A. Kishimoto

Development of a Dual-Sided Readout DOI-PET Module Using Large-Area Monolithic MPPC-Arrays

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) cubic crystals measuring 3×3×3 mm³ in size, separated by a layer of air approximately 10 μm-thick. When the light signals output from both ends are read with the 3×3 mm² MPPCs, the position of each crystal is clearly distinguished. The same measurements were also made using Ce-doped (Lu,Y)₂(SiO₄)O (Ce:LYSO), achieving a similarly good separation. We then fabricated thin Ce:GAGG 2-D crystal arrays consisting of two types: [A] 4 × 4 matrix of 3×3×3 mm³ pixels, and [B] 10 × 10 matrix of 0.8×0.8×5 mm³ pixels, with each pixel divided by a BaSO₄ reflector 0.2 mm-thick. Then four arrays are laid on top of each other facing the DOI direction through a layer of air 10 μm-thick. We demonstrated that the 3-D position of each Ce:GAGG pixel is clearly distinguished in both the 2-D and DOI directions for type A and B when illuminated by 662 keV gamma rays. Average energy resolutions of 9.8 ± 0.8% and 11.8 ± 1.3% were obtained for types A and B, respectively. These results suggest that our proposed method is simple and offers promise in achieving both excellent spatial and energy resolutions for future medical imaging, particularly in positron emission tomography (PET).

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.

Optimization and verification of image reconstruction for a Compton camera towards application as an on-line monitor for particle therapy

Particle therapy is an advanced cancer therapy that uses a feature known as the Bragg peak, in which particle beams suddenly lose their energy near the end of their range. The Bragg peak enables particle beams to damage tumors effectively. To achieve precise therapy, the demand for accurate and quantitative imaging of the beam irradiation region or dosage during therapy has increased. The most common method of particle range verification is imaging of annihilation gamma rays by positron emission tomography. Not only 511-keV gamma rays but also prompt gamma rays are generated during therapy; therefore, the Compton camera is expected to be used as an on-line monitor for particle therapy, as it can image these gamma rays in real time. Proton therapy, one of the most common particle therapies, uses a proton beam of approximately 200 MeV, which has a range of ~ 25 cm in water. As gamma rays are emitted along the path of the proton beam, quantitative evaluation of the reconstructed images of diffuse sources becomes crucial, but it is far from being fully developed for Compton camera imaging at present. In this study, we first quantitatively evaluated reconstructed Compton camera images of uniformly distributed diffuse sources, and then confirmed that our Compton camera obtained 3 %(1 σ) and 5 %(1 σ) uniformity for line and plane sources, respectively. Based on this quantitative study, we demonstrated on-line gamma imaging during proton irradiation. Through these studies, we show that the Compton camera is suitable for future use as an on-line monitor for particle therapy.

High resolution phoswich gamma-ray imager utilizing monolithic MPPC arrays with submillimeter pixelized crystals

We report the development of a high spatial resolution tweezers-type coincidence gamma-ray camera for medical imaging. This application consists of large-area monolithic Multi-Pixel Photon Counters (MPPCs) and submillimeter pixelized scintillator matrices. The MPPC array has 4 × 4 channels with a three-side buttable, very compact package. For typical operational gain of 7.5 × 105 at + 20 °C, gain fluctuation over the entire MPPC device is only ± 5.6%, and dark count rates (as measured at the 1 p.e. level) amount to ≤ 400 kcps per channel. We selected Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) and a brand-new scintillator, Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) due to their high light yield and density. To improve the spatial resolution, these scintillators were fabricated into 15 × 15 matrices of 0.5 × 0.5 mm2 pixels. The Ce:LYSO and Ce:GAGG scintillator matrices were assembled into phosphor sandwich (phoswich) detectors, and then coupled to the MPPC array along with an acrylic light guide measuring 1 mm thick, and with summing operational amplifiers that compile the signals into four position-encoded analog outputs being used for signal readout. Spatial resolution of 1.1 mm was achieved with the coincidence imaging system using a 22Na point source. These results suggest that the gamma-ray imagers offer excellent potential for applications in high spatial medical imaging.

First demonstration of multi-color 3-D in vivo imaging using ultra-compact Compton camera

In the field of nuclear medicine, single photon emission tomography and positron emission tomography are the two most common techniques in molecular imaging, but the available radioactive tracers have been limited either by energy range or difficulties in production and delivery. Thus, the use of a Compton camera, which features gamma-ray imaging of arbitrary energies from a few hundred keV to more than MeV, is eagerly awaited along with potential new tracers which have never been used in current modalities. In this paper, we developed an ultra-compact Compton camera that weighs only 580 g. The camera consists of fine-pixelized Ce-doped Gd3Al2Ga3O12 scintillators coupled with multi-pixel photon counter arrays. We first investigated the 3-D imaging capability of our camera system for a diffuse source of a planar geometry, and then conducted small animal imaging as pre-clinical evaluation. For the first time, we successfully carried out the 3-D color imaging of a live mouse in just 2 h. By using tri-color gamma-ray fusion images, we confirmed that 131I, 85Sr, and 65Zn can be new tracers that concentrate in each target organ.

High position resolution gamma-ray imagers consisting of a monolithic MPPC array with submillimeter pixelized scintillator crystals

We report on the development of two versatile, high spatial resolution gamma-ray imagers for medical imaging. One is a compact gamma-ray camera, the other is a tweezers type coincidence imaging system. These applications consisting of a large-area monolithic Multi-Pixel Photon Counter (MPPC) and submilIimeter pixelized scintillator matrices. The MPPC array has 4 × 4 channels with a three-side buttable, very compact package. Each channel has a photosensitive area of 3 × 3 mm2 and 3600 Geiger mode avalanche photodiodes (APD). For a typical operational gain of 7.5 × 105 at + 20 degrees, gain fluctuation over the entire MPPC device is only ± 5.6%, and dark count rates (as measured at the 1 p.e. level) amount to ≤ 400 kcps per channel. We particularly selected Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) and a brand-new scintillator, Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) due to their high light yield and density. To improve the spatial resolution, these scintilla tors were fabricated to 22 × 22 or 15 × 15 matrices of 0.5 × 0.5 mm2 pixels. These scintillator matrices were coupled to the MPPC array with an acrylic light guide with 1 mm thick, and signals were read out using the charge division resistor network, which compiles signals into four position-encoded analog outputs. The spatial resolution of 1.2 mm was achieved with the compact gamma-ray camera using collimated 57Co source, and a radiography image of a bearing was successfully obtained. On the other hand, the spatial resolution of 1.1 mm was achieved with the coincidence imaging system using a 22Na source. Furthermore the experimental measurements for a PET scanner was performed, and the spatial resolution of 0.91 mm was achieved. These results suggest that the gamma-ray imagers has excellent potential for their uses as a high spatial medical imaging, and also be promising for positron emission tomography (PET).

Development of a MPPC-based DOI-PET module with submillimeter 3-D resolution

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) cubic crystals measuring 3 × 3 × 3 mm³ in size, separated by a layer of air. When the light signals output from both ends are read with the MPPCs, the position of each crystal is clearly distinguished with a spatial uncertainty of 0.48 ± 0.03 mm. For 3-D measurements, we then fabricated three different type arrays: [A] 4 × 4 × 4 matrix of 3 × 3 × 3 mm³ pixels, [B] 5 × 5 × 5 matrix of 2 × 2 × 2 mm³ pixels, and [C] 10 × 10 × 10 matrix of 1 × 1 × 1 mm³ pixels, with each pixel divided by a BaSO₄ reflector in the 2-D direction and by a layer of air in the DOI direction. We demonstrated that the 3-D position of each Ce:GAGG pixel was clearly distinguished when illuminated by 662 keV gamma rays uniformly. Average energy resolutions of 9.8 ± 0.8 %,9.8 ± 0.9 %, and 13.2 ± 1.7 % were obtained for types A, B and C, respectively. These results suggest that our proposed method is simple and offers promise in achieving 1 mm 3-D spatial resolution for future medical imaging, partic

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.