高分辨micro-SPECT成像系统针孔准直器的优化设计

为了最大限度地减少γ射线在针孔准直器上的透射和散射效应,采用蒙特卡罗方法对单光子发射计算机断层(SPECT)成像系统使用的针孔准直器进行优化.模拟计算结果与实验测试得到的系统灵敏度和空间分辨率曲线相符,达到了高分辨小动物成像的要求.     

评价衍射增强成像空间分辨率的方法

衍射增强成像是X射线成像领域的前沿科技。相比于传统的吸收成像，衍射增强成像能大幅提高图像的衬度，尤其是对于由C、H、O、N等低原子序数元素构成的物体，这一特性使得衍射增强成像在医学诊断方面具有突出的应用价值。目前还没有系统评价衍射增强成像空间分辨率的方法。本文通过构建成像系统的调制传递函数模型，推导出衍射增强成像的空间分辨率计算公式，从而对衍射增强实验平台的整体性能进行综合评价。调制传递函数综合考虑了摇摆曲线几何、CCD像素尺寸、闪烁体荧光弥散效应对系统空间分辨率的固有影响，并详细分析了系统调制传递函数各因素对空间分辨率的影响规律，为衍射增强实验平台的物理设计及设备选型提供理论依据。     

SPECT的质量控制及有关参数的五年前后对比

选择能反映ＳＰＥＣＴ性能的九项指标（平面显像部分：固有空间分辨率，系统空间分辨率，固有均匀性，能量分辨率，线性度，最大计数率；断层显像部分：断层分辨率，断层均匀性，旋转中心漂移），用组织等效材料制备了点源、线源和断层分辨率模型，并应用断层均匀性模型、四象限铅栅模型和线性模型对我院１９８８年５月引进的Ｅｌｓｃｉｎｔ公司的ＡＰＥＸ４０９ＡＧＳＰＥＣＴ进行了测试，并将测试结果与１９８８年有关测试结果相比较。     

a-Si FPD在闪光X光照相中的应用可行性实验研究

非晶硅平板探测器(a-Si Flat Panel Detector)已在无损检测、医疗领域的连续式X光照相中得到了广泛应用,但在脉冲式(ns量级)闪光X光照相中的应用可行性及成像性能却未见报导.本文利用Varian公司生产的PaxScan 2520 HE型非晶硅平板探测器为接收系统,在450keV脉冲X光机上开展了闪光照相实验,考察了该探测器的应用可行性,测量了其探测灵敏度、像质计灵敏度、线扩散函数(LSF)及调制传递函数(MTF).实验结果表明该探测器可应用于闪光X光照相,且探测灵敏度较高,空间分辨率较好,是一种有前景的闪光X光照相图像接收系统.     

基于伴随粒子的快中子成像系统角分辨研究

不同于传统的快中子成像系统,采用伴随粒子成像技术无需机械准直即可消除大部分γ射线和散射中子的干扰,实现对厚重物体的高对比度成像。角分辨是影响系统成像质量的一项重要参数。通过理论分析,研究了入射离子的初始动量、靶点尺寸和探测器空间分辨等多个因素对系统角分辨的影响。利用基于GEANT4的模拟程序,计算了不同参数下被标记中子出射角分布的二维图像。分析及模拟结果表明,靶点直径和α探测器空间分辨率是影响角分辨的重要因素。为满足系统角分辨小于1°的设计目标,入射离子的初始动量变化范围应较小,靶点直径应小于1 mm,同时α探测器的空间分辨率应小于0.5 mm。     

基于伴随粒子的快中子成像系统角分辨研究

不同于传统的快中子成像系统，采用伴随粒子成像技术无需机械准直即可消除大部分γ射线和散射中子的干扰，实现对厚重物体的高对比度成像。角分辨是影响系统成像质量的一项重要参数。通过理论分析，研究了入射离子的初始动量、靶点尺寸和探测器空间分辨等多个因素对系统角分辨的影响。利用基于GEANT4的模拟程序，计算了不同参数下被标记中子出射角分布的二维图像。分析及模拟结果表明，靶点直径和α探测器空间分辨率是影响角分辨的重要因素。为满足系统角分辨小于1°的设计目标，入射离子的初始动量变化范围应较小，靶点直径应小于1 mm，同时α探测器的空间分辨率应小于0.5 mm。     

一个用于多重示踪技术的康普顿成像装置

提出了一个由位置灵敏度高纯锗探测器组成、用于多重示踪技术的康普顿成像装置。简要介绍了康普顿成像系统的工作原理以及影响其空间分辨率的各种因素。用蒙特卡洛方法模拟了所提出的康普顿成像装置对处于不同位置、发射不同能量γ射线的点源可达到的效率和空间分辨率。用现有的2个多单元高纯锗探测器进行了三个点源成像实验,证明所提出的康普顿成像装置适用于多重示踪技术。     

γ相机的计数率特性测试

计数率特性是γ相机的一项主要技术指标。本文参照NEMA标准,并根据国内γ相机用户的实际情况,介绍用双源法测定OMEGA 500γ相机的计数率特性的方法。 一、材料和方法 γ相机的计数率特性分为固有计数率特性和系统计数率特性。在各项参数的测定中,     

康普顿相机的GEANT4模拟与反投影图像重建

模拟构建了双层康普顿相机,利用康普顿散射原理,获取γ光子在两层探测器发生散射前后沉积的能量和位置信息,通过一定算法进行图像重建,获得放射源位置信息。该相机具有灵敏度高、体积小、应用范围广等优点。通过蒙特卡罗方法模拟了具有双层条状结构位置灵敏探测器(散射探测器、吸收探测器)组成的康普顿相机,其两层探测器分别由Si和Ge材料构成。利用反投影图像重建算法实现了放射源图像重建,当单点源与探测器距离为40 mm时,成像的效率为0.38%,位置分辨率达到8.0 mm,角分辨率达到3°。对于不同位置和不同个数点源的情况下,检验了反投影重建算法在康普顿相机放射源空间重建中的效果。     

高分辨率小动物SPECT成像系统的设计与性能评估

为实现低成本、高分辨率的小动物单光子发射断层成像(SPECT),设计了一种基于临床探测器和针孔准直器的小动物SPECT成像系统。综合考虑视野尺寸、空间分辨率和绝对探测效率等指标,分别给出了单针孔和七针孔准直器的两种最优化设计方案,并通过对比两种设计方案在全视野范围内的平均空间分辨率和探测效率,对成像性能进行了评估。结果表明,与单孔准直器相比采用七针孔准直器设计可同时获得更高的空间分辨率和探测效率。经蒙特卡罗模拟成像实验验证,该七针孔准直器在临床探测器上可达0.5mm级的超高空间分辨率,且成本远低于采用专用探测器的现有小动物SPECT系统。     

Experimental Results of the Dichotomic Sampling in Circular Ring Positron Emission Tomograph

Experimental verification of the new dichotomic sampling scheme was attempted and applied to a circular ring PET system to improve the sampling thereby the overall system resolution and its performances. In the experiment, two different types of aperture collimators were adopted for high resolution (HR) and very high resolution (VHR) imaging. In HR mode, a resolution of 6.5 mm FWHM was obtained without appreciable degradation in overall sensitivity which represents a threefold resolution improvement over the original system. In phantom studies with HR mode a sensitivity of 4500 counts/sec/?Ci/cc was obtained for a 20 cm diameter uniform phantom filled with water. VHR mode experiment was also conducted to observe the ultimate resolution capability of DICHOTOM-I system and resolution of 4.2 mm FWHM was obtained at the expense of sensitivity reduction by a factor of four from HR mode experiment.     

PET Performance Evaluation of an MR-Compatible PET Insert

A magnetic resonance (MR) compatible positron emission tomography (PET) insert has been developed in our laboratory for simultaneous small animal PET/MR imaging. This system is based on lutetium oxyorthosilicate (LSO) scintillator arrays with position-sensitive avalanche photodiode (PSAPD) photodetectors. The PET performance of this insert has been measured. The average reconstructed image spatial resolution was 1.51 mm. The sensitivity at the center of the field of view (CFOV) was 0.35%, which is comparable to the simulation predictions of 0.40%. The average photopeak energy resolution was 25%. The scatter fraction inside the MRI scanner with a line source was 12% (with a mouse-sized phantom and standard 35 mm Bruker 1H RF coil), 7% (with RF coil only) and 5% (without phantom or RF coil) for an energy window of 350-650 keV. The front-end electronics had a dead time of 390 ns, and a trigger extension dead time of 7.32 μs that degraded counting rate performance for injected doses above ~0.75 mCi (28 MBq). The peak noise-equivalent count rate (NECR) of 1.27 kcps was achieved at 290 μCi (10.7 MBq). The system showed good imaging performance inside a 7-T animal MRI system; however improvements in data acquisition electronics and reduction of the coincidence timing window are needed to realize improved NECR performance.     

多平行束准直器模拟研究

在平行束准直器中引用多通道技术设计了三通道的多平行束（Multi-parallel Beam,MPB）准直器。计算机模拟表明,MPB准直器在保持较高的空间分辨率下,有效地改善了系统的灵敏度。放射药物匀均分布的圆柱数字模具和Jaszczak数字模具（phantom）模拟表明,随重叠比例的增加,未出现重建环状伪影;虽然靠近准直器表面空间分辨率有所下降,但在FOV（Field of View）中心区域空间分辨率近似等同于平行束准直器,重建图像的匀均度有了改善。投影数据的重叠表明,MPB的投影有放大作用。该研究为高成像质量的临床SPECT成像提供一种新的准直器设计方法。     

Detector Identification with Four BGO Crystals on a Dual PMT

The current trend towards higher resolution positron emission tomography (PET) requires the development of small but efficient detectors. This paper presents the initial characterization of a detector composed of four equally spaced, 4.5mm wide bismuth germanate (BGO) crystals coupled to the face of a Hamamatsu R1548 dual photomultiplier tube (PMT). The intrinsic optical cross talk between the two segments of the dual PMT exhibits a spatial dependence that has been exploited for crystal identification. Misidentification of crystals due to the combined effects of compton scattering of gamma rays between crystals, crystal penetration of gamma rays, and the imperfection of the optical cross talk identification scheme has been studied under various conditions. The detector pair spatial resolution on axis was measured to be 2.9mm full width at half maximum (FWHM), and the intrinsic peak efficiency was found to be higher than that of the larger detectors currently in use in the Montreal Neurological Institute's (MNI) Positome III camera.     

Phantom experiments on a PSAPD-based compact gamma camera with submillimeter spatial resolution for small animal SPECT

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq (99m)Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.     

A compact gamma camera with scintillation array and parallel-hole collimator

A new compact gamma camera for small object imaging has been developed. It consists of a pixelized Nal（T1） scintillator array coupled to a position sensitive photomultiplier tube （Hamamatsu R2486） with a parallel-hole lead collimator. The compact camera has better spatial resolution than Anger camera. The average value of intrinsic spatial resolutions is 2.3 mm （FWHM）. The overall spatial resolution （FWHM） is 3, 5 and 6 mm at 0, 2.5 and 3 mm SCD （source-to-collimator distance）, respectively. The phantom studies with the compact camera have demonstrated that parallel-hole collimator gamma camera is a practical technique for nuclear medicine application.     

Optimization Design of Crystal Thickness Combination for Dual-layer-offset DOI Detector for Brain PET

Dual-layer-offset crystal array is a low cost and simple scheme for positron emission tomography (PET) system to achieve depth of interaction (DOI) information. Different thickness combinations of two layer crystals influence system performances. Monte Carlo simulations using GATE toolkit were implemented to optimize the design by evaluating the performances of the system with different thickness combinations in this study. Eleven magnetic resonance imaging (MRI) compatible brain PET inserted systems with an inner diameter of 345 mm, dual layer LYSO of 20 mm total thickness and inner layer thickness varied from 0 mm to 10 mm with a step size of 1 mm were built. Six point sources in the x-axis of the center slice from x=0 cm to x=10 cm with an interval of 2 cm were simulated. The sensitivity at the center was calculated and the radial resolutions were evaluated using the reconstructed points by the filtered back projection algorithm. The results show that dual-layer crystal design has a slight sensitivity decrease, but can significantly improve the radial spatial resolution and the resolution uniformity compared with the single-layer design. With the thickness increase of the inner crystal, the average radial spatial resolution decreases and then increases, and achieves minimal at inner layer with a thickness of 8 mm. In conclusion, 8 mm in the inner layer and 12 mm in the outer layer is the optimal design for the brain PET system with 20 mm dual-layer-offset crystal proposed in this study.     

双层错位DOI脑PET探测器晶体厚度组合优化设计

双层错位晶体阵列是正电子发射断层成像（PET）仪获取作用深度（DOI）信息的一种低成本且简便的方案。为获得晶体厚度最优组合方案，本文使用GATE软件进行蒙特卡罗仿真研究。模拟兼容核磁共振成像（MRI）的嵌入式脑PET，环内径345 mm，使用硅酸钇镥（LYSO）晶体，2层晶体总厚度20 mm，内层晶体从0 mm到10 mm共11种厚度（间隔1 mm）；进行点源模拟实验，点源位于中心断层x轴上，偏移从x=0 cm到x=10 cm共6种情况（间隔2 cm）。计算中心点灵敏度；采用滤波反投影算法重建图像，评估径向空间分辨率。结果显示，双层晶体的设计相比于单层晶体灵敏度略有下降，但系统径向空间分辨率显著提高且其均匀性得到改善。随着内层晶体厚度的增大，视野平均径向空间分辨率先减小后增大，在内层晶体厚度为8 mm时达到最小。综上所述，所设计的双层错位DOI脑PET探测器晶体厚度最优组合方案为内层晶体厚度8 mm、外层晶体厚度12 mm。