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Purpose To quantify the bias of shear wave speed (SWS) measurements in a viscoelastic phantom across six different ultrasound (US) systems and to compare the SWS with those from transient elastography (TE) and magnetic resonance elastography (MRE). Methods A viscoelastic phantom of stiffness representing fibrotic liver or healthy thyroid was measured with nine (linear probe) and 10 (convex probe) modes of six different US-based shear wave elastography (SWE) systems using linear and convex probes. SWS measurements of three regions of interest were repeated thrice at two focal depths, coupling the probe to the phantom using a jig. An MRE system using three motion-encoding gradient frequencies of 60, 90, and 120 Hz and TE were also used to measure the stiffness of the phantom. Results The SWS from different SWE systems had mean coefficients of variation of 9.0–9.2% and 5.4–5.6% with linear and convex probes, respectively, in viscoelastic phantom measurement. The focal depth was a less significant source of SWS variability than the system. The total average SWS obtained with US-SWE systems was 19.9% higher than that obtained with MRE at 60 Hz, which is commonly used in clinical practice, and 31.5% higher than that obtained with TE using the M probe. Conclusions Despite the measurement biases associated with the SWE systems, biases were not necessarily consistent, and they changed with the probes used and depth measured. The SWS of the viscoelastic phantom obtained using different modalities increased according to the shear wave frequency used.

Purpose To quantify the bias of shear wave speed (SWS) measurements in a viscoelastic phantom across six different ultrasound (US) systems and to compare the SWS with those from transient elastography (TE) and magnetic resonance elastography (MRE). Methods A viscoelastic phantom of stiffness representing fibrotic liver or healthy thyroid was measured with nine (linear probe) and 10 (convex probe) modes of six different US-based shear wave elastography (SWE) systems using linear and convex probes. SWS measurements of three regions of interest were repeated thrice at two focal depths, coupling the probe to the phantom using a jig. An MRE system using three motion-encoding gradient frequencies of 60, 90, and 120 Hz and TE were also used to measure the stiffness of the phantom. Results The SWS from different SWE systems had mean coefficients of variation of 9.0–9.2% and 5.4–5.6% with linear and convex probes, respectively, in viscoelastic phantom measurement. The focal depth was a less significant source of SWS variability than the system. The total average SWS obtained with US-SWE systems was 19.9% higher than that obtained with MRE at 60 Hz, which is commonly used in clinical practice, and 31.5% higher than that obtained with TE using the M probe. Conclusions Despite the measurement biases associated with the SWE systems, biases were not necessarily consistent, and they changed with the probes used and depth measured. The SWS of the viscoelastic phantom obtained using different modalities increased according to the shear wave frequency used.

我々が開発を進めているWGI(Whole Gamma Imaging)は、散乱検出器リングを吸収検出器リングへ挿入することでPETとコンプトンイメージングを融合した構造となっており、様々なガンマ線の利用を目指している。また、これまでに開発したWGI試作機は、コンプトンイメージング装置としては世界初のフルリング型ジオメトリを実現したものであった。ここで、コンプトン画像再構成の条件に着目すると、理論的にはフルリング型である必要性はなく、検出器を削減できれば設計の柔軟性向上や、コスト削減が期待できる。しかしながら、実際には散乱角度の検出限界やブロック型検出器の配置方法などの影響で、再構成条件は保証されていない。そこで、本研究では、WGI試作機の89Zr円柱ファントム測定データに対し、後処理によって検出器を制限し、909keVのコンプトン画像のアーチファクトを評価することで3次元画像再構成に必要なジオメトリ条件の検討を行った。その結果、散乱検出器、吸収検出器共に、測定対象上のすべての領域を180°以上囲うことが必要であると示唆された。, 第61回日本核医学会学術総会

We developed a whole gamma imaging (WGI) prototype composed of a scatterer ring insert and an absorber ring. It was the world’s first realization of a full-ring Compton camera geometry. In this study, we focused on the data sufficiency condition (DSC) for Compton image reconstruction. Contrary to PET which requires a full-ring geometry to satisfy the DSC, Compton imaging does not require a full ring theoretically. Reducing the detector-ring coverages can improve flexibility of the geometry design and reduce manufacturing costs although sensitivity is decreased. However, the DSC for reconstruction is not guaranteed in a practical situation due to actual absorber ring arrangement and detection limits in scattering angle. In this study, we experimentally assessed the DSC for the angular coverages. We evaluated image artifacts on reconstructed Compton images because, in general, images distort and exhibit artifacts when the DSC is not satisfied, degrading quantitative accuracy. We measured a cylindrical phantom filled with 89Zr solution of 10.2 MBq by the WGI prototype for 60 min and extracted 909-keV Compton events. We selected events related to the detector blocks marked as available to change detector angular coverages defined for the phantom bottom point. First, we changed the number of detectors in the scatterer ring, keeping the full-ring absorber. Next, we changed the absorber ring angular coverage by changing the number of detectors, keeping the scatterer ring at the number where the angular coverage just exceeded 180°. As a result, when the angular coverage of the scatterer detector was less than 180°, reconstructed images exhibited artifacts. The absorber ring detector with less than 180° coverage showed artifacts when used with the scatterer detector with more than 180° coverage. We concluded that angular coverages of more than 180° for both the scatterer and absorber detectors against any region on a subject are the DSC for Compton image reconstruction., IEEE NSS MIC 2021

昨年の発表では，我々が開発を進めているWGI(Whole Gamma Imaging) の小動物実証機におけるコンプトンイメージングによって，高精細なマウス画像が得られたことを報告した．高精細な画像が得られた要因の一つとして，WGI がフルリング型であることが考えられるが，実際にどの程度寄与しているのか明らかにはなっていない．そこで，本研究では，画像再構成に利用する測定データを限定し，フルリングの場合と比較することで，物体を囲むリングの範囲の変化が再構成画像へ与える影響を検討した．円筒型ファントムの測定データを用い，再構成画像を視覚的に評価した結果，散乱検出器に囲まれる範囲が180°未満の領域が物体にある場合，画像にアーチファクトが生じたが，物体全域が180°以上囲まれている場合には，フルリングと同様にアーチファクトのない画像が得られた．よって，WGI コンプトン画像再構成の完全性条件として，対象視野全域を180°以上囲む必要があることが示唆された．, 第40回日本医用画像工学会大会

This study was conducted in order to assess the intra- and interoperator reproducibility of shear-wave speed (SWS) measurement on elasticity phantoms and healthy volunteers using ultrasound-based point shear-wave elastography.

Purpose This study was conducted in order to assess the intra- and interoperator reproducibility of shear-wave speed (SWS) measurement on elasticity phantoms and healthy volunteers using ultrasound-based point shear-wave elastography. Materials and methods This study was approved by the institutional review board. Two operators measured the SWS of five elasticity phantoms and seven organs (thyroid, lymph node, muscle, spleen, kidney, pancreas, and liver) of 30 healthy volunteers with 1.0–4.5 MHz convex (4C1) and 4.0–9.0 MHz linear (9L4) transducers. The phantom measurements were repeated ten times, while the volunteer measurements were performed five times each. Intra- and interoperator reproducibility was assessed. Interoperator reproducibility was also evaluated with the 95% Bland–Altman limits of agreement (LOA). Results In phantoms, all intraclass correlation coefficients (ICCs) were above 0.90 and the 95% LOA between the two operators were less than ± 18%. In volunteers, intraoperator ICCs were > 0.75 for all regions except the pancreas. Interoperator ICC was above 0.75 for the right lobe of the liver (depth 4 cm) and the kidney, but the 95% LOA was less than ± 25% only for the liver. Conclusion Although excellent in phantoms, interoperator reproducibility was insufficient for all regions in the volunteers other than the right hepatic lobe at a depth of 4 cm. Clinicians should be aware of the 95% LOA when using SWS in patients. Key Points • Our phantom study indicated a high reproducibility for shear-wave speed (SWS) measurements with point shear-wave elastography (pSWE). • In volunteers, intraoperator reproducibility was generally high, but the interoperator reproducibility was not high enough except for the right hepatic lobe at 4 cm depth. • To evaluate interoperator reproducibility, the 95% limits of agreement (LOA) between operators should be considered in addition to the intraclass correlation coefficient (ICC). Electronic supplementary material The online version of this article (10.1007/s00330-019-06195-8) contains supplementary material, which is available to authorized users.

[Purpose] Compton imaging has been expected to be realized as collimator-less SPECT, but it has not yet been put to practical use in the field of nuclear medicine. Last year, in contrast, we had succeeded in obtaining 909-keV Compton images with a quality approaching that of PET in a mouse experiment using the whole gamma imaging (WGI) prototype. In this study, therefore, we aimed at experimentally identify the factors behind this success. [Method] The WGI prototype was composed of an absorber ring with 4-layer DOI detectors of 2.8×2.8×7.5 mm3 GSO crystals and a scatterer ring with non-DOI detectors of 0.9×0.9×6.0 mm3 GAGG crystals. We measured a cylindrical phantom with a 38 mm diameter filled with a 89Zr solution of 10.2 MBq by the WGI prototype for 60 min. We also measured a mouse placed at a close position to the upper side of the scatterer ring for 60 min, 22 hours after injection of 9.8 MBq 89Zr oxalate. To compare with the full-ring geometry, we simulated partial scatterer ring geometries with various angular coverage by limiting the number of the scatterer detectors used in the ring direction. Image artifacts in reconstructed images were visually evaluated. [Results] Strong streaking artifacts appeared in the direction from the scatterer detector toward the subjects when only one detector was used in the scatterer ring. The more the scatterer detectors covered, the better the streaking artifacts mitigated. We found that, when the cylindrical phantom was covered by more than 180° with the scatterer detectors, the streaking artifact was suppressed, and the shape of the cylinder was reproduced equivalently to the full-ring geometry. In the mouse imaging case, we also found that the streaking artifacts were suppressed when the subjects were covered more than 180° by the scatterer detectors. [Conclusion] We experimentally showed that it is necessary to cover the field-of-view with more than 180° with the scatterer detector to achieve accurate 3D Compton images., 第121回日本医学物理学会学術大会

C型部分PETリングの内側に散乱検出器を配置し、欠損情報をコンプトンイメージングの原理により補完することで、様々な応用が可能な開放部を持ちつつ、部分リングアーチファクトの低減が可能な装置を提案し、モンテカルロシミュレーションによって有効性を検討した。その結果、提案装置によりアーチファクト低減が可能であり、また、散乱検出器によって視野を広く囲う方がより効果が高いことが示された。, 応用物理学会春季学術講演会

我々は，PETとコンプトンイメージングの同時測定を可能とするWGIの開発を進めている．そして，2種類の異なる測定イベントを組み合わせることが可能なハイブリッド画像再構成手法を開発した．本手法を応用し，PETでは測定できないシングルガンマ線を組み合わせることによる高画質化，及び，開放部を設けた部分リング型装置を構成した際にPETのみでは避けられないアーチファクトの低減が可能であることを示した．, 医用画像研究会（MI） JAMITフロンティア2021

We evaluated the long-term stability of a newly developed viscoelastic phantom made of polyacrylamide (PAAm) gel for magnetic resonance elastography (MRE) and ultrasound-based shear-wave elastography (US SWE). The stiffness of the cylindrical phantom was measured at 0, 13 and 18 months. Storage and loss moduli were measured with MRE, and shear-wave speed (SWS) was measured with US SWE. Long-term stability was evaluated in accordance with the Quantitative Imaging Biomarker Alliance (QIBA) profiles for each modality. The initial storage and loss moduli of the phantom were 5.01±0.22 and 1.11±0.15 respectively, and SWS was 2.57±0.04 m/s. The weight of the phantom decreased by 0.6% over the 18 months. When measured with MRE, the stiffness of the phantom decreased and changes to the storage and loss moduli were -3.0% and -4.6% between 0 and 13 months, and -4.3% and 0.0% between 0 and 18 months. The US measurements found that SWS decreased by 2.4% over the first 13 months and 3.6% at 18 months. These changes were smaller than the tolerances specified in the QIBA profiles, so the viscoelastic PAAm gel phantom fulfilled the condition for long-term stability. This new phantom has the potential to be used as a quality assurance and quality control phantom for MRE and US SWE.

Elastography is a non-invasive technique for quantitatively measuring tissue viscoelasticity. To calibrate the elastography system or to evaluate bias and variance between several different elastography systems, a standardized viscoelastic phantom is needed. We have developed a viscoelastic dual-use phantom for ultrasound elastography (USE) and magnetic resonance elastography (MRE) that satisfies the QIBA acoustics specification.

Magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE) are imaging techniques to measure stiffness of the soft tissue using magnetic resonance imaging (MRI) and ultrasound images, respectively. The purpose of this study was to explore the feasibility of the MRE measurement to evaluate the change in supraspinatus (SSP) muscle stiffness before and after rotator cuff tear, and to compare the result with those of SWE. Six swine shoulders were used. The skin and subcutaneous fat were removed, and the stiffness value of the SSP muscle was measured by MRE and SWE. The MRE measurement was performed with 0.3 T open MRI and the vibration from a pneumatic driver system with active driver to a passive driver to create the shear wave in the tissue. The passive driver was placed on the center of the SSP muscle. The stiffness was estimated from the wave images using local frequency estimation methods. In the SWE measurement, the probe of the ultrasound was placed on the center of the SSP muscle. The shear wave propagation speed was measured at a depth of 1 cm from the surface, and the stiffness was calculated. After those measurements, the rotator cuff tendon was detached from the greater tuberosity, and MRE and SWE measurements were then performed in the same manner again. The differences in the stiffness values were compared between before and after the rotator cuff tendon tear on both the MRE and SWE measurements. The results indicated that stiffness values on MRE and SWE were 9.3 ± 1.8 and 10.0 ± 1.2 kPa respectively before the rotator cuff tear, and 7.3 ± 1.3 and 8.0 ± 0.8 kPa respectively after the tendon detachment. Stiffness values were significantly lower after the tendon detachment on both the MRE and SWE measurements (p 0.05). Our results demonstrated that stiffness values of the SSP muscle on MRE and SWE were lower after rotator cuff detachment. From this result, MRE may be a feasible method for quantification of the change in rotator cuff muscle stiffness.

We have been working on the development of a PET insert for existing magnetic resonance imaging (MRI) systems for simultaneous PET/MR imaging, which integrates radiofrequency (RF)-shielded PET detector modules with an RF head coil. In order to avoid interferences between the PET detector circuits and the different MRI-generated electromagnetic fields, PET detector circuits were installed inside eight Cu-shielded fiber-reinforced plastic boxes, and these eight shielded PET modules were integrated in between the eight elements of a 270-mm-diameter and 280-mm-axial-length cylindrical birdcage RF coil, which was designed to be used with a 3-T clinical MRI system. The diameter of the PET scintillators with a 12-mm axial field-of-view became 255mm, which was very close to the imaging region. In this study, we have investigated the effects of this PET/RF-coil integrated system on the performance of MRI, which include the evaluation of static field (Bo) inhomogeneity, RF field (B1) distribution, local specific absorption rate (SAR) distribution, average SAR, and signal-to-noise ratio (SNR). For the central 170-mm-diameter and 80-mm-axial-length of a homogenous cylindrical phantom (with the total diameter of 200mm and axial-length of 100mm), an increase of about a maximum of 3μT in the Bo inhomogeneity was found, both in the central and 40-mm off-centered transverse planes, and a 5 percentage point increase of B1 field inhomogeneity was observed in the central transverse plane (from 84% without PET to 79% with PET), while B1 homogeneity along the coronal plane was almost unchanged (77%) following the integration of PET with the RF head coil. The average SAR and maximum local SAR were increased by 1.21 and 1.62 times, respectively. However, the SNR study for both spin-echo and gradient-echo sequences showed a reduction of about 70% and 60%, respectively, because of the shielded PET modules. The overall results prove the feasibility of this integrated PET/RF-coil system for using with the existing MRI system.

We developed a full-ring “add-on PET” prototype which is brain-dedicated and consists of a RF-head coil with four-layer depth-of-interaction (DOI) PET detectors for integrated PET/MRI in order to evaluate performance of our previously proposed add-on PET system and to investigate the mutual influences between the individual PET and MRI modalities when they are integrated in simultaneous measurements. In this add-on PET prototype, the DOI detectors are mounted on the head coil and close to the patient head. As a result, higher sensitivity and higher spatial resolution can be achieved for the integrated PET/MRI, compared with conventional whole body PET/MRI systems. In addition, implementation cost can be reduced, tuning of the RF-coil can be optimized and PET and MRI images can be obtained simultaneously in exactly the same positions. Specifically, the full-ring prototype consists of eight DOI-PET detectors and a birdcage type head coil of a 3T MRI. The radius of the PET ring is 123.9 mm. The distance from the center to the RF-coil elements is 130.5 mm. The scintillator blocks consist of lutetium-yttrium oxyorthosilicate scintillators arranged in 19×6×4 layers with reflectors inserted between them. The size of each crystal element is 2.0 mm×2.0 mm ×5.0 mm. We evaluated performance of the full-ring prototype in simultaneous measurements of the integrated PET/MRI. We obtained spatial resolutions of 2.3 mm at the center of the field-of-view (FOV) and lower than 3.5 mm in the whole FOV. The energy resolution of 19.4% was obtained for 511 keV gamma-rays. In addition, we observed no degradation of PET performance caused by the MRI measurement. The signal-to-noise ratio (SNR) of the MRI image was 209.4 in simultaneous measurements with the PET. The maximum ΔB 0 and maximum difference of the secondary magnetic field due to the eddy current effect were smaller than 0.8 ppm and ±5.0 μT, respectively. We concluded that sufficient spatial resolution and detector performance of the PET were obtained for the add-on PET prototype even in the simultaneous measurements with the MRI. The influence of the PET detectors on the static magnetic field and the SNR of the MRI images were not problems, although the eddy current effect must be suppressed.

Magnetic resonance elastography (MRE) is a technique to identify the viscoelastic moduli of biological tissues by solving the inverse problem from the displacement field of viscoelastic wave propagation in a tissue measured by MRI. Because finite element analysis (FEA) of MRE evaluates not only the viscoelastic model for a tissue but also the efficiency of the inversion algorithm, we developed FEA for MRE using commercial software called ANSYS, the Zener model for displacement field of a wave inside tissue, and an inversion algorithm called the modified integral method. The profile of the simulated displacement field by FEA agrees well with the experimental data measured by MRE for gel phantoms. Similarly, the value of storage modulus (i.e., stiffness) recovered using the modified integral method with the simulation data is consistent with the value given in FEA. Furthermore, applying the suggested FEA to a human liver demonstrates the effectiveness of the present simulation scheme. Graphical abstract

This paper explores the impact of male and female living together, namely “marriage.” We applied the statistical matching method and estimate the scale merit of living together. In this method, different single households of the same statistical survey are matched referring to information of members in the existing household and create “virtual” households.

[Purpose] We have proposed a new concept of whole gamma imaging (WGI), which is a novel combination of PET and Compton imaging, and we have showed by simulation the feasibility of triple-gamma imaging targeting β+-γ emitters, which emit both a positron and a gamma ray almost at the same time. In this study, we developed a proof-of-concept prototype of the WGI and an imaging method for the triple-gamma imaging incorporating resolution modeling. [Method] We developed a WGI prototype by inserting a scatterer detector ring (20 cm in inner diameter, 20x2 (transverse x axial) detectors each consisted of a 24x24x1 array of 0.9x0.9x6-mm3 GAGG crystals and a 8x8 array of 3-mm multipixel photon counters (MPPCs)) into a PET (absorber) detector ring (66 cm in inner diameter, 40x4 detectors each consisted of a 16x16x4 array of 2.8x2.8x7.5-mm3 Zr-doped GSO crystals and 64-ch photo multiplier tubes). In the triple-gamma imaging, radioactivity source position can be determined as the intersection point between a line-of-response (positron) and a Compton cone (gamma ray) in the field of view. The half-cone angle of the Compton cone is equal to the scatter angle determined by the Klein-Nishina equation using energy information. We derived an equation necessary to determine the intersection point and developed a method to model blurring effect of the cone due to the energy resolution of the detectors. To demonstrate the effectiveness of the triple-gamma imaging, we measured a 22Na point source, which emits a positron and a 1274 keV gamma ray, placed at various positions by using the WGI prototype. [Results] We compared triple-gamma images generated by back-projection using the proposed model and using only LOR information for the same list-mode data set. The proposed method could generate clearer image. [Conclusion] We demonstrated that the triple-gamma imaging is possible with the WGI prototype and the proposed method., 第117回日本医学物理学会学術大会

Following the recovery of Buck’s original paper spreadsheets containing the data records for Land Utilization in China, there was a tremendous effort by a team of scholars and students from China and Japan to preserve the original records and digitize all data for further analysis. This chapter describes in detail the efforts undertaken in this work. The chapter details the story of the data, how it was discovered, the steps taken in preservation, and the painstaking task of converting paper spreadsheets to digital electronic spreadsheets.

We have proposed a new concept of whole gamma imaging (WGI), which is a combined Compton-PET system by inserting a scattering detector ring inside a PET detector ring. In addition to positron emitters, WGI can visualize single gamma emitters based on the Compton imaging method. Besides, for triple-gamma emitters such as 44Sc that emits a pair of 511 keV photons and a 1157 keV single gamma-ray almost at the same time, a direct imaging method would be possible; the source position will be calculated as an intersection point between a surface of the Compton cone and a line-of-response. In 2017, we succeeded in developing the world’s first WGI prototype. However, limited sensitivity for Compton imaging degraded the system sensitivity for triple-gamma emitters. Therefore, in this work, we simulated the WGI system to optimize the scatterer detector for higher sensitivity. Using GEANT4, we modeled the WGI geometry, in which a scatter ring (GAGG crystals, 20 cm inner diameter) was inserted into a PET ring (GSOZ crystals, 66 cm inner diameter). In order to improve the sensitivity of triplegamma imaging and reduce parallax error, we used 4-layer depth-of-interaction (DOI) detectors as the scatterer. Total thickness of the scatterer was increased from 6 mm to 18 mm. However, thick scatterer also increased detection efficiency of 511 keV photons in the scatterer. Therefore, we improved the triple-gamma detection method in order to use the scatterer detection events of 511leV for triple-gamma imaging. Events in the scatterer were identified as positron annihilation and Compton scattering of the 1157 keV gamma-ray based on their energy information. The simulation results showed that (1) the sensitivity of the WGI using the 18-mm thick GAGG scatterer with the new triple-gamma detection method was 2.7 times higher than that of our first prototype (using 6-mm thick GAGG), and (2) the use of 4-layer DOI kept the position resolution in the triple-gamma imaging.

This study aimed to apply magnetic resonance elastography (MRE) using micro-magnetic resonance imaging (micro-MRI) system for the measurements of viscoelastic modulus in soft matters. The rectangular specimens of 90 × 70 × 50 mm were made of agarose gel with five kinds of stiffness by changing concentrations. The specimens were oscillated with longitudinal waves transmitted by an elastic-bar from a vibration generator in a micro-MRI system. Since the viscoelastic properties depend on the excitation frequency and amplitude, the experimental conditions were selected in the range of 50-250 Hz and 0.1-0.5 mm. The viscoelastic modulus was expressed as storage shear modulus G´ and loss shear modulus G˝. As a result, G´ increased with the frequency and amplitude, and the difference of G´ between hard and soft gels was obtained. The viscoelastic modulus of agarose gels was measured using the MRE system under the excitation conditions. Furthermore, double-layer specimens composed of 0.6 and 2.0 wt% gels were examined as an application of the MRE system. The difference of wave pattern between the hard and soft parts was observed. The values of G´ in the soft parts of the double-layer specimens corresponded to the value of the single-layer specimen, but the values of G´ in the hard parts were varied.

PET is recognized as a successful method to pursue cancer diagnosis and molecular imaging. However, in order to meet emerging demands for widened imaging applications such as textbfin-situ real-time single-cell tracking, we need to break through the principle of PET itself. In this paper, therefore, we propose a new concept of whole gamma imaging (WGI), which is a novel combination of 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. Therefore, in addition to Compton imaging (single-gamma mode), missing pairs of annihilation photons in PET, at least one of which is undetected, can be used for imaging (PET mode). Further large sensitivity gain can be expected for triple gamma emitters such as $^{\mathbf {44}}$textbfSc, that emits a pair of 511 keV photons and a 1157 keV gamma ray almost at the same time (triple-gamma mode). In principle, only a single decay would be enough to localize the source position: (1) the coincidence detection of a pair of 511keV photons locates the source position along a line-of-response (LOR), and (2) the source position is identified as one of two intersection points of the LOR with a Compton cone after measuring the 1157 keV gamma ray. Using GEANT4, we simulated an “insert geometry”, in which a scatter ring (24 x 24 array of 1 x 1 x 6 mm$^{\mathbf {3}}$ GAGG crystals, 20 cm diameter and 5 cm long,) was inserted into a PET ring (16 × 16 × 4-DOI array of 2.9 × 2.9 × 7.5 mm$^{\mathbf {3}}$ GSOZ crystals, 66 cm diameter and 22 cm long). In the single-gamma mode, spatial resolution for the 511keV source obtained by 3D OSEM was 6.2 mm FWHM (center)-3.0 mm FWHM (8 cm off-center). In the triple-gamma mode, the position distribution of a $^{\mathbf {22}}$Na point source projected on a line-of-response was 7.3 mm FWHM at the 5 cm off-center position without applying any image reconstruction. From the simulation results, we were able to develop the first prototype of the WGI system.

Recently, various types of PET-MRI systems have been developed by a number of research groups. However, almost all of the PET detectors used in these PET-MRI systems have no depth-of-interaction (DOI) capability. The DOI detector can reduce the parallax error and lead to improvement of the performance. We are developing a new PET-MRI system which consists of four-layer DOI detectors positioned close to the measured object to achieve high spatial resolution and high scanner sensitivity. As a first step, we are investigating influences the PET detector and the MRI system have on each other using a prototype four-layer DOI-PET detector. This prototype detector consists of a lutetium yttrium orthosilicate crystal block and a 4 × 4 multi-pixel photon counter array. The size of each crystal element is 1.45 mm × 1.45 mm × 4.5 mm, and the crystals are arranged in 6 × 6 elements × 4 layers with reflectors. The detector and some electric components are packaged in an aluminum shielding box. Experiments were carried out with 3.0 T MRI (GE, Signa HDx) and a birdcage-type RF coil. We demonstrated that the DOI-PET detector was normally operated in simultaneous measurements with no influence of the MRI measurement. A slight influence of the PET detector on the static magnetic field of the MRI was observed near the PET detector. The signal-to-noise ratio was decreased by presence of the PET detector due to environmental noise entering the MRI room through the cables, even though the PET detector was not powered up. On the other hand, no influence of electric noise from the PET detector in the simultaneous measurement on the MRI images was observed, even though the PET detector was positioned near the RF coil.

We are developing a PET-MRI system which consists of PET detectors integrated with the head coil of the MRI in order to realize high spatial resolution and high sensitivity in simultaneous measurements. In the PET-MRI system, the PET detectors which consist of a scintillator block, photo-detectors and front-end circuits with four-layer depth-of-interaction (DOI) encoding capability are placed close to the measured object. Therefore, the proposed system can achieve high sensitivity without degradation of spatial resolution at the edge of the field-of-view due to parallax error thanks to the four-layer DOI capability. In this paper, we fabricated a prototype system which consists of a prototype four-layer DOI-PET detector, a dummy PET detector and a prototype birdcage type head coil. Then we used the prototype system to evaluate the performance of the four-layer DOI-PET detector and the reciprocal influence between the PET detectors and MRI images. The prototype DOI-PET detector consists of six monolithic multi-pixel photon counter (MPPC) arrays (S11064-050P), a readout circuit board, two scintillator blocks and a copper shielding box. Each scintillator block consists of four layers of Lu1.8Gd0.2SiO5:Ce (LGSO) scintillators and reflectors are inserted between the scintillation crystals. The dummy detector has all these components except the two scintillator blocks. The head coil is dedicated to a 3.0 T MRI (MAGNETOM Verio, Siemens) and the two detectors are mounted in gaps between head coil elements. Energy resolution and crystal identification performance of the prototype four-layer DOI-PET detector were evaluated with and without MRI measurements by the gradient echo and spin echo methods. We identified crystal elements in all four layers from a 2D flood histogram and energy resolution of 15–18% was obtained for single crystal elements in simultaneous measurements. The difference between the average energy resolutions and photo-peak positions with and without MRI measurements was lower than 0.3 percentage points and 1.7% for all layers. The results indicated that these performances were sufficient as PET detectors for the proposed PET-MRI system and there was no influence from the MRI measurements on the PET imaging in the simultaneous measurements. The signal-to-noise ratio of the MRI image and static magnetic field of the MRI were also evaluated with and without measurements of the PET detectors. The maximum decrease of the static magnetic field due to the LGSO scintillators was approximately 1 ppm. The signal-to-noise ratio decreased from 242.80 without the PET detector to 44.948 in simultaneous measurements.

Several MRI-based attenuation correction methods have been reported for PET/MRI; these methods are expected to make efficient use of high-quality anatomical MRIs and reduce the radiation dose for PET/MRI scanning. The accuracy of the attenuation map (μ-map) from an MRI depends on the accuracy of tissue segmentation and the attenuation coefficients to be assigned (μ-values). In this study, we proposed an MRI-based μ-value estimation method with a non-rotational radiation source to construct a suitable μ-map for PET/MRI. The proposed method uses an accurately segmented tissue map, the partial path length of each tissue, and detected intensities of attenuated radiation from a fixed-position (rather than a rotating) radiation source to obtain the μ-map. We estimated the partial path length from a virtual blank scan of fixed-point radiation with the same scanner geometry using the known tissue map from MRI. The μ-values of every tissue were estimated by inverting a linear relationship involving the partial path lengths and measured radioactivity intensity. Validation of the proposed method was performed by calculating a fixed- point data set based upon real a real transmission scan. The root-mean-square error between the μ-values derived from a conventional transmission scan and those obtained with our proposed method were 2.4±1.4%, 17.4±9.1% and 6.6±4.3% for brain, bone and soft tissue other than brain, respectively. Although the error estimates for bone and soft tissue are not insignificant, the method we propose is able to estimate the brain μ-value accurately and it is this factor that most strongly affects the quantitative value of PET images because of the large volumetric ratio of the brain.

We are developing a novel PET detector with 3D isotropic resolution called a crystal (X'tal) cube. The X'tal cube detector consists of a crystal block all 6 surfaces of which are covered with silicon photomultipliers (SiPMs). We have developed a prototype detector with 3D isotropic 1 mm resolution. On the other hand, when the X'tal cubes are arranged to form a PET scanner, insensitive inter-detector gaps made by the SiPM arrays should not be too wide, or, better yet, they should be removed. Reduction of the number of SiPMs will also be reflected in the production costs. Therefore, reducing the number of faces to be connected to the SiPMs has become our top priority. In this study, we evaluated the effect of reducing the number of SiPMs on the positioning accuracy through numerical simulations. Simulations were performed with the X'tal cube, which was composed of a 6 × 6 × 6 array of Lu2x Gd2(1-x)SiO5:Ce crystal elements with dimensions of (3.0 mm)(3). Each surface of the crystal block was covered with a 4 × 4 array of SiPMs, each of which had a (3.0 mm)(2) active area. For material between crystal elements, we compared two: optical glue and an air gap. The air gap showed a better crystal identification performance than did the optical glue, although a good crystal identification performance was obtained even with optical glue for the 6-face photodetection. In conclusion, the number of photodetection faces could be reduced to two when the gap material was air.

Positron emission tomography (PET) has become a popular imaging method in metabolism, neuroscience, and molecular imaging. For dedicated human brain and small animal PET scanners, high spatial resolution is needed to visualize small objects. To improve PET resolution, we are developing X'tal cube, which is our new PET detector to achieve isotropic 3D positioning detectability. It consists of a segmented 3D crystal block of which all surfaces are covered with semiconductor photo-sensors. We have shown that the X'tal cube can achieve (1 mm)3 uniform crystal identification performance. To improve further spatial resolution, we intend to develop the sub-millimeter X'tal cube. However, one problem that prevents its commercial mass production is handling the small crystal pieces, i.e., cutting out and assembling them. In this work, to evaluate to benefit of the sub-millimeter X'tal cube, we simulated dedicated brain and small animal PET scanners with X'tal cubes using a Monte Carlo simulation and calculated spatial resolutions with the X'tal cubes with from (0.5 mm)3 to (1.0 mm)3 cubic crystals. For spatial resolution evaluation, a point source emitting 511 keV photons was simulated with all physical processes involved during emission and interaction of positrons. The brain PET scanner has a detector ring of 275 mm in diameter composed of 48 detectors. The small animal PET scanner has a detector ring of 82 mm in diameter composed of 14 detectors. The simulated data were projected to the sinogram and reconstructed using the 2D filtered backprojection. For the small animal PET scanner, we showed that sub-millimeter spatial resolution is possible using the X'tal cube with cubic crystals smaller than 0.9 mm even with the physical processes. On the other hand, for the brain PET scanner, spatial resolutions increased 20% larger than these of the small animal PET scanner due to the non-collinearity effect. However, for both PET scanners, we showed that the spatial resolution significantly improved, as cubic crystals were smaller.

We are developing a novel PET/MRI system. In this system, PET detectors are closely located to the MR RF-coil. To reduce the electromagnetic interaction between the PET detectors and the RF-coil, the PET detectors are covered with conductive shield boxes. However, when the magnetic field around the shield box is changed by MRI field gradient pulses, an eddy current is generated in the shield box. The eddy current produces the secondary magnetic field (ΔB 0 ), resulting in degraded MR image quality. The interference effect may depend on the static magnetic field strength of the MRI. In our previous work, we evaluated the signal-to-noise ratio and the eddy current for several shield materials with a 3 T clinical MRI system. The results showed that carbon fiber roving had high shielding performance and suppression capacity for the eddy current this paper, we evaluated performance of the carbon fiber roving in different static magnetic field strengths of the MRI. The results showed that carbon fiber roving had high shield performance regardless of the static magnetic field strength. From the experiment using the 3 T MRI, we clarified that the ΔB 0 of carbon fiber roving was equivalent to ΔB 0 of FRP. The slew rate of the gradient magnetic field of 0.3 T MRI is lower than that of the 3 T MRI, therefore, the eddy current and ΔB 0 of the 0.3 T MRI are lower than those of the 3 T MRI. Thus, we judged that carbon fiber roving was valid as the shield material of shield boxes regardless of magnetic field strength of the MRI.

We are developing a PET/MRI system based on 4-layered depth-of-interaction (DOI) detectors integrated with a birdcage RF-coil. The PET detectors are placed close to the objective to achieve both high sensitivity and high spatial resolution even at the edge of the field of view (FOV). In a previous study, we reported the first prototype which had a single detector ring to show a proof-of-concept and lower than 1.6 mm spatial resolution was achieved. However, the axial FOV of the prototype was only 1 cm, which could not be extended due to the size of the readout circuit boards. In this paper, we finally constructed a new RF-coil for the second prototype, which is combined with 24 DOI-PET detectors. We evaluated performance in simultaneous measurements. The second prototype consisted of an RF-coil and 24 PET detector units with two PET detectors each. The detector units were shielded with carbon fiber shielding boxes. Each PET detector consisted of a 14 × 14 × 4-layer array of lutetium fine silicate (LFS) crystals (1.9 mm × 1.9 mm × 4.0 mm), an 8 × 8 multi-pixel photon counter module (MPPC, S11206-0808FC(X)) and a readout circuit board with an ASIC. The RF coil was dedicated to a 3T MRI (MAGNETOM Verio, Siemens). We conducted performance tests of the second prototype in simultaneous measurements. As a result, the influence of the MRI measurements on the PET performance was found to be negligible. In addition, secondary magnetic field due to the eddy current effect can be reduced, compared with the previous prototype system.

In positron emission tomography (PET), 3D iterative image reconstruction methods have a huge computational burden. In this paper, we developed a list-mode image reconstruction method using graphics processing units (GPUs). Efficiency of acceleration for GPU implementation largely depends on the method chosen, where a reduced number of conditional statements and a reduced memory size are required. On the other hand, accurate system models are required to improve the quality of reconstructed images. Various accurate system models for conventional CPU implementation have been proposed, but these models basically require many conditional statements and huge memory size. Therefore, we developed a new system model which matches GPU implementation better. In this model, the detector response functions, which vary depending on each line of response (LOR), are pre-computed in CPUs and modeled by sixth-order polynomial functions in order to reduce the memory size occupied in GPUs. Each element of a system matrix is obtained on-the-fly in GPUs by calculating the distance between an LOR and a voxel. Therefore the developed system model enables efficient GPU implementation of the accurate system modeling with a reduced number of conditional statements and a reduced memory size. We applied the developed method to a small OpenPET prototype, in which 4-layered depth-of-interaction (DOI) detectors were used. For image reconstruction, we used the dynamic row-action maximum likelihood algorithm (DRAMA). Compared with a conventional model for GPU implementation, in which DRFs are given as a Gaussian function of fixed width, we saw no remarkable difference for DOI data, but for non-DOI data the proposed model outperformed the conventional at the peripheral region of the field-of-view. The proposed model had almost the same calculation time as the conventional model did. For further acceleration, we tried parallel GPU implementation, and we obtained 3.8-fold acceleration by using 4 GPUs.

We have developed a new three-dimensional (3D) position sensitive radiation detector, called the X'tal cube. The X'tal cube is composed of a scintillation crystal block and a number of multi-pixel photon counters (MPPCs) which are coupled on all six sides of the block. The block is segmented into cubes and no reflector is used between the segments. Scintillation light originating in a crystal segment accordingly propagates three-dimensionally along the alignment of the crystal segments and is efficiently detected by the MPPCs. The X'tal cube could be used as the detector element of a PET system, for instance. We constructed two prototypes of the X'tal cube and evaluated their performance using gamma-ray sources. The crystal block of each prototype is composed of a 3D array of Lu2xGd2(1-x)SiO5: Ce (LGSO, x = 0.9) crystal segments. Each crystal volume is 3.0 mm x 3.0 mm x 3.0 mm. MPPCs of a 3.0 mm x 3.0 mm active area are coupled to each surface of the crystal block. In this work, we show that all crystal segments are identified by a simple Anger-type calculation performed on the MPPC signals for both prototypes. The X'tal cube provides high spatial resolution in three dimensions regardless of the incident angle of the radiation.

The OpenPET and its real-time imaging capability have great potential for real-time tumor tracking in medical procedures such as biopsy and radiation therapy. For the real-time imaging system, we intend to use the one-pass list-mode dynamic row-action maximum likelihood algorithm (DRAMA) and implement it using general-purpose computing on graphics processing units (GPGPU) techniques. However, it is difficult to make consistent reconstructions in real-time because the amount of list-mode data acquired in PET scans may be large depending on the level of radioactivity, and the reconstruction speed depends on the amount of the list-mode data. In this study, we developed a system to control the data used in the reconstruction step while retaining quantitative performance. In the proposed system, the data transfer control system limits the event counts to be used in the reconstruction step according to the reconstruction speed, and the reconstructed images are properly intensified by using the ratio of the used counts to the total counts. We implemented the system on a small OpenPET prototype system and evaluated the performance in terms of the real-time tracking ability by displaying reconstructed images in which the intensity was compensated. The intensity of the displayed images correlated properly with the original count rate and a frame rate of 2 frames per second was achieved with average delay time of 2.1 s.

We are developing a novel, general purpose isotropic-3D PET detector X'tal cube which has high spatial resolution in all three dimensions. The research challenge for this detector is implementing effective detection of scintillation photons by covering six faces of a segmented crystal block with silicon photomultipliers (SiPMs). In this paper, we developed the second prototype of the X'tal cube for a proof-of-concept. We aimed at realizing an ultimate detector with 1.0 mm(3) cubic crystals, in contrast to our previous development using 3.0 mm(3) cubic crystals. The crystal block was composed of a 16 × 16 × 16 array of lutetium gadolinium oxyorthosilicate (LGSO) crystals 0.993 × 0.993 × 0.993 mm(3) in size. The crystals were optically glued together without inserting any reflector inside and 96 multi-pixel photon counters (MPPCs, S10931-50P, i.e. six faces each with a 4 × 4 array of MPPCs), each having a sensitive area of 3.0 × 3.0 mm(2), were optically coupled to the surfaces of the crystal block. Almost all 4096 crystals were identified through Anger-type calculation due to the finely adjusted reflector sheets inserted between the crystal block and light guides. The reflector sheets, which formed a belt of 0.5 mm width, were placed to cover half of the crystals of the second rows from the edges in order to improve identification performance of the crystals near the edges. Energy resolution of 12.7% was obtained at 511 keV with almost uniform light output for all crystal segments thanks to the effective detection of the scintillation photons.

The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings. In this paper, we developed the first small prototype to show a proof-of-concept of OpenPET imaging. Two detector rings of 110 mm diameter and 42 mm axial length were placed with a gap of 42 mm. The basic imaging performance was confirmed through phantom studies; the open imaging was realized at the cost of slight loss of axial resolution and 24% loss of sensitivity. For a proof-of-concept of PET image-guided radiation therapy, we carried out the in-beam tests with (11)C radioactive beam irradiation in the heavy ion medical accelerator in Chiba to visualize in situ distribution of primary particles stopped in a phantom. We showed that PET images corresponding to dose distribution were obtained. For an initial proof-of-concept of real-time multimodal imaging, we measured a tumor-inoculated mouse with (18)F-FDG, and an optical image of the mouse body surface was taken during the PET measurement by inserting a digital camera in the ring gap. We confirmed that the tumor in the gap was clearly visualized. The result also showed the extension effect of an axial field-of-view (FOV); a large axial FOV of 126 mm was obtained with the detectors that originally covered only an 84 mm axial FOV. In conclusion, our initial imaging studies showed promising performance of the OpenPET.