羽石 秀昭

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.

Depth of interaction detector is essential to achieve high spatial resolution and high sensitivity. Our group has developed an isotropic 3D detector, X'tal cube. For the crystal block, a laser processing was applied to a monolithic crystal to generate laser-processed boundaries (LPBs) inside the crystal block. The LPB is expected to give appropriate photon scattering effect as well as reduction of assembling steps compared with gluing small pieces of crystals. In this paper, we introduced the LPB to our former 4-layer DOI detector based on the reflector control, which projects 3D positions of interaction crystal to a 2D position histogram of the Anger-type calculation. The key point of proposed detector are that we set photo detectors at only one side and applied LPBs to crystals. To investigate features of four layer DOI detector combined with LPBs, we compared three conditions for the gaps between crystals in the 2 x 2 array: the air, the room temperature vulcanizing (RTV) rubber and the LPB. In the LPB case, actual crystal size was 4 mm x 4 mm x 5 mm, and the 2 x 2 segmentations were made by the laser processing. For these three patterns, we carried out both experiments and simulations. Almost all crystal responses were separated in the proposed detector with the LPB, although separation was not clearer than results with the other conditions because responses corresponded to the 2 x 2 crystal array had a tendency to get together. On the other hand, clusters can be discriminated clearer at detector edge.

In laparoscopic surgery, to identify the location of lesions and blood vessels inside organs, an ultrasound probe which can be inserted through small incision is used. However, since surgeons must observe the laparoscopic and ultrasound images both at the same time, it is difficult to understand the correspondence between the ultrasound image and real space. Therefore, to recognize the correspondence between these two images intuitively, we developed a system for overlaying ultrasound image on the laparoscopic image. Since the tip of the probe is flexed freely, we acquired the probe tip position and orientation using a method for detecting the probe angle from laparoscopic image and information obtained from optical tracking sensor. As a result of an experiment using a wire phantom, overlaying error of ultrasound images was found to be 0.97 mm. Furthermore, the rate of probe angle detection was evaluated through an animal experiment to be 83.1%.

体幹部臓器の呼吸性体動を経時的断層像として得られれば，これを疾患の診断や放射線治療，核医学画像の画質改善等に利用できる可能性がある．われわれは，この目的のために2次元時系列MR画像を適切に合成して，3次元時系列MR画像すなわち4次元MR画像を構築する方法を考案した．これは，異なるスライスで得られた時系列画像群を，これと直交する時系列画像を利用して同期させ合成する方法である．本稿ではまずその基本原理を説明するとともに実際に得られた4次元MR画像を例示する．また，この手法をベースとして，より短時間でのデータ収集法について検討した結果についても触れる．さらに，得られた4次元MR画像から横隔膜を抽出し，その体軸方向の変位量やスピードを可視化・解析する手法についても紹介する．

The world's first open type PET, named OpenPET, is being developed at the National Institute of Radiological Sciences. We are aiming to employ the OpenPET in radiotherapy and to track the respiratory motion of a tumor in the thoracoabdominal region of a patient. By using PET images, we expect that the tumor itself can be directly visualized without using radio-opaque markers. Our prototype OpenPET system can output reconstructed images at about 2 frames per second with about a 2 s delay; this delay is mainly due to the reconstruction calculation time. Therefore we presented a time delay correction method with belt type piezoelectric sensor for tumor tracking for the OpenPET with support vector regression method (SVR) in the last IEEE NSS-MIC meeting. In this paper, we improved accuracy of this method. In the modified method, we added body surface displacement velocity as a secondary independent variable of the regression line so that it represents hysteresis effect. In addition, to supplement SVR's prediction posterior probability, we introduced hidden Markov model (HMM). As the result, the mean and maximum tracking error was decreased 1.25 mm and 4.70 mm in the most effective case, respectively.

We are developing the OpenPET which can provide an open space to make the patient observable and accessible during PET measurements. In addition, we have proposed a real-time imaging system for the OpenPET which is expected to be used in PET-guided tumor tracking radiation therapy. We conducted a feasibility study on tumor tracking using 18F-FDG by Monte Carlo simulation of a human body-sized OpenPET and showed that the tumor tracking is feasible with a time window of 0.5 s if the tumor contains sufficient radioactivity. Although we have demonstrated real-time tumor tracking using a small OpenPET prototype, the delay was 2 s. On a human body scale, the image matrix size become huge compared to the body size of the small prototype and delay can be increased. Therefore, further improvement of the reconstruction speed is essential. From this viewpoint, image reconstruction of the entire field-of-view (FOV) is time consuming and not required for the purpose of tumor tracking.In this study, we developed a region-of-interest (ROI) reconstruction method for the OpenPET and evaluated the tumor tracking accuracy. Although conventional wisdom states that the ROI reconstruction requires a priori information of a small region inside the ROI and 180 degrees scanning without angular truncation for accurate image reconstruction, the computer simulation showed that the ROI reconstruction method without any a priori information had equivalent accuracy in terms of tumor tracking compared with the case reconstructing the entire region. The ROI reconstruction method was more than 10 times faster than the entire FOV reconstruction method.

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed two geometries for our open-type PET scanners named OpenPET. The original OpenPET had a physically opened field-of-view (FOV) between two detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wide axial FOV including the gap. But, this geometry is not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we proposed a single-ring OpenPET (SROP) geometry which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors compared to DROP. Also, we proposed two arrangement types, a slanted ellipse (SE) type where oval detector rings are slanted and stacked and an axial shift (AS) type where block detectors originally forming a conventional PET scanner are axially shifted little by little. Previously, we developed the SE-SROP prototype, and showed its good performance for in-beam PET. However, the SE-SROP decreased sensitivity slightly relative to the ring-type PET and needed a large gantry in the axial direction for conventional PET studies. On the other hand, AS-SROP can transform conventional ring-type PET scanners into stacked detectors with a gradual axial shift. In this work, we developed and evaluated a small AS-SROP prototype for proof-of-concept. The AS-SROP prototype was designed with 2 cylinder-shaped detector rings of 16 DOI detectors. The system sensitivity and spatial resolution were measured and were 5.1% and 2.6 mm, respectively, for the OpenPET mode. For the conventional PET mode, the system sensitivity and spatial resolution were 7.3% and 2.2 mm, respectively. We concluded that the AS-SROP geometry has a good potential for not only in-beam imaging but also conventional PET studies.

A novel DOI detector with a stack of planer scintillators (SOPS) allows easier position estimation and simpler fabrication. To investigate suitable optical characteristics of the proposed detector, we constructed a Monte Carlo simulator. In this paper, we studied the relationship between the surface roughness of monolithic scintillator and the spatial resolution. We defmed the parameter of scintillator's surface roughness as roughness rate which was used to statistically determine triangle patch's angle in the simulator. We simulated a six layer SOPS and found that smoother surface leads to higher spatial resolution for x and y direction. Furthermore we applied laser-processed boundaries to the SOPS (SOPS with LPB). We confirmed that SOPS with LPB provided better spatial resolution although it leads to additional process in fabrication to some extent.

膝関節の主な疾患である変形性膝関節症の早期診断のために軟骨成分比の変化の検出が求められている。本論文では、軟骨成分の含有量評価を目的に、軟骨の主成分であるプロテオグリカン(PG)とコラーゲンのファントムを作成し、解析を行った。撮影装置には、臨床用MRIとMR顕微鏡の2種類の装置を使用した。臨床機では複数のシーケンスで撮影し、各成分含有量と基準溶液との信号強度比の相関性を評価した。一方、MR顕微鏡では、PGのファントムの緩和時間を測定し評価を行った。この結果、PGのファントムにおいて、PG含有量とT1 CUBEの信号強度比に高い相関が見られた。また、PGのT1緩和時間は含有量に伴って短縮し、T1 CUBEの信号強度比と整合性のある結果を得られた。変形性膝関節症の診断として、変性軟骨と同信号値のファントムを患者と同時に撮影し、信号強度比を取得することで、生体内軟骨の成分含有量を高精度に評価できる可能性がある。(著者抄録)

横隔膜の呼吸性運動を主成分分析(PCA)および一般化N次元主成分分析(GND-PCA)を用いて統計解析した。まずわれわれが以前提案した交差プロファイル法によって得られた4次元MRIから横隔膜領域を抽出した。異なる形状の横隔膜の統計解析を行うために、抽出した横隔膜を水平面に射影した後、その形状を長方形に規格化した。この長方形のマトリクスには横隔膜の体軸方向座標が入り、これが1呼吸周期分存在するため、個々のデータは3次元データとなる。従来のPCAはこれを長い1次元ベクトルとして扱い、GND-PCAはこれを3次元のまま扱う。10名の健常者の呼吸性運動に対して、それぞれの手法でモデル化を行いLeave-one-out法で評価したところ、PCAの場合に平均誤差が4mmを超えるのに対して、GND-PCAの適切なパラメータ設定で平均誤差を1mm程度に抑えられることがわかった。(著者抄録)

We are developing the OpenPET which can provide an open space observable and accessible to the patient during PET measurements. In addition, we have proposed the real-time imaging system for the OpenPET which is expected to be used in PET-guided tumor tracking radiation therapy and demonstrated its tracking ability using a point source and a small OpenPET prototype. However, tumor tracking in the human body still remains as a challenging task when we use F-18-FDG which is the best available tracer for tumors because of its background activity, scatter and attenuation in the body. In this study, we assess conditions under which tumor tracking is feasible in the human body by using the 4D XCAT phantom which is a realistic 4D human whole body phantom. To simulate realistic F-18-FDG distributions, we assigned standardized uptake values (SUVs) to normal organs based on the literature. We conducted Monte Carlo simulation of a human-sized OpenPET geometry by using Geant4 Application for Tomographic Emission (GATE) ver. 6.1 assuming a measurement at 100 minutes after the F-18-FDG injection of 370 MBq and a spherical tumor with the diameter of 10 to 30 mm and SUV of 3 to 10. List-mode data were generated for each 0.5 s time frame of a respiratory cycle of 5 s. Image reconstruction was done in a frame-by-frame manner and tumor position was automatically extracted for each time frame by a pattern matching technique. Tumor movement in the 4D XCAT phantom was about 17 mm at the maximum. The mean error of the tumor positions extracted from the reconstructed images was similar to the PET image resolution when the tumor size was 20 mm or more and SUV was 5 or more. We showed that tumor tracking by the OpenPET is feasible even in the human body scale and for realistic conditions.

The use of circulant matrix as the sensing matrix in compressed sensing (CS) scheme has recently been proposed to overcome the limitation of random or partial Fourier matrices. Aside from reducing computational complexity, the use of circulant matrix for magnetic resonance (MR) image offers the feasibility in hardware implementations. This paper presents the simulation of compressed sensing for thoracic MR imaging with circulant matrix as the sensing matrix. The comparisons of reconstruction of three different type MR images using circulant matrix are investigated in term of number of samples, number of iteration and signal to noise ratio (SNR). The simulation results showed that circulant matrix works efficiently for encoding the MR image of respiratory organ, especially for smooth and sparse image in spatial domain. © 2012 Universitas Ahmad Dahlan.

We propose a VR-based injection training system using Simulated Patient (SP) with an original haptic needle which can represent a haptic expression. SP is trained to realistically portray a real patient. In order to increase a realistic sensation and effect of training, it is important to train with real human. Therefore we propose a new training system which fuse SP and virtual training system. In the proposed system, trainee can virtually puncture the SP using the haptic needle. In addition, the haptic needle can represent a haptic expression of needle insertion of the virtual anatomical model. We developed the haptic needle which can represent a reaction force of puncturing. A reaction force of needle insertion is generated by clipping needle by using computer-controlled electromagnet and permanent magnet, and then a friction force is accrued. By using the proposed system, trainee can feel virtual puncture as well as operating for real patient.

膝関節の主な疾患である変形性膝関節症の早期診断のために軟骨成分比の変化の検出が求められている．本論文では，MRI画像を用いた軟骨成分の含有量評価に向けた基礎研究として，軟骨の主成分であるプロテオグリカン(PG)とコラーゲンのファントムを作成し，MRI画像解析を行った．撮影装置には，臨床用MRIとMR顕微鏡の2種類の装置を使用した．臨床機では複数のシーケンスで撮影し，各成分含有量と基準溶液との信号強度比の相関性を評価した．一方，MR顕微鏡では，PGのファントムの緩和時間を測定し評価を行った．この結果，PGのファントムにおいて，PG含有量と等方的3D高速スピンエコー撮像法であるT₁CUBEで信号強度比に高い相関が見られた．また，PGのT₁緩和時間は含有量に伴って短縮し，T₁CUBEの信号強度比と整合性のある結果を得られた．変形性膝関節症の診断として，変性軟骨と同信号値のファントムを患者と同時に撮影し，信号強度比を取得することで，生体内軟骨の成分含有量を高精度に評価できる可能性がある．

The world's first open type PET, named OpenPET, is being developed at the National Institute Radiological Sciences. We are aiming to employ the OpenPET in radiotherapy and to track the respiratory motion of a tumor in the thoracoabdominal region of a patient. By using PET images, we expect that the tumor itself can be directly visualized without using radio-opaque markers, but this goal has not been achieved yet. Our demonstration results using a small prototype OpenPET system showed that the system can output reconstructed images at about 2 frames per second with about a 2 s delay; this delay is mainly due to the reconstruction calculation time. In this study, we developed a time delay correction method for tumor tracking for the OpenPET. Since it is difficult to correct the time delay using only the tumor location 2 s before, we introduced another sensor to acquire the respiration phase for correction. In the proposed method, the relationship between tumor motion and the additional sensor output signal was calculated by support vector regression (SVR) and the time delay was corrected by using the regression line which represents the relationship. We simulated this time delay correction method with computer-generated PET images which had practical respiration motions obtained from clinical MRI. As a result, we could track the tumor within almost 1.5 mm mean error when we assumed the use of a belt type respiratory motion sensor. We also found from the simulation that disturbance of the relationship can be detected by gates. Thus we could avoid the tracking errors which are caused by disturbance of the correlation.

リアルタイムPETイメージングを検出器リング間に物理的な開放空間を持つOpenPETによって行うことで、放射線治療中リアルタイムに腫瘍追跡や放射能分布の測定が可能となる。本研究では、画像再構成の高速化のために3D one-pass list-mode DRAMAを、GPGPUの技術を用いて実装し、また、安定化のためにデータ転送制御システムを導入したアーキテクチャを提案し実装を行った。開発したシステムでは、再構成処理で使用されるデータ量の上限を再構成の処理速度に応じて設定し、定量性確保のために使用したデータ量と検出されたデータ量の比によって再構成像を補償する。今回、提案システムをOpenPET小型試作機に実装し、線源追跡の性能評価を行った。その結果、大きな遅延なく毎秒2フレーム以上での再構成像の表示が可能で、表示された画像強度がカウントレートの変化に追随して変化していることを確認した。(著者抄録)

重粒子線がん治療は、がん組織に線量を集中する理想的な放射線がん治療法である一方、その特徴を最大限に引き出すためには、患者体内の照射ビームを3次元画像化する手段が不可欠である。そこで我々は、治療しながらPET計測できる世界初の開放型PET装置「OpenPET」を提案した。2つの検出器リングを離して配置し、開放領域を3次元画像化する。今回、小型試作機を開発し、重粒子線がん治療イメージングのコンセプトをファントム実験にて実証したので報告する。具体的には、重粒子線がん治療装置HIMACにおいて、放射化した治療ビームを検出器リングの隙間を通してアクリルファントムに照射し、ファントム内のビームの分布を、照射しながら3次元画像化した。今後、ヒトサイズの実証機の開発を目指す。また、肺がんなど動く標的のトラッキングの実現に向けて、リアルタイムイメージングシステムの研究も進めている。(著者抄録)

Lung motion due to respiration causes image degradation in medical imaging, especially in nuclear medicine which requires long acquisition time. We have developed a method for image correction between the respiration-gated (RG) PET images in different respiration phases or breath-hold (BH) PET images in inconsistent respiration phase. In the method, the RG or BH PET images in different respiration phase are deformed under two criteria; similarity of image density distribution and smoothness of estimated motion vector field (MVF). However, only these criteria may cause un-natural motion estimation of lung. In this paper, assuming the use of a PET-CT scanner, we add another criterion that is the similarity to the motion direction estimated from inhalation and exhalation CT images. The proposed method was applied to XCAT phantom image data and seven patients' BH-PET image data. Successful registration results were obtained by the proposed method.

One of the challenging applications of PET is for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed an open-type PET scanner, Open PET. The original OpenPET has a physically opened field-of-view (FOV) between two detector rings which irradiation beams pass through. This dual-ring OpenPET has a wide axial FOV including the gap. Therefore this geometry is not necessarily the most efficient when it is applied to in-beam PET in which only a limited FOV around the irradiation field is required. In this paper, we proposed new single-ring OpenPET geometry as more efficient geometry dedicated to in-beam PET. The detector ring of the proposed geometry is a cylinder both ends of which are cut by parallel aslant planes. The proposed geometry can be made compact so that the beam port can be placed close to the patient.One of the problems for the proposed geometry is arranging the rectangular block detectors. We proposed two arrangement types, a slanted ellipse type where oval detector rings are slanted and stacked and an axial shift type where block detectors originally forming a conventional PET scanner are axially shifted little by little. We compared the two types through numerical simulations. The simulated systems were a small prototype that we designed for a proof-of-concept and a human sized system. We evaluated the effect of utilizing depth-of-interaction (DOl) measurement in both sized system in both types.In the small sized simulations, both types resulted in acceptable image quality with DOl capability.In the human sized simulation, on the other hand, the effect of DOl utilizing was not clearly shown because the size of spots was too large compared with the size of the crystals. Therefore, our future work is further investigation the quality of reconstructed images for both types in the human sized simulation.

X'tal cube is a next generation DOI detector for PET with high resolution and sensitivity that we are developing. It is constructed from a cubic scintillation crystal and silicon photomultipliers (SiPMs) coupled on various positions of six surfaces of the cube. A laser processing technique is applied to produce 3D optical boundaries composed of micro-cracks inside a monolithic scintillator crystal. In this paper, in order to design optical characteristics properly, we developed a Monte Carlo simulator which is able to arrange laser-processed optical boundaries (LPB) arbitrarily. Optical characteristics of LPB were measured using a set of laser and photo-diode and then modeled for simulator. Effectiveness of the simulator was confirmed from a comparison with the experiment using a cubic LYSO monolithic crystal with 6 x 6 x 6 segments. Furthermore, the simulator was accelerated by parallel computing with a general purpose GPU (GPGPU). Calculation speed with GPGPU was 350 to 430 times faster than that with only CPU.

We propose an intersection profile method for reconstructing a 4D-MRI of respiratory organ motion from time sequential images of 2D-MRI. In the proposed method, first, time sequential MR images in many coronal planes set to widely cover the lung region are acquired as the data slices. Second, the time sequential MR images in a proper sagittal plane are acquired as the navigator slice. 4D-MRI is reconstructed by extracting and combining a proper respiratory pattern from each data slice which is most similar to an adequately selected respiratory pattern in the navigator slice on the intersection between the navigation slice and each data slice. Successful visualization of the respiratory organ motion is demonstrated and the validation of reconstruction is also presented. Such a 4D-MRI has a great potential for many medical applications. In this paper, we further propose to construct a diaphragmatic function map from a 4D-MRI reconstructed by the intersection profile method to evaluate diaphragmatic motion quantitatively. Experimental results using three healthy volunteers and three patients are shown.

筆者らは，2次元時系列MR画像から4次元MR画像を構築する新たな手法として交差プロファイル法を開発した．交差プロファイル法では，自然呼吸下において，Data sliceとして肺野領域を広くカバーするように複数断面のCoronal面時系列MR画像と，Navigator sliceとしてData sliceと直交するSagittal面時系列MR画像を1断面撮像する．次に，各Data sliceとNavigator sliceが交差する位置で2次元時空間画像を作成し，2次元時空間画像上で正規化相互相関テンプレートマッチングの手法を用いて，Data sliceからNavigator sliceと同じ呼吸パターンを抽出して撮像した全断面の同期を取るものである．本論文では，交差プロファイル法について概説するとともに，本手法によって得られた4次元MR画像の利用法として，横隔膜機能画像の生成について紹介する．

A breath-hold (BH) acquisition method is easy of operation and recently studied actively. In the method, a patient is asked to hold his/her breath for 10 to 30 s as the image acquisition is performed. This is repeated several times and the obtained images are summed. In practice, however, a patient cannot necessarily hold his/her breath at the same timing of breathing. In such a case, the summed images still has a blur. In this paper, we propose to apply our image registration method developed before for respiratory-gated images to BH images. The BH images are nonlinearly motion-corrected so as to match a reference BH image and summed. Through the experiment with ten patient BH image data, we confirmed that the proposed method is effective in obtaining a non-blurred image.

The video capture of surgery is becoming widely used but the colors reproduced by conventional RGB-based video systems differ from the original. In this work, 6-band multispectral video was applied to the open surgery for high-fidelity color reproduction, and medical doctors visually evaluated the reproduced image quality. As a result, 1) the 6-band video system was rated significantly higher in "color reproducibility," "fidelity," and "material appearance." 2) The perceived color differences between 6-band/RGB and 6-band/3-band were significant. 3) Color videos from 6-band data were transmitted via network, and approximately enough quality was obtained with 15Mbps bit rate. These results show the potential of multispectral technology for the improvement of surgical video quality.

The X'tal cube is our new PET detector which is composed of a segmented crystal block and multi-pixel photon counters (MPPCs). It detects scintillation photons in three dimensions by arranging MPPCs on multiple surfaces of the crystal block. It is possible to collect scintillation photons efficiently for higher energy, spatial, and timing resolution. We have previously reported that 3 mm isotropic spatial resolution was obtained using the X'tal cube. In this study, we achieved 1 mm isotropic spatial resolution. The developed X'tal cube detector was composed of 1.0 x 1.0 x 1.0 mm(3) LYSO crystals, which were structured into 16 x 16 x 16 arrays. Each surface of the crystal block was covered with 4x4 MPPCs with 3.0 mm x 3.0 mm sensitive area. We evaluated crystal identification and energy performance, and the results showed that the X'tal cube has the expected 1 mm isotropic spatial resolution.

We have previously developed a method that combines a mini gamma camera with an optical camera to synthesize the two kinds of images and help surgeons to easily identify sentinel lymph nodes on the gastric surface in the cancer resection surgery. Since it is assumed in this method that the object surface is flat, the application is limited. In this article, we extend this method so as to estimate the three-dimensional shape of the object from stereo optical cameras. A prototype system with a mini gamma camera and two optical cameras was constructed and a preliminary experiment was performed.

本稿において， X線CR画像による膝関節の大腿骨と脛骨の骨軸の自動抽出法を提案する． 開発した画像処理法は， あらかじめ必要な情報として右肢か左肢かの情報のみで， 次の2点に特徴をもつ方法である． 第1は輝度の横断分布による骨の境界点の抽出法で， 第2は骨の形状に基づく縦断方向の骨境界線の抽出法である． この自動解析法は健常者を含む変形性ひざ関節症患者の18例のX線CR下肢画像に適用され， 専門医による大腿脛骨外側角の照合データから有効であることが示された．

A four-layer depth-of-interaction (DOI) detector was proposed from and developed at the research project at the National Institute of Radiological Sciences in Japan, with the aim of achieving high resolution and high spatial sensitivity. Previously, we had developed a Monte Carlo simulation for a DOI detector with 2 x 2 x 4 crystal elements. In this study, in order to simulate the final version of the DOI detector of the project, which uses a larger number of crystal elements, we have developed a much faster and simpler simulator. In this paper the algorithm of the simplified simulator as well as the previously proposed Monte Carlo method is presented and the validation of the simplified simulator through comparisons with the full Monte Carlo simulation and with some experimental results is described.

PET scanners are an important component in functional brain imaging devices. Several PET scanners have been developed during the last two decades for brain research studies. We are developing a high-performance brain PET scanner, jPET-D4. This scanner is designed to achieve not only high spatial resolution but also high sensitivity using four-layered depth-of-interaction (DOI) detectors. The scanner has five block detector rings with the ring diameter of 390 mm and each block detector ring consists of 24 DOI detectors. In previous work, we have demonstrated that 3 mm FWHM uniform spatial resolution within the field-of-view could be realized. In this paper, we describe the jPET-D4 system and evaluate its performance. The average energy resolution for 120 DOI detectors is optimized to 16 % ± 1.0 %. The scatter fraction for this system is 40 % with an energy window of 400 600 keV. The sensitivity for the point source is 102 kcps/MBq (10.2 %) with a 400 keV LLD. Maximum noise equivalent count rate (NECR) is 154 kcps at 11 kBq/ml with a 10 ns coincidence time window. We evaluated scatter fraction and NECR following procedures based on NEMA NU2-1994. These first evaluation measurements indicate the jPET-D4 has good performance.

Previously we have proposed a motion correction method for avoiding both motion blur and image noise in respiratory-gated lung SPECT images [1][2]. In a PET/CT system, attenuation correction can be achieved by making an attenuation map from a CT image. In PET/CT system using respiratory-gated PET, however, when a CT image acquired under a breath-hold condition is used for attenuation correction of PET in different respiratory phase, voxel values of the reconstructed PET image may be incorrect, especially in a region that has large respiratory motion. In this paper, we propose an attenuation correction method for this case. The effectiveness of the proposed method was tested using the 4D-NCAT phantoms [3] and a marked stability in the voxel values around the diaphragm was observed. The proposed method was also applied to clinical data.