Search

Purpose The positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system that implements the radiofrequency (RF) built-in body coil of the MRI system as a transmitter is designed to be RF-transparent, as the coil resides outside the RF-shielded PET ring. This approach reduces the design complexities (e.g., large PET ring diameter) related to implementing a transmit coil inside the PET ring. However, achieving the required field transmission into the imaging region of interest (ROI) becomes challenging because of the RF shield of the PET insert. In this study, a modularly RF-shielded PET insert is used to investigate the RF transparency considering two electrical configurations of the RF shield, namely the electrical floating and ground configurations. The purpose is to find the differences, advantages and disadvantages of these two configurations. Methods Eight copper-shielded PET detector modules (intermodular gap: 3 mm) were oriented cylindrically with an inner diameter of 234 mm. Each PET module included four-layer Lutetium-yttrium oxyorthosilicate scintillation crystal blocks and front-end readout electronics. RF-shielded twisted-pair cables were used to connect the front-end electronics with the power sources and PET data acquisition systems located outside the MRI room. In the ground configuration, both the detector and cable shields were connected to the RF ground of the MRI system. In the floating configuration, only the RF shields of the PET modules were isolated from the RF ground. Experiments were conducted using two cylindrical homogeneous phantoms in a 3 T clinical MRI system, in which the built-in body RF coil (a cylindrical volume coil of diameter 700 mm and length 540 mm) was implemented as a transceiver. Results For both PET configurations, the RF and MR imaging performances were lower than those for the MRI-only case, and the MRI system provided specific absorption ratio (SAR) values that were almost double. The RF homogeneity and field strength, and the signal-to-noise ratio (SNR) of the MR images were mostly higher for the floating PET configuration than they were for the ground PET configuration. However, for a shorter axial field-of-view (FOV) of 125 mm, both configurations offered almost the same performance with high RF homogeneities (e.g., 76 ± 10%). Moreover, for both PET configurations, 56 ± 6% larger RF pulse amplitudes were required for MR imaging purposes. The increased power is mostly absorbed in the conductive shields in the form of shielding RF eddy currents; as a result, the SAR values only in the phantoms were estimated to be close to the MRI-only values. Conclusions The floating PET configuration showed higher RF transparency under all experimental setups. For a relatively short axial FOV of 125 mm, the ground configuration also performed well which indicated that an RF-penetrable PET insert with the conventional design (e.g., the ground configuration) might also become possible. However, some design modifications (e.g., a wider intermodular gap and using the RF receiver coil inside the PET insert) should improve the RF performance to the level of the MRI-only case.

The positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system that implements the radiofrequency (RF) built-in body coil of the MRI system as a transmitter is designed to be RF-transparent, as the coil resides outside the RF-shielded PET ring. This approach reduces the design complexities (e.g., large PET ring diameter) related to implementing a transmit coil inside the PET ring. However, achieving the required field transmission into the imaging region of interest (ROI) becomes challenging because of the RF shield of the PET insert. In this study, a modularly RF-shielded PET insert is used to investigate the RF transparency considering two electrical configurations of the RF shield, namely the electrical floating and ground configurations. The purpose is to find the differences, advantages and disadvantages of these two configurations.Eight copper-shielded PET detector modules (intermodular gap: 3 mm) were oriented cylindrically with an inner diameter of 234 mm. Each PET module included four-layer Lutetium-yttrium oxyorthosilicate scintillation crystal blocks and front-end readout electronics. RF-shielded twisted-pair cables were used to connect the front-end electronics with the power sources and PET data acquisition systems located outside the MRI room. In the ground configuration, both the detector and cable shields were connected to the RF ground of the MRI system. In the floating configuration, only the RF shields of the PET modules were isolated from the RF ground. Experiments were conducted using two cylindrical homogeneous phantoms in a 3 T clinical MRI system, in which the built-in body RF coil (a cylindrical volume coil of diameter 700 mm and length 540 mm) was implemented as a transceiver.For both PET configurations, the RF and MR imaging performances were lower than those for the MRI-only case, and the MRI system provided specific absorption ratio (SAR) values that were almost double. The RF homogeneity and field strength, and the signal-to-noise ratio (SNR) of the MR images were mostly higher for the floating PET configuration than they were for the ground PET configuration. However, for a shorter axial field-of-view (FOV) of 125 mm, both configurations offered almost the same performance with high RF homogeneities (e.g., 76 ± 10%). Moreover, for both PET configurations, 56 ± 6% larger RF pulse amplitudes were required for MR imaging purposes. The increased power is mostly absorbed in the conductive shields in the form of shielding RF eddy currents; as a result, the SAR values only in the phantoms were estimated to be close to the MRI-only values.The floating PET configuration showed higher RF transparency under all experimental setups. For a relatively short axial FOV of 125 mm, the ground configuration also performed well which indicated that an RF-penetrable PET insert with the conventional design (e.g., the ground configuration) might also become possible. However, some design modifications (e.g., a wider intermodular gap and using the RF receiver coil inside the PET insert) should improve the RF performance to the level of the MRI-only case.

Marker-less head motion correction methods have been well-studied; however, no reports discussing potential issues in positional calibration between a PET system and an external sensor remain limited. In this study, we develop a method for positional calibration between the PET system and an external range sensor to achieve practical head motion correction. The basic concept of the developed method involves using the subject's face model as a marker not only for head motion detection but also for the system positional calibration. The face model of the subject, which can be obtained easily using the range sensor, can also be calculated from a computed tomography (CT) image of the same subject. The CT image, which is acquired separately for attenuation correction in PET, has the same coordinates as the PET image because of the appropriate matching algorithm between CT and PET images. The proposed method was implemented in the helmet-type PET and the motion correction accuracy was assessed quantitatively using a mannequin head. The phantom experiments demonstrated the performance of the developed motion correction method; high-resolution images with no trace of the applied motion were obtained as if no motion was provided. Statistical analysis supported the visual assessment results in terms of the spatial resolution, contrast recovery; uniformity, and the results implied that motion with correction slightly improved image quality compared with the motionless case. The tolerance of the developed method against potential tracking errors had a minimum 10% difference in the amplitude of the rotation angle.

2021年8月25日に行われた新春座談会

Pioneering investigations on the usage of positron-emission-tomography (PET) for the monitoring of ion beam therapy with light (protons, helium) and heavier (stable and radioactive neon, carbon and oxygen) ions started shortly after the first realization of planar and tomographic imaging systems, which were able to visualize the annihilation of positrons resulting from irradiation induced or implanted positron emitting nuclei. And while the first clinical experience was challenged by the utilization of instrumentation directly adapted from nuclear medicine applications, new detectors optimized for this unconventional application of PET imaging are currently entering the phase of (pre)clinical testing for more reliable monitoring of treatment delivery during irradiation. Moreover, recent advances in detector technologies and beam production open several new exciting opportunities which will not only improve the performance of PET imaging under the challenging conditions of in-beam applications in ion beam therapy, but will also likely expand its field of application. In particular, the combination of PET and Compton imaging can enable the most efficient utilization of all possible radiative emissions for both stable and radioactive ion beams, while positronium lifetime imaging may enable probing new features of the underlying tumour and normal tissue environment. Thereby, PET imaging will not only provide means for volumetric reconstruction of the delivered treatment and in-vivo verification of the beam range, but can also shed new insights for biological optimization of the treatment or treatment response assessment.

Purpose To reduce postoperative complications, intraoperative lymph node (LN) diagnosis with 18F-fluoro-2-deoxy-D-glucose (FDG) is expected to optimize the extent of LN dissection, leading to less invasive surgery. However, such a diagnostic device has not yet been realized. We proposed the concept of coincidence detection wherein a pair of scintillation crystals formed the head of the forceps. To estimate the clinical impact of this detector, we determined the cut-off value using FDG as a marker for intraoperative LN diagnosis in patients with esophageal cancer, the specifications needed for the detector, and its feasibility using numerical simulation. Methods We investigated the dataset including pathological diagnosis and radioactivity of 1073 LNs resected from 20 patients who underwent FDG-positron emission tomography followed by surgery for esophageal cancer on the same day. The specifications for the detector were determined assuming that it should measure 100 counts (less than 10% statistical error) or more within the intraoperative measurement time of 30 s. The detector sensitivity was estimated using GEANT4 simulation and the expected diagnostic ability was calculated. Results The cut-off value was 620 Bq for intraoperative LN diagnosis. The simulation study showed that the detector had a radiation detection sensitivity of 0.96%, which was better than the estimated specification needed for the detector. Among the 1035 non-metastatic LNs, 815 were below the cut-off value. Conclusion The forceps-type coincidence detector can provide sufficient sensitivity for intraoperative LN diagnosis. Approximately 80% of the prophylactic LN dissections in esophageal cancer can be avoided using this detector.

Purpose To reduce postoperative complications, intraoperative lymph node (LN) diagnosis with 18F-fluoro-2-deoxy-D-glucose (FDG) is expected to optimize the extent of LN dissection, leading to less invasive surgery. However, such a diagnostic device has not yet been realized. We proposed the concept of coincidence detection wherein a pair of scintillation crystals formed the head of the forceps. To estimate the clinical impact of this detector, we determined the cut-off value using FDG as a marker for intraoperative LN diagnosis in patients with esophageal cancer, the specifications needed for the detector, and its feasibility using numerical simulation. Methods We investigated the dataset including pathological diagnosis and radioactivity of 1073 LNs resected from 20 patients who underwent FDG-positron emission tomography followed by surgery for esophageal cancer on the same day. The specifications for the detector were determined assuming that it should measure 100 counts (less than 10% statistical error) or more within the intraoperative measurement time of 30 s. The detector sensitivity was estimated using GEANT4 simulation and the expected diagnostic ability was calculated. Results The cut-off value was 620 Bq for intraoperative LN diagnosis. The simulation study showed that the detector had a radiation detection sensitivity of 0.96%, which was better than the estimated specification needed for the detector. Among the 1035 non-metastatic LNs, 815 were below the cut-off value. Conclusion The forceps-type coincidence detector can provide sufficient sensitivity for intraoperative LN diagnosis. Approximately 80% of the prophylactic LN dissections in esophageal cancer can be avoided using this detector.

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 positron emission tomography (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 field of view (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 heavy ion medical accelerator in Chiba 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 has a capability to realize versatile PET applications by utilizing its wide-open space and mobility in addition to high spatial resolution with sufficiently good sensitivity. -9mm]Please consider rephrasing the sentence “We concluded the developed system” for clarity.

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 (

例えばMRI に後付けできるアドオンPET など，リングの一部を開放化したC 型PET によりPET の応用が広がる可能性がある．その一方で，測定データの欠損に起因した強いアーチファクトが再構成画像に発生してしまう．そこでわれわれは，開放部と対向する位置に散乱検出器を追加して，欠損情報をコンプトンカメラの原理により補うC 型コンプトンPET を提案し，開発を進めている．本研究では，C型コンプトンPET のアーチファクト低減効果をモンテカルロシミュレーションによって検討した．具体的には，半径20 cm，開放部の角度が115 度のC 型PET の内側に，散乱検出器を半径15 cm で円弧状に配置したジオメトリーを模擬した．視野中心に配置した円柱ファントムを再構成し，定量的に評価した結果，視野領域全体における円柱内部の画素値の割合が89％から95％に増加したことから，アーチファクト低減に有効であることが示された．

To improve positron emission tomography (PET) image quality, depth-of-interaction (DOI) information and time-of-flight (TOF) information are key technologies. In this work, we developed the DOI-TOF detector based on our original single-ended readout scheme with the continuous layered gadolinium fine aluminum garnet (GFAG) array. The size of each GFAG crystal is $1.45\times 1.45\times 20$ mm3. The multipixel photon counter (MPPC) used as a photo sensor has a surface area of $3.0\times 3.0$ mm2. One pair of crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Each pair is coupled with paired MPPCs of different patterns. Crystal identification is obtained from paired MPPCs of different patterns and output rates. By limiting the spread of scintillation light, the proposed DOI detector based on the local centroid calculation was expected to improve crystal response, including the edge. For performance comparison, we prepared three discrete layers of the GFAG array with the same total thickness. For the continuous layer, the DOI resolution, the energy resolution and the timing resolution of the pair of detectors are 4.7 mm, 14%, and 402 ps, respectively. Their respective values are 29%, 20%, and 33% better than those with the three discrete layers.

Background 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. Methods A mouse model with large (>1 cm) resectable pancreatic cancer of xPA-1-DC cells expressing red fluorescent protein was used. OpenPET-guided surgery was conducted 24 h after intraperitoneal administration of 64Cu-labeled cetuximab (7.4 MBq/mouse). For comparison, similar surgical procedures were conducted, and conventional tumor resection was attempted using only the naked eye (control). Survival rate after OpenPET-guided surgery was compared to that after control operations. Results Intraoperative OpenPET guidance enabled detection and resection of small residual tumors. Ten residual tumor specimens (3-10 mm in diameter) were intraoperatively isolated with OpenPET guidance (n = 7 mice). All isolated specimens showed tumor RFP signals. No resection of tumor tissue was performed in control group because the tumor could not be clearly detected with the naked eye alone. Mice after OpenPET-guided surgery showed significantly longer survival rates than those in control group. Conclusions OpenPET-guided surgery with 64Cu-labeled-cetuximab enabled intraoperative identification and resection of intrapancreatic small residual tumors. This technology could be useful to prevent tumor residuals during surgery and improve pancreatic cancer survival.

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.

多様な核医学用シンチレータについて、ごく簡単に概説した。

Positron emission tomography (PET) is useful to detect malignant tumors by imaging glucose or amino acid metabolism. Whole-body PET with the detector arrangement in a large diameter is currently used for brain imaging, however, the detector arrangement in small diameter is better for brain imaging. Stereotactic radiosurgery (SRS) treats brain tumor with maintaining cognitive function. Therefore, it needs more dedicated PET to identify small lesions and their localization. Then, we developed the world's first brain-dedicated PET with detectors arranged in a hemisphere of 28 cm diameter, attaining 2.2 mm in spatial resolution. This development was performed with ATOX CO., LTD and the brain-dedicated PET has been commercialized as VrainTM. Ten normal volunteers (22 - 45 age male) underwent 18FDG-PET using a whole-body PET (Discovery MI, GE) and the brain-PET for 10-min each, which were started 30-min and 45-min after FDG injection, respectively. A phantom with 10-22 diameter hot spheres and background (radioactivity ratio, 4:1) was also acquired by both PET systems, then estimated the radioactivity ratio in case of 2-mm diameter hot sphere. As a results, the inferior colliculus, substantia nigra, red nucleus, and brain stem raphe nucleus were identified with the brain-PET. The inferior colliculus was identified with the whole-body PET, but other nuclei were not. Based on phantom study, it was estimated that radioactivity in a 2-mm diameter sphere was measured with 1.4 - 2.9 times higher contrast than whole-body PET. The substantia nigra and the red nucleus has 5.0-6.0mm in short diameter on MRI T2WI axial image. The raphe nucleus extends laterally to 2.5mm on postmortem brain specimens. We concluded that our brain-dedicated PET can identify approximately 2.5mm diameter tumor clearly and will be particularly useful in SRS.

Objective. We aim to evaluate a method for estimating 1D physical dose deposition profiles in carbon ion therapy via analysis of dynamic PET images using a deep residual learning convolutional neural network (CNN). The method is validated using Monte Carlo simulations of 12C ion spread-out Bragg peak (SOBP) profiles, and demonstrated with an experimental PET image. Approach. A set of dose deposition and positron annihilation profiles for monoenergetic 12C ion pencil beams in PMMA are first generated using Monte Carlo simulations. From these, a set of random polyenergetic dose and positron annihilation profiles are synthesised and used to train the CNN. Performance is evaluated by generating a second set of simulated 12C ion SOBP profiles (one 116 mm SOBP profile and ten 60 mm SOBP profiles), and using the trained neural network to estimate the dose profile deposited by each beam and the position of the distal edge of the SOBP. Next, the same methods are used to evaluate the network using an experimental PET image, obtained after irradiating a PMMA phantom with a 12C ion beam at QST’s Heavy Ion Medical Accelerator in Chiba facility in Chiba, Japan. The performance of the CNN is compared to that of a recently published iterative technique using the same simulated and experimental 12C SOBP profiles. Main results. The CNN estimated the simulated dose profiles with a mean relative error (MRE) of 0.7% ± 1.0% and the distal edge position with an accuracy of 0.1 mm ± 0.2 mm, and estimate the dose delivered by the experimental 12C ion beam with a MRE of 3.7%, and the distal edge with an accuracy of 1.7 mm. Significance. The CNN was able to produce estimates of the dose distribution with comparable or improved accuracy and computational efficiency compared to the iterative method and other similar PET-based direct dose quantification techniques.

放射線の飛行時間差情報を活用して画像改善を図るTime-of-flight（TOF）PETにおいて、時間分解能（TOF分解能 ）の改善が求められている。本研究では、市販装置の最速値である214psのTOF分解能を超す ことを目的とし、シンチレータと半導体受光素子（SiPM）のパラメータ最適化を行った。具体的には、化学研磨されたfast-LGSO結晶(3.1×3.1×20 mm^3)とMPPC 受光素子を一対一に光学結合し、MPPCの印加電圧を61 V、波高弁別の閾値を5%に最適化することで184 psのTOF分解能を得た。, 第59回アイソトープ・放射線研究発表会

PET検出器の基礎原理をシンチレーターから光センサーまで含めて説明する。シンチレーターのサイズがPETの性能と画質に与える影響を説明する。 BGO, LYSO, GAGGを用いた最近のTOF-PET検出器の性能を発表する。, 未来PET Bench to Clinicalワークショップ2022

位置弁別機能を持った新しい鉗子型ミニPETについて、深層学習を用いた位置推定の原理とその機能が放射能推定値に与える影響を説明する。さらに、光学シミュレーションを用いて、ハードウェアをどのように構成・最適化していくかなどの今後の展望について説明する。, 未来PET Bench to Clinical ワークショップ 2022

[Objectives] A PET system can be more efficient in terms of size and cost when it is designed as an organ-dedicated or application-specific system. Considering the potential need for brain PET, we proposed the concept of a hemispherical detector arrangement, which enables higher sensitivity with fewer number of detectors than a conventional cylindrical geometry has. A laboratory benchtop prototype has been evolved into a complete brain PET system, which has been commercialized in Japan under the product name of VrainTM. In this work, we characterize the system and carry out its first human imaging tests. [Methods] The VrainTM has 54 detectors. A hemispherical part is formed by 45 detectors and the other 9 detectors are located at the backside of the neck. Each detector is a one-to-one combination of lutetium-based scintillators (crystal element size: 4.1×4.1×10 mm3) and silicon photomultipliers (SiPMs) (active area size: 4×4 mm2). The inner diameter of the bottom detector ring is 279 mm. The crystal thickness is optimized by simulation to reduce the parallax error and to improve the TOF performance. The energy window is also optimized to 450–590 keV to get better image quality. We evaluated the physical performance of the VrainTM according to the NEMA NU 2-2018 standards. Some measurements were added and others were modified to suit the hemispherical detector arrangement. Finally, we carried out the first healthy volunteer imaging. 18F-FDG of 3.7 MBq/kg was intravenously injected to a male subject (42 years old). The PET measurement duration was 10 min starting at 45 min after injection. PET images were reconstructed using the OSEM with 4 iterations and 8 subsets. A Gaussian filter of 4 mm FWHM was applied to PET images. CT images, which were obtained by another PET/CT scanner, were used for attenuation and scatter corrections. [Results] The 2.2 mm rods of a small rod phantom were resolved. The TOF resolution at the peak noise-equivalent count ratio (NECR) (9.8 kBq/mL) was 256 ps and the TOF gain in sensitivity was calculated as 5.2 (20 cm diameter / 3.8 cm localization accuracy). The effective sensitivity was 21.8 kcps/MBq and the effective peak NECR was 129 kcps at 9.8 kBq/mL. The sensitivity was higher at the upper position in the axial direction. For the brain-sized image quality phantom evaluation, the percent contrast of the 10 mm sphere was 48% with the percent background variability of 8.6%. For the human FDG PET imaging, the whole brain was successfully imaged without any artifacts. Gray and white matters were clearly visualized with a high contrast. [Conclusion] The helmet-type brain PET system VrainTM showed 2.2 mm spatial resolution, 256 ps TOF resolution, and excellent image quality. The brain FDG PET images of the healthy volunteer were high contrast and low noise. Further studies are warranted to assess the potential clinical impact of PET imaging with the VrainTM., SNMMI 2022 Annual Meeting

Objective. For PET imaging systems, a smaller detector ring enables less intrinsic spatial resolution loss due to the photon non-collinearity effect as well as better balance between production cost and sensitivity, and a hemispherical detector arrangement is more appropriate for brain imaging than a conventional cylindrical arrangement. Therefore, we have developed a brain-dedicated PET system with a hemispherical detector arrangement, which has been commercialized in Japan under the product name of VRAINTM. In this study, we evaluated imaging performance of VRAIN. Approach. The VRAIN used 54 detectors to form the main hemispherical unit and an additional half-ring behind the neck. Each detector was composed of a 12 × 12 array of lutetium fine silicate crystals (4.1 × 4.1 × 10 mm3) and a 12 × 12 array of silicon photomultipliers (4 × 4 mm2 active area) with the one-to-one coupling. We evaluated the physical performance of VRAIN according to the NEMA NU 2–2018 standards. Some measurements were modified so as to fit the hemispherical geometry. In addition, we performed 18F-FDG imaging in a healthy volunteer. Main results. In the phantom study, the VRAIN showed high resolution for separating 2.2 mm rods, 229 ps TOF resolution and 19% scatter fraction. With the TOF gain for a 20 cm diameter object (an assumed head diameter), the peak noise-equivalent count rate was 144 kcps at 9.8 kBq ml−1 and the sensitivity was 25 kcps MBq−1. Overall, the VRAIN provided excellent image quality in phantom and human studies. In the human FDG images, small brain nuclei and gray matter structures were clearly visualized with high contrast and low noise. Significance. We demonstrated the excellent imaging performance of VRAIN, which supported the advantages of the hemispherical detector arrangement.

肺移植後早期のFDG-PETから慢性拒絶の兆候をいち早く掴むことを課題として，肺移植モデルラットを対象にFDG-PET画像と炎症の病理診断結果を関連付ける研究を行った．今回，ImageNetで学習済みのVGG16を使用し，肺移植後３週目のFDG-PET画像から６週目の病理診断結果を予測したところ，最も精度の高かったエポックで感度96%，特異度91％と高い精度で予測することができたので報告する．さらに入力画像の特徴量を削減することで予測精度への影響を分析し，病理予測には左肺下部の情報が重要である可能性が示された．

頭部PET装置を対象とした体動補正手法の開発を行った．提案手法では，高精度なトラッキングを行うためにレンジセンサを用いた体動の測定を行う．この時必要となるセンサとPET装置のキャリブレーションにおいて，事前の測定等を必要とせず，被験者を撮像した実データのみからマーカレスでキャリブレーションが可能な手法の開発を行った．提案手法を，ファントムを用いた実験により検証し，動きを伴う場合においても，静的な状態とほぼ同等の高解像度PET画像が得られることを実証した．

To improve positron emission tomography (PET) image quality, depth-of-interaction (DOI) information and timeof-flight (TOF) information are key technologies. In this work, we developed the DOI-TOF detector based on our original singleended readout scheme with the continuous layered gadolinium fine aluminum garnet (GFAG) array. The size of each GFAG crystal is 1.45×1.45×20 mm3. The multi-pixel photon counter (MPPC) used as a photo-sensor has a surface area of 3.0×3.0 mm2. One pair of crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Each pair is coupled with paired MPPCs of different patterns. Crystal identification is obtained from paired MPPCs of different patterns and output rates. By limiting the spread of scintillation light, the proposed DOI detector based on the local centroid calculation was expected to improve crystal response including the edge. For performance comparison, we prepared three discrete layers of the GFAG array with the same total thickness. For the continuous layer, the DOI resolution, the energy resolution and the timing resolution of the pair of detectors are 4.7 mm, 14% and 402 ps, respectively. Their respective values are 29%, 20% and 33% better than those with the three discrete layers.

PETを用いた小動物イメージングは創薬や病態解明研究に活用されている。一方で、全ての研究者にPETが十分に行き届いているとは言い難い。例えば、特定エリアでの管理が必要な遺伝子操作マウスのPET撮像、粒子線照射直後のラットのPET撮像、マウスやラットの全身薬物動態の観察、微小腫瘍内の不均一薬剤集積の可視化、限られたスペースへの小動物用PET装置の導入、といった様々なアンメットニーズが存在する。このような多様な研究ニーズに応えるべく、サブミリ分解能を有する多目的用途のポータブル小型PET装置を開発した。 独自に開発した22 mm角のcrosshair light-sharing (CLS) 検出器（1.45×1.45×15 mm3のGFAGシンチレータ14×14アレイと3×3 mm2の半導体光センサSiPM 8×8アレイを接合）を使用した。相互作用を起こした深さによって光の分配パターンに差がでるように反射材が組んであり、深さ方向の検出位置（DOI）を5 mm単位で識別できる。128個の検出器を用いて体軸方向視野207 mmの8リング装置を製作し、CLS-PETと名付けた（下図a）。ガントリやデータ収集機器は台車上に配置し、1人で移動できるポータブル仕様とした（重量約100 kg）。撮像場所を選ばないよう、100V電源1つで動作する仕様とした。空間分解能評価として、22Na密封ロッドファントム（1 MBq）の30分撮像を行った。小動物全身撮像の実証として、健常ラットに18F-NaF（37 MBq）を投与し、60分後より60分撮像を行った。ファントムでは0.75 mmロッドが識別でき、サブミリ分解能を達成した（下図b）。ラットNaF PETでは、骨構造が明瞭に描出された全身画像を得た（下図c）。 サブミリ分解能を有する多目的ポータブル装置CLS-PETを開発した。PET室に縛られないポータブル装置のため様々なニーズに対応でき、新しいPET応用研究を開拓できる可能性がある。, 第16回日本分子イメージング学会総会・学術集会参加発表

[Purpose] We developed the world’s first helmet-type time-of-flight (TOF) PET, which had a hemispherical detector arrangement. The developed system is characterized as its high-performance and compactness as well as an unusual seat-type design which enables keeping the subject’s eye view wide. Here, we report the basic performance characteristics and the first FDG imaging results of healthy volunteers. [Methods] The helmet-type PET consisted of 54 detectors. The 45 detectors were arranged to form a hemisphere and the other 9 detectors were placed to form a half-ring behind the neck. The detector was composed of 12×12 lutetium fine silicate (LFS) scintillators (4×4×10 mm3) connected to a 12×12 silicon photomultiplier (SiPM) array with one-to-one coupling. The energy window was 450–590 keV and the coincidence time window was 3.6 ns. Image reconstruction was performed using 3D-OSEM with 4 iterations and 8 subsets. The performance was characterized based on the NEMA NU2 standards. The coincidence timing resolution was measured using a 22Na point source. 18F-FDG PET and MRI were performed for 11 normal control subjects. The PET data were measured for 10 minutes starting at 45 minutes after injection of 285±23 MBq. [Results] The coincidence timing resolution was 235 ps. With the sensitivity gain of 5.7 (= 20-cm-diameter object / 3.5 cm TOF localization), the effective sensitivity was 24 cps/kBq and the effective noise-equivalent count ratio was 120 kcps@9 kBq/mL. The spatial resolution of the FBP image was 2.9 mm at the central position (1 cm offset) of the field-of-view. For the human FDG PET images, the gray matter was clearly visualized with a high contrast to the white matter. Small brain nuclei such as the substantia nigra, the red nucleus, and the superior colliculus were able to be identified. [Conclusion] We have successfully demonstrated first human imaging on the helmet-type PET., 第78回日本放射線技術学会総会学術大会

Objective Head motions during brain PET scan cause degradation of brain images, but head fixation or external-maker attachment become burdensome on patients. Therefore, we have developed a motion correction method that uses a 3D face-shape model generated by a range-sensing camera (Kinect) and by CT images. We have successfully corrected the PET images of a moving mannequin-head phantom containing radioactivity. Here, we conducted a volunteer study to verify the effectiveness of our method for clinical data. Methods Eight healthy men volunteers aged 22–45 years underwent a 10-min head-fixed PET scan as a standard of truth in this study, which was started 45 min after 18F-fluorodeoxyglucose (285 ± 23 MBq) injection, and followed by a 15-min head-moving PET scan with the developed Kinect based motion-tracking system. First, selecting a motion-less period of the head-moving PET scan provided a reference PET image. Second, CT images separately obtained on the same day were registered to the reference PET image, and create a 3D face-shape model, then, to which Kinect-based 3D face-shape model matched. This matching parameter was used for spatial calibration between the Kinect and the PET system. This calibration parameter and the motion-tracking of the 3D face shape by Kinect comprised our motion correction method. The head-moving PET with motion correction was compared with the head-fixed PET images visually and by standard uptake value ratios (SUVRs) in the seven volume-of-interest regions. To confirm the spatial calibration accuracy, a test–retest experiment was performed by repeating the head-moving PET with motion correction twice where the volunteer’s pose and the sensor’s position were different. Results No difference was identified visually and statistically in SUVRs between the head-moving PET images with motion correction and the head-fixed PET images. One of the small nuclei, the inferior colliculus, was identified in the head-fixed PET images and in the head-moving PET images with motion correction, but not in those without motion correction. In the test–retest experiment, the SUVRs were well correlated (determinant coefficient, r2 = 0.995). Conclusion Our motion correction method provided good accuracy for the volunteer data which suggested it is useable in clinical settings.

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.