T. Nishiyama

Demonstration of three-dimensional imaging based on handheld Compton camera

Compton cameras are potential detectors that are capable of performing measurements across a wide energy range for medical imaging applications, such as in nuclear medicine and ion beam therapy. In previous work, we developed a handheld Compton camera to identify environmental radiation hotspots. This camera consists of a 3D position-sensitive scintillator array and multi-pixel photon counter arrays. In this work, we reconstructed the 3D image of a source via list-mode maximum likelihood expectation maximization and demonstrated the imaging performance of the handheld Compton camera. Based on both the simulation and the experiments, we confirmed that multi-angle data acquisition of the imaging region significantly improved the spatial resolution of the reconstructed image in the direction vertical to the detector. The experimental spatial resolutions in the X, Y, and Z directions at the center of the imaging region were 6.81 mm ± 0.13 mm, 6.52 mm ± 0.07 mm and 6.71 mm ± 0.11 mm (FWHM), respectively. Results of multi-angle data acquisition show the potential of reconstructing 3D source images.

Performance and field tests of a handheld Compton camera using 3-D position-sensitive scintillators coupled to multi-pixel photon counter arrays

After the nuclear disaster in Fukushima, radiation decontamination has become particularly urgent. To help identify radiation hotspots and ensure effective decontamination operation, we have developed a novel Compton camera based on Ce-doped Gd3Al2Ga3O12 scintillators and multi-pixel photon counter (MPPC) arrays. Even though its sensitivity is several times better than that of other cameras being tested in Fukushima, we introduce a depth-of-interaction (DOI) method to further improve the angular resolution. For gamma rays, the DOI information, in addition to 2-D position, is obtained by measuring the pulse-height ratio of the MPPC arrays coupled to ends of the scintillator. We present the detailed performance and results of various field tests conducted in Fukushima with the prototype 2-D and DOI Compton cameras. Moreover, we demonstrate stereo measurement of gamma rays that enables measurement of not only direction but also approximate distance to radioactive hotspots.

Gamma-ray visualization module

Gamma ray cameras can easily locate radiation hotspots where decontamination is required. Among them, the Compton camera that utilizes the Compton scattering is compact and lightweight because no radiation shielding is required. We have developed a Compton camera for quick visualization of the radioactive contamination. It features high detection efficiency by utilizing gamma ray detectors which is a combination of Multi-Pixel Photon Counter (MPPC) array and Gadolinium Aluminum Gallium Garnet (GAGG) scintillator arrays.

Development of prototype PET scanner using dual-sided readout DOI-PET modules

In our previous work, we proposed a novel design for a gamma-ray detector module capable of measuring the depth of interaction (DOI). In this paper, we further developed DOI-PET detector modules and a data acquisition system, and evaluated its performance. Each detector module was composed of a 3-D scintillator array and two large-area monolithic Multi-Pixel Photon Counter (MPPC) arrays coupled to both ends of the 3-D scintillator array, leading to only 8-ch signal outputs from a module. The 3-D scintillator array was composed of 9 × 9 × 7 matrices of 1.0 × 1.0 × 3.0 mm3 Ce:GAGG crystals. The detector module showed good energy resolution of 10.6% as measured at 511 keV and a high average peak to valley ratio higher than 8 for each pixel crystal identified in the X-Y-Z directions. Moreover, we evaluated the spatial resolution of a virtual 18-ch PET gantry simulated by using two detector modules that were flexibly controlled using both the X-stage and θ-stage in 20-degree steps. By measuring a 22Na point source (0.25 mm in diameter), we showed that spatial resolution substantially degrades from 1.0 mm to 7.8 mm (FWHM; as measured at 0 mm and 28 mm off-center) with a non-DOI configuration, whereas the corresponding values for the DOI configuration were 0.9 mm and 1.5 mm, respectively (FWHM; as measured at 0 mm and 28 mm off-center). This preliminary study confirms that our DOI-PET module is useful for future high spatial resolution and compact small-animal PET scanners without radial image distortions at the peripheral regions of the field of view (FOV).

Current status and optimization of handy compton camera using 3D position-sensitive scintillators

After the Japanese nuclear disaster in 2011, a large amount of radioactive isotopes was released and still remains a serious problem in Japan. To help identify radiation hotspots and ensure effective decontamination operation, we are developing a novel Compton camera weighing only 1.9 kg and measuring just 14×14×15 cm3 in size. Despite its compactness, the camera realizes a wide 180° field of vision, Δθ ~ 10°(FWHM) angular resolution, and offers excellent sensitivity that can image a hotspot producing a 5 μSv/h dose at a distance of three meters, every 10 sec. Our key technology using 3D position-sensitive scintillators coupled with thin monolithic MPPC arrays has made this innovation possible for the first time. In this paper, we present the detailed optimization and simulation of the Compton camera currently under production with Hamamatsu Photonics.

Development of high-precision color gamma-ray image sensor based on TSV-MPPC and diced scintillator arrays

We developed a high-precision color gamma-ray image sensor with fine spatial resolution that is cost-effective, widely applicable, and very sensitive by using a diced cerium-doped Gd3Al2 Ga3O12 (Ce:GAGG) scintillator array coupled with a 3.0 × 3.0 mm2/pixel 8 × 8 MPPC-array. The proposed image sensor can measure the energy of individual X-ray photons transmitted through an object. The pixel size of the Ce:GAGG scintillator array is 0.2 mm, and the pixels are separated by 50-μm-wide micro-grooves. The image sensor has a size of 20 × 20 mm2 and thickness of 1.0 mm, and it achieved an excellent spatial resolution of 0.3-0.4 mm and energy resolutions of 12% and 18% (FWHM) for 122 and 59.5 keV gamma-rays, respectively. We conducted an experiment to determine the local effective atomic number of metals by using dual-energy gamma-ray sources. In addition, we developed a color-composite image using mixed images taken at three energies (31, 59.5, and 88 keV).