET scanners have been developed. However, all previous prototypes were based on a cylindrical geometry, which is not the most efficient for brain imaging. Making the detector ring as small as possible is essential in PET, because sensitivity can be increased with a limited number of detectors. Therefore, IPT developed the world’s first helmet-chin PET, in which DOI detectors are arranged to form a hemisphere, for compact, high-sensitivity, high-resolution, and low-cost brain PET imaging.

  In 2017, progressing towards a more patient-friendly system, we moved the chin detector unit to the back of the neck position. This change was based on our simulation, which showed that the effect of additional detectors depends on the number of detectors and does not depend significantly on their position. In practice, a 7% improvement in sensitivity was obtained because the safety margin, which was required for the chin-detectors, could be removed for the neck detectors.

  Add-on PET for MRI

  

  One of the major innovations made in recent years is the combined PET/MRI, but utilization of DOI detectors has not been studied well. DOI measurements are essential for PET in order to exploit the improved spatial resolution and sensitivity as well as reduced production costs. Therefore, we proposed an add-on PET, which is a RF coil combined with DOI-PET detectors.

  In order to make the PET detector ring as small as possible while placing electronic parts such as photodetectors and front-end circuits outside the RF coil, PET detector modules were placed between spokes of the birdcage RF coil. For each detector module, electronic parts were covered with a shielding box with a hole in front of the photodetectors, and scintillators were stuck out of the shielding box to allow their placement inside the birdcage coil. In theory, the proposed birdcage coil integrated with PET detectors can be applied to any existing MRI.

  Following the development of a head-sized prototype in 2016, an oval detector arrangement was investigated, in particular we focused on the RF transmission performance, for the potential extension to whole-body PET/MRI imaging.

  Whole Gamma Imaging (WGI)

  

  PET is recognized as a successful imaging method, but in order to meet emerging demands such as in-situ single-level cell tracking, we need to break through an inherent limitation in the principle of PET itself. Therefore, in collaboration with Prof. Katia Parodi at Ludwig-Maximilians-Universität München and others, with support by the NIRS International Open Laboratory program and the QST President Grant, IPT started realization of the new concept of whole gamma imaging (WGI) in 2016.

  WGI is a concept utilizing all detectable gamma rays for imaging by combining PET and Compton imaging. An additional detector ring, which is used as the scatterer, is inserted in a conventional PET ring so that single gamma rays can be detected by the Compton imaging method. As a wide range of radioisotopes can be visualized, WGI is expected to enable imaging of the targeted radioisotope therapy. In addition, triple gamma emitters such as 44Sc, that emits a pair of 511 keV photons and a 1157 keV gamma ray almost at the same time, are selected as an imaging target. In theory, localization from a single decay is possible by identifying the intersection point between a coincidence line and a Compton cone.

  In 2017, following the simulation we accomplished in 2016, we succeeded in prototyping the first WGI system and showed a proof-of-concept of WGI, by visualizing a 44Sc source generated by the Department of Radiopharmaceuticals Development at NIRS. We showed the localization accuracy of the 12.7mm FWHM which was obtained as the intersection point between a coincidence line and a Compton cone. Further improvement will be possible by optimizing the detectors.