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Using time-of-flight (TOF) information in image reconstruction improves image signal-to-noise ratio and quantitative accuracy in positron emission tomography (PET). A new TOF-PET detector module with a 3.2 mm pitch lutetium fine silicate (LFS) scintillator array one-to-one coupled to a 16×16 (256-ch) silicon photomultiplier (SiPM) array was made commercially available (C13500 series, Hamamatsu Photonics K.K.). In this study, as a candidate detector module for the next generation brain-dedicated PET, we changed the scintillator length of 20 mm to 10 mm according to our helmet-type PET geometry and investigated the basic performance of the PET detector module, including its energy resolution, sensitivity, coincidence response function (CRF) and coincidence timing resolution (CTR). All performance values were compared with those of the 4.2 mm pitch 144-ch detector module which was selected for our current helmet-type PET prototype. The energy resolution was 12% at 511 keV. The sensitivity was about 10% lower compared with those of the 4.2 mm pitch module. The average full width at half maximum (FWHM) of the CRFs was 1.9 mm. The CTRs had values of 235–241 ps with various energy windows after applying timing calibration. The CTR was not significantly changed (<10 ps) at the single count rate of 412 kcps or lower.

Myocardial perfusion imaging (MPI) with PET plays a vital role in the management of coronary artery disease. High sensitivity systems can contribute to maximizing the potential value of PET MPI; therefore, we have proposed two novel detector arrangements, an elliptical geometry and a D-shape geometry, that are more sensitive and more compact than a conventional large-bore cylindrical geometry. Here we investigate two items: the benefits of the proposed geometries for cardiac imaging; and the effects of scatter components on cardiac PET image quality. Using the Geant4 toolkit, we modeled four time-of-flight (TOF) PET systems: an 80-cm-diameter cylinder, a 40-cm-diameter cylinder, a compact ellipse, and a compact D-shape. Spatial resolution and sensitivity were measured using point sources. Noise equivalent count rate (NECR) and image quality were examined using an anthropomorphic digital chest phantom. The proposed geometries showed higher sensitivity and better count rate characteristics with a fewer number of detectors than the conventional large-bore cylindrical geometry. In addition, we found that the increased intensity of the scatter components was a big factor affecting the contrast in defect regions for such a compact geometry. It is important to address the issue of the increased intensity of the scatter components to develop a high-performance compact cardiac TOF PET system.

In pancreatic cancer surgery, accurate identification and resection of intrapancreatic residual tumors are quite difficult. We have developed a novel open-typed PET system (called 'OpenPET'), which enables high-resolution PET-guided surgery in real time, and demonstrated that OpenPET-guided surgery with intraperitoneally administered 64Cu-labeled anti-epidermal growth factor receptor antibody cetuximab is useful to detect and resect primary pancreatic cancer. Here, we investigated applicability of OpenPET-guided surgery for unexpected residual intrapancreatic tumors and examined its survival benefit over conventional surgery.

The goal of whole gamma imaging (WGI) is to utilize all detectable gamma rays for imaging. By inserting a scatterer ring into a PET ring, the WGI detects not only 511 keV annihilation photons of positron emitters but also single gamma rays with various energies as Compton events. The Compton events include one photon of a pair of the annihilation photons in that the other photon is not detected, as well as a single gamma ray photon emitted from a non-pure positron emitter. To realize the WGI concept, it is necessary to develop an image reconstruction method combining the two different types of events. In this study, therefore, we developed a hybrid image reconstruction algorithm which utilized PET and Compton events efficiently in each iteration based on the list-mode ordered subset expectation maximization framework. For each sub-iteration of the proposed algorithm, a weighted sum of update images individually calculated for PET and Compton events was used as an update image. To demonstrate the effectiveness, we used our WGI prototype to measure a contrast phantom filled with a 89Zr solution of 9.4 MBq having a hot to background ratio of 4:1. We extracted 511 keV PET events and 909 keV Compton events by setting different energy windows in software-based coincidence detection. Reconstructed images obtained using both PET and Compton events were compared with those using PET events only and those using Compton events only. The contrast recovery against background variability for each iteration was used as a figure of merit. The results showed the image reconstructed using both PET and Compton events had better image contrast compared with other images., IEEE NSS&MIC 2020

To make a high-sensitivity brain PET system with a limited number of detectors, we have proposed and developed a helmet-type TOF-PET prototype for which the coincidence timing resolution (CTR) was 245 ps. The energy resolution of 12.6% was obtained with the energy window of 420-590 keV. For such a compact system, the scattered coincidence counts increase due to the increased solid angle coverage. Our aim in this study was to investigate an optimal energy window of this prototype in order to obtain better count rate performance and better image quality. CTR, noise-equivalent count rate with TOF gain (TOF-NECR), and image quality were evaluated with 16 different patterns of the energy windows (range, 400-700 keV). A brain-sized image quality (IQ) phantom and a hemispherical Hoffman phantom were used for image quality evaluation. The highest TOF-NECR was obtained with the energy window of 440-590 keV, and the CTR was improved to 236.1 ps. Comparing the image quality between the previous (420-590 keV) and the optimized (440-590 keV) energy windows, the background variability of the IQ phantom image was reduced by 5.4%, and the contrast in the Hoffman phantom image was increased by 14.5%. In conclusion, we significantly enhanced imaging performance of the helmet-type TOF-PET prototype by using the optimized energy window. We found that the lower energy window setting was a key factor to get a higher TOF-NECR and better image quality in compact TOF-PET systems., 2020 IEEE Nuclear Science Symposium & Medical Imaging Conference

Compton imaging or Compton camera imaging has been studied well, but its advantages in nuclear medicine and molecular imaging have not been demonstrated yet. Therefore, the aim of this work was to compare Compton imaging with positron emission tomography (PET) by using the same imaging platform of whole gamma imaging (WGI). WGI is a concept that combines PET with Compton imaging by inserting a scatterer ring into a PET ring. This concept utilizes diverse types of gamma rays for 3D tomographic imaging. In this paper, we remodeled our previous WGI prototype for small animal imaging, and we developed an image reconstruction method based on a list-mode ordered subset expectation maximization algorithm incorporating detector response function modeling, random correction and normalization (sensitivity correction) for either PET and Compton imaging. To the best of our knowledge, this is the world's first realization of a full-ring Compton imaging system. We selected 89Zr as an imaging target because a 89Zr nuclide emits a 909 keV single-gamma ray as well as a positron, and we can directly compare Compton imaging of 909 keV photons with PET, a well-established modality. We measured a cylindrical phantom and a small rod phantom filled with 89Zr solutions of 10.3 MBq and 10.2 MBq activity, respectively, for 1 h each. The uniform radioactivity distribution of the cylindrical phantom was reconstructed with normalization in both PET and Compton imaging. Coefficients of variation for region-of-interest values were 4.2% for Compton imaging and 3.3% for PET; the difference might be explained by the difference in the detected count number. The small rod phantom experiment showed that the WGI Compton imaging had spatial resolution better than 3.0 mm at the peripheral region although the center region had lower resolution. PET resolved 2.2 mm rods clearly at any location. We measured a mouse for 1 h, 1 d after injection of 9.8 MBq 89Zr oxalate. The 89Zr assimilated in the mouse bony s

We developed a human-scale single-ring OpenPET (SROP) system, which had an open space allowing us access to the subject during measurement. The SROP system consisted of 160 4-layer depth-of-interaction detectors. The open space with the axial width of 430 mm was achieved with the ring axial width of 214 mm and the ring inner diameter of 660 mm. The detectors were axially shifted to each other so that the detector ring was aligned along a plane horizontally tilted by 45∘ against the axial direction. The system was developed as a mobile scanner to be used not only in clinical PET rooms but also in charged-particle therapy treatment rooms as well as animal experiment rooms. Almost uniform spatial resolution better than 3 mm throughout the entire FOV was realized with an iterative image reconstruction method. Peak absolute sensitivity was 3.1%, and there was a region with sensitivity better than 0.8% for a length of more than 700 mm. An in-beam imaging experiment conducted at the HIMAC showed that the system was operable even at the highest beam intensity available for heavy-ion therapy. In addition, we conducted entire-body monkey dynamic imaging utilizing the long region inside the gantry by positioning a monkey along the direction having the longest FOV tilted by 45∘ against the axial direction. We concluded the developed system realizes versatile PET applications because of its wide-open space and mobility as well as high spatial resolution with sufficiently good sensitivity.

Breast-dedicated PET systems are categorized into two types, a ring-shaped detector arrangement or a two-panel parallel detector arrangement. Although there are some advantages in the parallel arrangement, PET images are blurred due to the limited angular coverage. In this work, to compensate for the angular coverage issue while keeping the open space to access the breast, we proposed a third type, a round-edge detector arrangement. Using the Geant4 simulation, we evaluated the imaging performance of the three detector arrangements: a ring arrangement, a round-edge arrangement, and a parallel arrangement. Adding TOF and DOI measurement capabilities was also investigated. Each simulated system was composed of the same 50 detector blocks, for which the scintillator was a 30×30 array of 1.5×1.5×15 mm3 GAGG. We evaluated sensitivity, spatial resolution and image quality (contrast and noise). TOF (timing resolution: 500-ps and 300-ps) and DOI (7.5-mm-thickness × 2-layer) information were applied in the image reconstruction to investigate their impacts on image quality. As a result, we saw that the round-edge arrangement improved the spatial resolution and slightly reduced the image blurring, compared to the parallel one. In the non-ring arrangements, the 500-ps TOF slightly reduce the image blurring, while the 300-ps TOF eliminated it. Using the 2-layer DOI information, the contrasts of small spheres were increased in all arrangements. In conclusion, our proposed round-edge arrangement could improve the spatial resolution and reduce the image blurring with the help of TOF and DOI information. The 300-ps TOF performance was needed to eliminate the image blurring and the DOI measurement capability was preferable to obtain higher contrasts of small lesions., 2020 IEEE Nuclear Science Symposium & Medical Imaging Conference

An error was introduced during the production of this article. In the published paper, figures 9 and 10 display extra axes which should not be there. The following are the corrected figures.

GATE Monte Carlo optical simulation has been used widely as an excellent tool for the design and optimization of positron emission tomography (PET) detectors. Recently, our group has started to develop a small animal depth-of-interaction (DOI) PET scanner with a trapezoid geometry crystal for highresolution and high-sensitivity small animal imaging. However, modeling of a DOI PET detector with a trapezoid geometry crystal has not been studied yet because of the complex geometry. The aim of this study was to model this trapezoid geometry detector with a mixed SiPM pitch using GATE Monte Carlo optical simulation. The trapezoid geometry dual-ended readout PET detector consisted of a 13 × 11 LYSO crystal array, a 7×6 SiPM array (pixel pitch = 2.4 mm) and an 8×4 SiPM array (pixel pitch = 3.2 mm). The 20 mm long LYSO crystal had front and back pitches of 1.18 mm and 1.87 mm, respectively. The effect of the crystal surface roughness on the DOI resolution was investigated. The DOI resolution was calculated by using the signal amplitude ratio of the two SiPMs. We analyzed the GATE optical simulation output file by using a custom-written MATLAB code. In conclusion, we could identify all crystals in the 13×11 LYSO crystal array in the 2D flood map. The DOI resolution of 1.1 mm could be obtained by using a saw-cut LYSO crystal. In the future, we plan to validate the GATE optical simulation results by experimental results.

This work presents an iterative method for the estimation of the absolute dose distribution in patients undergoing carbon ion therapy, via analysis of the distribution of positron annihilations resulting from the decay of positron-emitting fragments created in the target volume. The proposed method relies on the decomposition of the total positron-annihilation distributions into profiles of the three principal positron-emitting fragment species - 11C, 10C and 15O. A library of basis functions is constructed by simulating a range of monoenergetic 12C ion irradiations of a homogeneous polymethyl methacrylate phantom and measuring the resulting one-dimensional positron-emitting fragment profiles and dose distributions. To estimate the dose delivered during an arbitrary polyenergetic irradiation, a linear combination of factors from the fragment profile library is iteratively fitted to the decomposed positron annihilation profile acquired during the irradiation, and the resulting weights combined with the corresponding monoenergetic dose profiles to estimate the total dose distribution. A total variation regularisation term is incorporated into the fitting process to suppress high-frequency noise. The method was evaluated with 14 different polyenergetic 12C dose profiles in a polymethyl methacrylate target: one which produces a flat biological dose, 10 with randomised energy weighting factors, and three with distinct dose maxima or minima within the spread-out Bragg peak region. The proposed method is able to calculate the dose profile with mean relative errors of 0.8%, 1.0% and 1.6% from the 11C, 10C, 15O fragment profiles, respectively, and estimate the position of the distal edge of the SOBP to within an average of 0.7 mm, 1.9 mm and 1.2 mm of its true location.

The purpose of this work is to develop a validated Geant4 simulation model of a whole-body prototype PET scanner constructed from the four-layer depth-of-interaction detectors developed at the National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Japan. The simulation model emulates the behaviour of the unique depth of interaction sensing capability of the scanner without needing to directly simulate optical photon transport in the scintillator and photodetector modules. The model was validated by evaluating and comparing performance metrics from the NEMA NU 2-2012 protocol on both the simulated and physical scanner, including spatial resolution, sensitivity, scatter fraction, noise equivalent count rates and image quality. The results show that the average sensitivities of the scanner in the field-of-view were 5.9 cps kBq−1 and 6.0 cps kBq−1 for experiment and simulation, respectively. The average spatial resolutions measured for point sources placed at several radial offsets were 5.2± 0.7 mm and 5.0± 0.8 mm FWHM for experiment and simulation, respectively. The peak NECR was 22.9 kcps at 7.4 kBq ml−1 for the experiment, while the NECR obtained via simulation was 23.3 kcps at the same activity. The scatter fractions were 44% and 41.3% for the experiment and simulation, respectively. Contrast recovery estimates performed in different regions of a simulated image quality phantom matched the experimental results with an average error of -8.7% and +3.4% for hot and cold lesions, respectively. The results demonstrate that the developed Geant4 model reliably reproduces the key NEMA NU 2-2012 performance metrics evaluated on the prototype PET scanner. A simplified version of the model is included as an advanced example in Geant4 version 10.5.