菅 幹生

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.

我々が開発したWGI(Whole Gamma Imaging)の小動物実証機は,世界初のフルリング型コンプトンイメージングシステムであり,これまでに高精細なマウス画像が得られたことを報告した.フルリングにより高感度化を達成したことが要因の一つとして考えられるが,理論的にコンプトン画像再構成の条件としては必ずしもフルリング型である必要性はなく,検出器の削減により,感度は犠牲になるが,コスト削減や設計の柔軟性向上が期待できる.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の検出器配置の影響等で,再構成条件は保証されていない.そこで,本研究では,試作装置の測定データを限定することで部分リング化し,フルリングの場合と比較することで,コンプトン画像再構成に必要なジオメトリ条件を実験的に検討した.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,散乱検出器もしくは吸収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,物体全域が散乱検出器と吸収検出器の共に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,散乱検出器,吸収検出器共に,対象視野全域を180°以上共通の角度範囲として囲む必要があることが示唆された.(著者抄録)

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.

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.

<p>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．</p>

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.

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.

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.

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.

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 triple-gamma 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.

We are developing a PET/MM 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 DOt-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 x 14 x 4-layer array of lutetium fine silicate (LFS) crystals (1.9 mm x 1.9 mm x 4.0 mm), an 8 x 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 MM (MAGNETOM Verio, Siemens). We conducted performance tests of the second prototype in simultaneous measurements. As a result, the influence of the MM 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.

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 255 mm, 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 200 mm and axial-length of 100 mm), an increase of about a maximum of 31ST 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. (C) 2017 Elsevier Inc. All rights reserved.

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 ΔB0 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.

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 in-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 Sc-44, 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(3) GAGG crystals, 20 cm diameter and 5 cm long,) was inserted into a PET ring (16 x 16 x 4-DOI array of 2.9 x 2.9 x 7.5 mm(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 Na-22 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.

Objectives To compare shear-wave speed (SWS) measured by ultrasound-based point shear-wave elastography (pSWE) and MR elastography (MRE) on phantoms with a known shear modulus, and to assess method validity and variability.Methods 5 homogeneous phantoms of different stiffnesses were made. Shear modulus was measured by a rheometer, and this value was used as the standard. 10 SWS measurements were obtained at 4 different depths with 1.0-4.5MHz convex (4C1) and 4.0-9.0MHz linear (9L4) transducers using pSWE. MRE was carried out once per phantom, and SWSs at 5 different depths were obtained. These SWSs were then compared with those from a rheometer using linear regression analyses.Results SWSs obtained with both pSWE as well as MRE had a strong correlation with those obtained by a rheometer (R-2>0.97). The relative difference in SWS between the procedures was from -25.2% to 25.6% for all phantoms, and from -8.1% to 6.9% when the softest and hardest phantoms were excluded. Depth dependency was noted in the 9L4 transducer of pSWE and MRE.Conclusions SWSs from pSWE and MRE showed a good correlation with a rheometer-determined SWS. Although based on phantom studies, SWSs obtained with these methods are not always equivalent, the measurement can be thought of as reliable and these SWSs were reasonably close to each other for the middle range of stiffness within the measurable range.

© 2015 IEEE. A new detector for PET using multi-pixel photon counters (MPPCs) was developed for MRI compatible applications. This module has an 8 × 8 MPPC array, each segment has a 3 mm × 3 mm active area, and the pitch of the array is 4.1 mm in both directions. A temperature sensor is attached to the back of the array for temperature compensation. The MPPC array is connected to the front-end circuit with a detachable flexible printed circuit cable (FPC), which provides flexibility for detector arrangement. The front-end circuit consists of preamplifiers, a register network, buffer amplifiers, a built-in high voltage (HV) unit, and an embedded microprocessor unit. The HV unit is a down-regulator and requires an external HV supply. The preamplifier also has a sum output, which can be used for timing pick-off and energy discrimination. With LYSOs, the timing performance was evaluated using flexible printed cables of two different lengths. Set in a copper shield box, energy spectra and flood images were evaluated with a 3T-MRI Very little interference was observed during simultaneous MRI measurements.

The single-ring OpenPET (SROP), for which the detector arrangement has a cylinder shape cut by two parallel planes at a slant angle to form an open space, is our original proposal for in-beam PET. In this study, we developed a small prototype of an axial-shift type SROP (AS-SROP) with a novel transformable architecture for a proof-of-concept. In the AS-SROP, detectors originally forming a cylindrical PET are axially shifted little by little. We designed the small AS-SROP prototype for 4-layer depth-of-interaction detectors arranged in a ring diameter of 250 mm. The prototype had two modes: open and closed. The open mode formed the SROP with the open space of 139 mm and the closed mode formed a conventional cylindrical PET. The detectors were simultaneously moved by a rotation handle allowing them to be transformed between the two modes. We evaluated the basic performance of the developed prototype and carried out in-beam imaging tests in the HIMAC using C-11 radioactive beam irradiation. As a result, we found the open mode enabled in-beam PET imaging at a slight cost of imaging performance; the spatial resolution and sensitivity were 2.6 mm and 5.1% for the open mode and 2.1 mm and 7.3% for the closed mode. We concluded that the AS-SROP can minimize the decrease of resolution and sensitivity, for example, by transforming into the closed mode immediately after the irradiation while maintaining the open space only for the in-beam PET measurement.

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 (Delta B-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 Delta B-0 of carbon fiber roving was equivalent to Delta B-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 Delta B0 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.

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.

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 x 4 multi-pixel photon counter array. The size of each crystal element is 1.45 mm x 1.45 mm x 4.5 mm, and the crystals are arranged in 6 x 6 elements x 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.

The aim of this study is to conduct experimental evaluation of the effect of PET module on magnetic resonance imaging (MRI) system's radiofrequency (RF) field transmission (B-1) distribution for a separate head PET/RF-coil modality developed by integrating PET modules (in cylindrical format) in between the 8-elements of a birdcage RF coil for human brain imaging in a 3T Siemens Magnetom Verio MRI system [1]. Knowledge of field will enable us for ensuring quality control of RF coil, correcting results of various quantitative methods, validating theoretical models of electromagnetic field calculations, etc. [2,3].

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 Lu 1.8Gd0.2SiO5: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. © 2014 Elsevier B.V.

Magnetic resonance elastography (MRE) was developed for detecting the region and the stage of disease by changing in the hardness of human tissue or organ. The MRE technology requires an external excitation system for generating transverse waves to the subject in the gantry of MRI. Stiffness of the organ is quantitatively calculated from the wave patterns through a mathematical model. Techniques to measure viscoelastic property of soft materials have been developed. For example, rheometer and ultrasound elastography has been generally used for gel materials and soft tissues. For example, rheometer and ultrasound elastography has been generally used for gel materials and soft tissues. Magnetic resonance elastography have an important role to measure a distribution of quantitative viscoelastic property under a keeping shape of soft material objects and an influence of frequency. In this study, we focused the viscoelastic property of gel material and soft tissue measured by the MRE based on the micro MRI (0.3 T) designed from a high-power vibration generator and a bartype vibration transmitter. The MRE represent the viscoelastic property at a complex modulus G*=G’+iG”. The storage shear modulus G’ and the loss shear modulus G” shows elasticity and viscosity, respectively. Specimens made by gelatin gel and bovine liver and muscle. We report about influence of excitation frequency that is used 62.5 to 500 Hz and specimen boundary condition that fix each plane of the specimen. The G’ of gelatin gel increase with the frequency. We will discuss about the boundary condition of the specimen and the soft tissue.

We are developing a new PET scanner based on the "OpenPET" geometry, which consists of two detector rings separated by a gap. One item to which attention must be paid is that OpenPET image reconstruction is classified into an incomplete inverse problem, where low-frequency components are truncated. In our previous simulations and experiments, however, the OpenPET imaging was made feasible by application of iterative image reconstruction methods. Therefore, we expect that iterative methods have a restorative effect to compensate for the lost frequency. There are two types of reconstruction methods for improving image quality when data truncation exists: one is the iterative methods such as the maximum-likelihood expectation maximization (ML-EM) and the other is an analytical image reconstruction method followed by the method of convex projections, which has not been employed for the OpenPET. In this study, therefore, we propose a method for applying the latter approach to the OpenPET image reconstruction and compare it with the ML-EM. We found that the proposed analytical method could reduce the occurrence of image artifacts caused by the lost frequency. A similar tendency for this restoration effect was observed in ML-EM image reconstruction where no additional restoration method was applied. Therefore, we concluded that the method of convex projections and the ML-EM had a similar restoration effect to compensate for the lost frequency. © 2014 Japanese Society of Radiological Technology and Japan Society of Medical Physics.

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 x 6 x 6 array of Lu2xGd2(1-x)SiO 5:Ce crystal elements with dimensions of (3.0 mm)3. Each surface of the crystal block was covered with a 4 x 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. © Japanese Society of Radiological Technology and Japan Society of Medical Physics 2013.

PET detectors based on light sharing, which can obtain spatial resolution smaller than the photo-detector size with a simple calculation to estimate the center of gravity, have often been studied, but the difficulty in identifying inter-crystal scattering (ICS) has not been resolved yet. In particular, ICS identification is strongly required to make full use of a minutely segmented crystal block such as used in the X'tal cube. The maximum likelihood (ML) method that calculates the likelihood function with response functions of photo-detector signals is suitable for the X'tal cube. However, the method does not take ICS into account. Therefore, in this paper, we developed a new method taking ICS into account for the X'tal cube detector. The proposed method was based on the ML method, and unknown positions were expanded to two to estimate scattering position and absorbed position for the ICS event. We evaluated performance of the proposed method using simulated data. We simulated the X'tal cube composed of the 6 x 6 x 6 array of (3 mm)(3) LGSO crystals. Each surface of the crystal block was covered with 4 x 4 MPPCs, each with a 3 x 3 mm(2) active area. As the results, the proposed method promises high ICS event identification, but crystal identification performance of the proposed method is almost equal to previous methods.

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 (mu-map) from an MRI depends on the accuracy of tissue segmentation and the attenuation coefficients to be assigned (mu-values). In this study, we proposed an MRI-based mu-value estimation method with a non-rotational radiation source to construct a suitable mu-map for PET/MRI. The proposed method uses an accurately segmented tissue map, the partial path length or each tissue, and detected intensities of attenuated radiation from a fixed-position (rather than a rotating) radiation source to obtain the mu-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 mu-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 mu-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 mu-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. (C) 2013 Elsevier B.V. All rights reserved.

The OpenPET is our original idea that realizes the world's first open-type 3D PET scanner for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Even with a full-ring geometry, the OpenPET has an open gap between its 2 detector rings through which the treatment beam passes. Following our initial 2008 proposal, we developed a small prototype in 2010 to show a proof-of-concept. Now, we report the development of a prototype whole-body OpenPET. The key technology which enabled the OpenPET realization is our original, 4-layered depth-of-interaction detector. In order to measure a radiation from the limited activity produced though fragmentation reactions, Zr-doped GSO (GSOZ), which contains less natural radioactivity, was chosen for the scintillators instead of Lu-based scintillators although timing performance was compromised. In order to compensate for the limited light yield, on the other hand, we used 64-channel flat-panel PMTs with a super-bialkali photocathode, which had a 30% higher quantum efficiency. In order to enable stable in-beam PET measurement even under high background radiations, voltage divider circuits were designed to provide 5 times higher linearity. Additionally, to avoid severe radiation damage, we did not use gain control ASICs in the front-end circuits, and position analyzer circuits were placed with a 15-m cable extension. The prototype consists of 2 detector rings, and each detector ring has 2 sub-rings of 40 detectors. Each detector consists of 16 x 16 x 4 array of GSOZ (2.8 x 2.8 x 7.5mm(3)). The portable gantry has a compact design; each detector ring has a 940 mm outer diameter and 171 mm thickness for the detector inner bore of 640 mm diameter and 113 mm thickness. The system was tested with a carbon beam irradiation at a clinical intensity. Phantom images were obtained by applying a GPGPU-based, list mode iterative reconstruction algorithm with geometrical detector response modeling.

Many aspects of medical imaging require innovative measures to improve the quality of diagnosis and treatment. The FERMI Project is promoting these innovations through new developments and improvements of high-dimension, high-definition, quantification of physical or physiological parameters of biological objects and integration of multimodal medical images. The FERMI Project consists of three sub projects: (1) development of essential imaging technologies in medicine; (2) development of dynamic imaging technologies; and (3) integration of spatial information in medical treatment. In this paper, some ongoing research studies are reviewed. These include: ultrasound-based tissue characterization; magnetic resonance elastography; 4D-MRI reconstruction; knee motion analysis from X-ray fluoroscopy and CT images; projector-based assistance for laparoscopic surgery; and a surgical navigation system with three-dimensional ultrasound imaging.

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 the world's first, open-type 3D PET scanner "OpenPET" for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Following our first idea of a dual-ring Open PET (DROP), we proposed our second-generation geometry, single-ring OpenPET (SROP), which is more efficient than DROP in terms of manufacturing cost and sensitivity. In this paper, we have developed a SROP prototype based on a novel detector arrangement, in which block detectors originally forming a conventional PET scanner were axially shifted little by little. Sixteen detector units each of which consists of two depth-ofinteraction detectors are arranged to form a perfect circle, 25cm in diameter. Detector units have an axial shifting mechanism so that they can be transformed into the SROP; adding this mechanism to the units allows us to use the scanner as a conventional (i.e., non-open) PET when in-beam PET measurements are not required. After confirming its basic imaging performance using a phantom filled with F-18 solution, we carried out in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC). In addition to the usual carbon (C-12) beam, we applied RI beams of C-11 and C-10. Stopping positions of primary particles were directly imaged with the RI beam irradiation, while the stopping position distribution of secondary particles was imaged with the C-12 beam irradiation. Phantom study results with pencil beam irradiation of about 2.5Gy showed that beam stopping positions can be measured with the precision better than 2mm with the C-11 beam irradiation followed by 20 mm PET measurement. With the C-10 beam, PET measurement time could be reduced to 1/10 while still maintaining the precision. For both C-11 and C-10, there is room for further reduction of PET measurement time.

Several MRI-based attenuation correction methods have been reported for PET/MRI. The accuracy of the attenuation map (μ-map) from an MRI image depends on correctness of the segmentation of tissue and the attenuation coefficients to be assigned (μ-values). However, an MRI image does not reflect the attenuation of radiation and inaccurate assignment of μ-values affects the quantitative assessment of functional images of PET. Although installation of a transmission scan function on the PET/MRI can provide an accurate μ-map, it restricts the design of the scanner, increases the manufacturing cost and takes additional scanning time. In this study, we implemented an MRI-based μ-value estimation method with a non-rotational radiation source to construct the proper μ-map for PET/MRI and assessed it based on clinical data sets. The proposed method uses the accurately segmented tissue map, the partial path length of each tissue, and detected intensities of attenuated radiation from a fixed-position radiation source which usually rotates around the subject to obtain the μ-map with the tomographic procedure. According to the Lambert-Beer law, attenuated intensity is described as the function of partial path length and μ-values of every tissue. The partial path length could be estimated by the simulation of fixed-point radiation with the same scanner geometry using the known tissue map from MRI. The μ-values of every tissue could be estimated by inverting the function. The simulation results, based upon measurement data, showed the errors between μ-values of the conventional transmission scan and our proposed method were 2.3±0.9%, 18.6±8.0% and -11.1±5.5% for brain, bone and soft tissue other than brain, respectively. Although there were over- and under-estimations for bone and soft tissue, respectively, the present method is able to estimate the brain μ-value accurately in clinical situations and that strongly affects the quantitative value of PET images because of the large volumetric ratio of the brain. © 2013 IEEE.

We are developing a new PET/MRI system based on our novel concept: an MR head coil integrated with PET detectors. To reduce electromagnetic interaction between the PET detectors and the MRI coil, the PET detectors are covered with conductive shield boxes. However, when the magnetic field around the shield box is changed by field gradient, eddy current is generated in the shield box, and that produces secondary magnetic field degenerating image quality. In this study, we quantitatively evaluated the secondary magnetic field induced by the eddy current. We created a map of the secondary magnetic field in the whole measuring system (ABO map). The ABO maps with and without shield boxes contrast were nearly the same. This suggests that the effect of the shield-box-inducing eddy current is smaller than that of the eddy current generated by the whole measurement system.

We are developing a PET system integrated with a birdcage RF coil for PET-MRI to realize highly sensitive PET system for simultaneous measurements. The PET detectors which consist of a scintillator block, photo sensors and front-end circuits with four-layer DOI encoding capability are placed close to the objective. Therefore, the proposed system can achieve high sensitivity without degradation spatial resolution at the edge of the field of view due to parallax error thanks to the four-layer DOI capability. If a shielding material is inside the RF coils, the RF pulse is blocked by the shielding material and then complete images cannot be obtained. Therefore, each RF coil element is inserted between the scintillator crystal blocks and then medially located in the shielding materials. We developed an one-pair prototype system for performance evaluation of PET detector for PET-MRI. The one-pair prototype system consists of two PET modules and a birdcage RF-coil. The PET modules consist of six monolithic multi-pixel photon counter (MPPC) array (S11064-050P), a readout circuit, two four-layer DOI arrays and shielding box. The monolithic MPPC has 4 x 4 readout pixels. The size of readout pixels is 3 x 3 mm(3). The six MPPC arrays arraigned on a line are soldered on the readout circuit board. 96ch readout signals from each MPPC pixel are reduced to 4 ch signals by a weighted sum circuit and fed to amplifiers on the circuit board. The RF-coil is dedicated to a 3.0 T MRI. The PET detector can be mounted on gaps between the RF-coil elements and the distance of the PET detectors can change from 23.0 cm to 27.0 cm. In this paper, we present about details of the one-pair prototype system and results of evaluation experiment with the one-pair prototype.

Several MRI-based attenuation correction methods have been reported for PET/MRI. The accuracy of the attenuation map (mu-map) from an MRI image depends on correctness of the segmentation of tissue and the attenuation coefficients to be assigned (mu-values). However, an MRI image does not reflect the attenuation of radiation and inaccurate assignment of mu-values affects the quantitative assessment of functional images of PET. Although installation of a transmission scan function on the PET/MRI can provide an accurate mu-map, it restricts the design of the scanner, increases the manufacturing cost and takes additional scanning time. In this study, we implemented an MRI-based mu-value estimation method with a non-rotational radiation source to construct the proper mu-map for PET/MRI and assessed it based on clinical data sets. The proposed method uses the accurately segmented tissue map, the partial path length of each tissue, and detected intensities of attenuated radiation from a fixed-position radiation source which usually rotates around the subject to obtain the mu-map with the tomographic procedure. According to the Lambert-Beer law, attenuated intensity is described as the function of partial path length and mu-values of every tissue. The partial path length could be estimated by the simulation of fixed-point radiation with the same scanner geometry using the known tissue map from MRI. The mu-values of every tissue could be estimated by inverting the function. The simulation results, based upon measurement data, showed the errors between mu-values of the conventional transmission scan and our proposed method were 2.3 +/- 0.9%, 18.6 +/- 8.0% and -11.1 +/- 5.5% for brain, bone and soft tissue other than brain, respectively. Although there were over- and under-estimations for bone and soft tissue, respectively, the present method is able to estimate the brain mu-value accurately in clinical situations and that strongly affects the quantitative value of PET images because of the large volumetric ratio of the brain.

We are developing a new PET/MRI system based on our novel concept: an MR head coil integrated with PET detectors. To reduce electromagnetic interaction between the PET detectors and the MRI coil, the PET detectors are covered with conductive shield boxes. However, when the magnetic field around the shield box is changed by field gradient, eddy current is generated in the shield box, and that produces secondary magnetic field degenerating image quality. In this study, we quantitatively evaluated the secondary magnetic field induced by the eddy current. We created a map of the secondary magnetic field in the whole measuring system (ΔB0 map). The ΔB0 maps with and without shield boxes contrast were nearly the same. This suggests that the effect of the shield-box-inducing eddy current is smaller than that of the eddy current generated by the whole measurement system. © 2013 IEEE.

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.

One of the challenging applications of positron emission tomography (PET) is in-beam PET, which is an in situ dose monitoring method for charged particle therapy. It is known that the activity of positron emitters produced through fragmentation reaction is generally low. Image reconstruction from low count data would require alternative algorithms to suppress noise in images, such as the maximum a posteriori (MAP) reconstruction algorithm, which is often used to improve an image reconstruction problem by introducing penalty functions. In particular, the total variation (TV) norm has been proposed as a penalty function to suppress noise while preserving edges. In this study, we applied a MAP-TV image reconstruction algorithm to in-beam PET imaging, and we evaluated the effect of TV constraint for the in-beam PET image in terms of detection performance of distal edge a of positron emitter distribution along beam irradiation.We applied the one-step-late algorithm combined with the TV norm to a new small prototype of the single-ring OpenPET (SROP), which is our second generation OpenPET geometry. The small SROP prototype consisted of two oval detector rings which were slanted by 45 degree and stacked. We carried out initial in-beam experiments in the Heavy Ion Medical Accelerator in Chiba (HIMAC). In the experiment, we used a C-12 beam. The target was a rectangular parallelepiped phantom (40 x 40 mm(2) and 100 mm long) made of polymethyl methacrylate (PMMA). We calculated averaged peak position of profiles obtained in reconstructed images. In the experimental results, reconstructed images became smoother and less noisy with stronger constraint. It was shown that the MAP-TV algorithm enables smoothness and less noisy in reconstructed images even from the low count.

We are developing a novel PET detector with 3D isotropic resolution, the X'tal cube. Our original idea was to cover all 6 surfaces of a segmented crystal block with multi-pixel photon counters (MPPCs) for effective scintillation photon acquisition. On the other hand, modification toward practical use is important in terms of assembling efficiency and production costs. In addition, when the X'tal cubes are arranged to form a PET scanner, insensitive inter-detector gaps made by the MPPC arrays should not be too wide or better yet, they should be removed. Therefore, reducing the number of faces to be connected to the MPPCs has become our top priority. In this paper, we evaluated the effect of reducing the number of MPPCs on positioning accuracy through numerical simulations. We optimized the X'tal cube in terms of the gap material of crystal segments and the number of MPPC connection faces. We simulated the X'tal cube with (3.0 mm)(3) crystal segments; the crystal block was composed of 6 x 6 x 6 array of (3.0 mm)(3) LGSO crystals. Each surface of the crystal block was covered with a 4 x 4 array of MPPCs, each of which had a 3.0 x 3.0 mm(2) active area. Outer surfaces of the crystal block, except for the partitions that the MPPCs were arranged on, were covered with reflectors. For material between crystal elements, we compared optical glue and air gap. The air gap showed better crystal identification performance than the optical glue, although good crystal identification performance was obtained even with optical glue for the 6-face MPPC connection. We showed that the number of MPPC connection faces could be reduced to two when the gap material was air.

The OpenPET geometry is our original idea to visualize a physically opened space even with a full ring geometry. One of our targets is in-beam PET, which is a method for in situ monitoring of charged particle therapy. In our initial idea, the OpenPET had a physically opened field-of-view (FOV) between two detector rings separated by a gap. Originally, the OpenPET was proposed to provide a stress-less brain imaging device. For a dedicated in-beam PET scanner, this dual-ring OpenPET is a good candidate, but it is not necessarily the most efficient geometry because it has a wide FOV (i.e., a gap FOV plus two in-ring FOVs) while only a limited FOV around the irradiation field is required in actual use of in-beam PET. At the last conference, therefore, we proposed a single-ring OpenPET (SROP) dedicated for in-beam PET as our 2nd generation geometry. The detector ring of the SROP geometry was the cylinder both ends of which were cut by parallel aslant planes. In this paper, we developed a small prototype of the SROP for a proof-of-concept. It consisted of 2 ellipse-shaped detector rings, each of which had 16 detectors. Each ellipse-shaped detector ring had a major axis of 281.6 mm and a minor axis of 207.5 mm. The rings were slanted by 45 deg and staggered to obtain an open space of 74.5 mm width. We carried out initial in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC) using a C-11 beam as well as a C-12 beam. PET measurement started at the beginning of the irradiation, and continued for 20 min after the irradiation. For about 3Gy irradiation, a 6 mm range difference was clearly detected with the C-11 beam irradiation. Our initial imaging studies showed promising performance of the SROP prototype.

We are developing a PET system integrated with a birdcage RF coil for PET-MRI. The integrated system is intended to realize a highly sensitive PET system for simultaneous measurements. In the proposed PET-MRI system, PET detectors which consist of scintillator crystal blocks, photo sensors and front-end circuits with four-layer DOI encoding capability are placed close to the objective. Therefore, the proposed system can achieve high sensitivity without degradation of spatial resolution at the edge of the FOV due to parallax error. The photo sensors and front-end circuits should be shielded to minimize noises from the MRI and noise influence on the MRI imaging. Elements of the RF coil are inserted between the crystal blocks inside of the shielding material so as not to interfere with the RF-pulse.At the last MIC conference, we demonstrated the possibility of realizing the proposed PET-MRI system with a prototype PET detector and a commercial RF-coil. The prototype PET detector consisted of a LGSO crystal block and a 4 x 4 MPPC array. The crystals were arranged in a 4 x 4 x 4 layer with four-layer DOl capability. The detector and electrical circuit were packaged in an aluminum shielding box. Since they were located inside the MRI magnetic field, most circuit elements were made of nonmagnetic materials.As a next step, we constructed a new RF-coil system which can be mounted on PET detectors between each coil element. We carried out experiments with the prototype RF-coil and the four-layer DOl detector. The prototype RF-coil has eight RF-coil elements and the PET detector can be mounted on gaps between them. Only one detector was mounted in the gap positioned at one side of the prototype RF-coil. We evaluated the performances from the 2D flood histogram, energy resolution and MRI images. We observed no degradations of the performance of the PET detector and MRI image in simultaneous measurements.