磁共振扩散张量成像的可视化方法研究进展

磁共振扩散张量成像(DT-MRI)与传统的扩散加权成像(DWI)相比能够更加准确地反映分子扩散的方向.大脑白质区的水分子扩散表现出显著的各向异性,因而DT-MRI技术在显示白质神经纤维和功能束的走行方向以及三维形态等方面具有极大的优越性.主要介绍了该技术的基本原理及目前常用的数据集可视化的方法.     

正常成人近段坐骨神经的弥散张量成像**

背景：弥散张量成像及神经纤维束示踪的出现为外周神经细微结构的显示及定量分析提供了新的方法。 目的：前瞻性分析健康成人大腿近段坐骨神经纤维束示踪、弥散张量成像的可行性及最佳成像参数。 方法：采用单次激发自旋回波-平面回波技术对28名健康志愿者双侧坐骨神经进行弥散张量成像及神经纤维束示踪,b值分别为1 200,1 400,1 600 s/mm2。 结果与结论：弥散张量成像及神经纤维束示踪成功者26名,成功率93%,神经纤维束示踪图上能清晰显示近段坐骨神经,与T1WI上解剖图像融合较好。两侧坐骨神经具有相同的弥散特征：随着b 值增加,信噪比逐渐减少,b值为1 200 s/mm2,信噪比值最大为142.72±32.25,神经纤维束长度最长,所占体素最大,但不同b值的弥散张量参数无差异(P > 0.05),且两侧坐骨神经弥散张量参数无差异。说明正常成人大腿近段坐骨神经的弥散张量成像及经纤维束示踪是可行的,可清晰显示坐骨神经走行及弥散特征;最佳b值为1 200 s/mm2。 关键词：弥散张量成像;坐骨神经;磁共振成像;神经纤维束示踪术;成年人 doi:10.3969/j.issn.1673-8225.2012.09.030     

扩散张量MR成像研究

本文一方面综述了8年来国际上扩散张量磁共振成像(DTI)的研究进展情况。包括总结了到目前为止所使用的4种提高DTI成像精度的方法，并指出精度的提高依赖于成像中脉冲序列的优化、实验方法的完善和后处理算法两个方面。文中比较了目前研究扩散张量的两种基本模型：扩散张量模型和q-空间模型。指出这两种模型侧重应用于不同的场合。前者侧重于研究器官或宏观组织中的扩散，而后者侧重于研究小到细胞尺度(μm量级)的扩散行为。两者在应用研究方面是互补的，所要求的实验条件是有差别的。另一方面结合文献对扩散张量模型的实验条件进行了理论分析。认为b因子取得过高并不合理。并用DTI实验数据和结果进行了初步验证。为了改进扩散张量模型本文探讨了考虑多指数衰减的设想。文中综述了DTI的导出量和一些实验结果，在此基础上分析了设计和优化各向异性指标(DAI)的原则。对于DTI的重要导出结果神经纤维柬成像(fiber tractography)重点分析了造成其成像精度不高的主要因素，指出了改进纤维束传导方向甄选算法和寻求纤维束方向场的几何性质是两种可能的解决办法。     

应用DTI技术对人脑白质纤维束的初步研究

目的建立扩散张量纤维束成像对人脑白质纤维的显示方法,并应用中国数字化可视人体数据进行对照观察,验证扩散张量成像(DTI)方法的可靠性。方法选择5名健康志愿者进行DTI成像,采用DtiStudio软件进行分析处理,重建出部分各向异性(FA)图、容积比(VR)图、相对各向异性(RA)图、表面扩散系数(ADC)图以及二维彩色张量图。应用中国数字化可视人体数据集断面图像、FA图及彩色FA图进行对照观察,利用fibertracking纤维跟踪软件及3DMRI软件进行三维重建显示脑内主要白质纤维束,辨认脑内白质纤维束的位置、形态。结果应用DTI纤维束成像可以清晰准确地描绘脑白质内主要神经纤维束的解剖图谱,包括联络纤维如弓形纤维、钩束、扣带束、上纵束和下纵束,连合纤维如胼胝体、前连合和穹隆,投射纤维如锥体束、视放射、内侧丘系等。DTI纤维束成像结果与已知解剖知识、中国可视化人体断面图像具有很好的一致性。结论应用DTI纤维束成像可以清晰准确地描绘脑白质内主要神经纤维束的解剖图谱,其结果与中国可视化人体断面图像、已知解剖知识是一致的,应用DTI纤维束成像研究脑内纤维连通性是可靠的。     

基于DT-MRI的纤维跟踪及可视化研究

磁共振扩散张量成像(DT-MRI)是一种新的成像技术,比传统的扩散加权成像(DWI)能够更加准确地反映分子扩散的方向.在脑白质这样具有大量纤维束的组织中,水分子的扩散表现出显著的各向异性,从而有可能从张量信息的各向异性入手,跟踪得到白质纤维束的走行方向.本文介绍了DTI的原理、数据采集与处理方法,提出了一种可变步长的纤维跟踪方法,并以VC /ITK/VTK为开发工具实现了DTI分析和纤维跟踪与可视化.     

磁共振DTI技术和锰离子示踪方法活体定位鼠脑神经纤维

本文应用磁共振(MR)DTI的方法清晰准确描绘出鼠脑白质内主要神经纤维的位置、方向和形态,包括胼胝体、前联合、内囊、外囊、视束、大脑脚、锥体束等。皮质内注射MnCl2后MR成像,可见皮质脊髓束、皮质丘脑束、皮质纹状体束和纹状体黑质通路被示踪,可重复性地表现为特定部位的MR3D-FSPGR图像和T1W像高信号改变,相应区域T2WI为低信号。LFB染色各纤维束位置、形态与DTI-DEC张量图和MnCl2示踪MR图像结果相一致。因此,磁共振DTI技术和锰离子示踪的方法都能够对鼠脑主要神经纤维和投射通路进行定位,能够作为活体示踪脑内神经纤维束的手段。     

DTI技术在脑梗死与肌力功能相关性研究中的应用

大脑皮质脊髓束分布区脑梗死可引起其下行神经纤维发生华勒氏变性（WD）及对侧肢体肌力功能障碍。扩散张量成像（DTI）技术是目前能检测到WD病理变化的有效方法。文章对皮质脊髓束WD的病因及病理变化进行简要介绍,主要针对DTI技术在脑梗死后皮质脊髓束WD与肌力功能相关性研究的进展进行综述,并阐述DTI在脑梗死与肌力功能相关性研究中的应用的不足。随着MRI机器场强的提高,如3.0 T MRI、7.0 T MRI的临床应用,可有效解决信噪比低、容易失真变形及磁场不均匀等问题。DTI技术不但能通过平均扩散率（ADC）、容积比率（VR）及分数各向异性系数（FA）值等进行量化分析,还可进行三维立体可视性分析,DTI技术开辟了脑梗死与肌力相关性研究的有效研究途径,其前景将非常广阔。     

应用白质束成像显示缺氧缺血性脑病患儿的视放射

目的：利用白质束成像显示HIE患儿的视放射，并与正常幼儿进行比较。方法：对8例HIE患JL和8例正常幼儿进行了弥散张量成像扫描，应用白质束成像技术对他们的双侧视放射进行追踪显示其形态，并对HIE患儿和正常幼儿的视放射进行比较。结果：自质束成像显示正常幼儿的双侧视放射能够到达皮层下，并可见分支，而HIE患儿的双侧视放射明显短，分支难以显示：结论：白质束成像在显示脑白质中纤维束的走行和形态，能反映儿童发育中纤维束的异常。     

磁共振扩散张量成像在评价儿童脑内部分皮质脊髓束发育中的应用

目的 应用磁共振扩散成像技术探讨儿童皮质脊髓束年龄和性别差异及其变化规律。 方法 90例无中枢神经系统症状及体征且颅脑磁共振检查正常的儿童（年龄5d～18岁）。按年龄分为5组: 婴儿组（组1,≤1岁）,幼儿组（组2,＞1～3岁）,学龄前组（组3,＞3～6岁）,学龄组（组4,＞6～12岁）,青春发育期组（组5,＞12～18岁）。每个年龄组内再按性别分为男、女2组。各组儿童分别行头部扩散张量成像, 根据感兴趣法选取皮质脊髓束感兴趣区并重建,测量重建的皮质脊髓束的扩散张量参数并进行统计学分析。 结果 各年龄组的表观扩散系数(ADC)值、分数各向异性(FA)值、纤维示踪平均长度、体积以及示踪的纤维束数量不完全相同（P<0.01）;组间的两两比较间发现,1组与2组间ADC值、FA值及示踪纤维平均长度差异具有统计学意义;2组与3组间FA值差异具有统计学意义;3组与4组间ADC值、FA值、示踪纤维平均长度、纤维束数目及体积差异均具有统计学意义。ADC值与年龄间呈负相关,余参数与年龄间呈正相关。结论 儿童皮质脊髓束发育具有阶段性,且具有阶段性特征;磁共振扩散张量技术可用于观测儿童皮质脊髓束,评价其发育状况。     

弥散张量成像在周围神经病变的应用及进展

弥散张量成像（diffusion tensor imaging，DTI）及纤维束示踪术（diffusion tensor tracking，Dr丌）是在常规弥散基础上发展起来的新兴技术，可通过监测水分子的随机运动和测量各向异性来提供组织的显微结构和组织架构等有价值的信息，具有常规MRI技术无法比拟的优势。目前已广泛应用于中枢神经系统多种疾病的诊断及预后评价中。近年来，DTI及DTT示踪技术已逐步应用于周围神经及肌肉病变的诊断。由于神经纤维的组织学特性，沿着神经轴突长轴方向弥散略高于垂直于它们的短轴方向，导致各向异性扩散存在差异，因此，周围神经病变适合采用DTI及DTr技术对细微结构进行评估及量化研究。     

弥散张量纤维束成像在脑干肿瘤手术的应用

目的评估磁共振弥散张量成像(DTI)技术在脑干肿瘤手术中的应用价值。方法报告6例脑干肿瘤患者,依据DTI确定不同的手术方案。结果肿瘤全切除4例,次全切除2例;病理检查证实胶质瘤4例,海绵状血管瘤2例;术前Rankin量表平均为2.22分,术后评分下降0.56分,平均为1.66分。结论 DTI能揭示脑干肿瘤与受累白质纤维束的关系,有助于优化手术方案;为提高手术效果,建议有条件可以考虑将DTI列为脑干肿瘤术前常规检查项目。     

磁共振弥散张量成像的质量控制

磁共振弥散张量成像（DTI）是诊断和分析神经系统疾病的重要方法,但其目前发展受空间分辨率、信噪比及图像质量等限制。本文对近年来MRI系统基础质量保证方法、DTI参数定性测量方法、用于扩散成像的软件方法以及不同纤维跟踪方法等进行综述。     

Color-coded diffusion-tensor-imaging of posterior cingulate fiber tracts in mild cognitive impairment

Different processes like microvascular dysfunction, free radical toxicity, beta-amyloid deposits, and Wallerian degeneration can cause functionally relevant disturbances of cerebral neuronal networks by myelin degeneration. Color-coded diffusion-tensor-imaging (ccDTI) allows the structural identification and quantification of myelinated fiber tracts. Particularly, posterior cingulate fiber tracts, which are regarded as important neuronal substrates of the network representing memory processing can be localized only imprecisely by conventional magnetic resonance imaging techniques. The posterior cingulate bundles were assessed by ccDTI in 17 patients with amnestic mild cognitive impairment (MCI), 25 patients with Alzheimer's dementia (DAT), and 21 age-matched controls. Additionally, DTI values were correlated with memory performance in the delayed verbal recall test. Fractional anisotropy and mean diffusivity differed significantly between MCI and controls, as well as between DAT and controls. Performance in the delayed verbal recall test of the entire study group correlated significantly with posterior cingulate bundle anisotropy and diffusivity. Using ccDTI seems, hence, a favorable strategy to detect and quantify the structural integrity of posterior cingulate white matter in MCI. Alterations of DTI parameters substantiate the involvement of white matter pathology in the development of MCI. Moreover, ccDTI could serve as in vivo method to investigate age and disease-related myelin alterations as potential morphological substrates of cognitive dysfunction.     

Diffusion Tensor Imaging of White Matter Tracts in the Dog Brain

Diffusion weighted imaging sequences are now widely available on Magnetic Resonance Imaging (MRI) scanners. Diffusion Tensor Imaging (DTI) of the brain is able to show white matter tracts and is now commonly used in human medicine to study brain anatomy, tumors, structural pathways,… The purpose of this study was to show the interest of DTI to reveal the white matter fibers in the dogs' brain. DTI MR Images for this study were obtained with a 3 T system of 4 dogs euthanized for other reasons than neurological disorders. Combined fractional anisotropic (FA) and directional maps were obtained in the first 2 hours after death. The heads were amputated immediately after scanning and stored in 10% formalin until preparation for dissection. An experienced anatomist tracked white matter tracts with clinical relevance using the scanner software. The selected tracts were REFVIDume rendered and correlated with gross dissection. Using DTI we were able to track relevant neurological connections, such as the corticospinal tract, the optic and the cerebellar tract. The three dimensional anatomy is better presented using modern visualization techniques. DTI seems to be a valuable tool in order to present clinically relevant white matter tracts to neurological clinicians and researchers. Anat Rec, 296:340–349, 2013. © 2013 Wiley Periodicals, Inc.     

Fiber tracking: principles and strategies - a technical review

The state of the art of reconstruction of the axonal tracts in the central nervous system (CNS) using diffusion tensor imaging (DTI) is reviewed. This relatively new technique has generated much enthusiasm and high expectations because it presently is the only approach available to non-invasively study the three-dimensional architecture of white matter tracts. While there is no doubt that DTI fiber tracking is providing exciting new opportunities to study CNS anatomy, it is very important to understand its limitations. In this review we therefore assess the basic principles and the assumptions that need to be made for each step of the study, including both data acquisition and the elaborate fiber reconstruction algorithms. Special attention is paid to situations where complications may arise, and possible solutions are reviewed. Validation issues and potential future directions and improvements are also discussed.     

磁共振扩散张量成像评价小儿脑病的初步应用

目的应用扩散张量成像（DTI）来进一步评价小儿脑疾病，探讨DTI的诊断价值。方法对15例患各种脑病的小儿进行MRI检查，其中男性10例，女性5例，年龄为生后3天至11岁（平均年龄4．8岁）。采用Philips Intera Achieva 3．0 Tesla的超导MRI仪，用回波平面成像（EPI）的DTI技术，b为800s／mm^2，15个方向。观察彩色分数各向异性（FA）图和三维彩色编码图。结果15例患儿中，发育畸形2例，分别是胼胝体发育不良、巨脑回；脑室旁白质软化（PVL）7例；缺氧缺血性脑病3例；脑软化2例；脑积水1例。在巨脑回病例，常规MRI见右侧脑回发育不良，呈巨脑回畸形，右侧脑室扩大，在DTI上见右侧病变区白质束明显较对侧少。胼胝体发育不良病例在DTI张量图见胼胝体菲薄。在PVL和缺氧缺血性脑病病例均可见白质纤维束在放射冠颜色混杂，方向性混乱。脑软化病例可见白质纤维束部分中断。在脑积水病例可见白质束受压推移。结论DTI能够显示白质束的走向、绕行、交叉及推挤、中断等异常，可能对今后评估小儿脑病的预后转归有帮助。     

Diffusion tensor imaging in acquired blind humans

Retinal implants as a future possible therapy of blindness rely on an intact neural transmission from the retina to the primary visual cortex. By now it remains unknown, in how far the absence of afferent input in blindness affects also the organization of the optic radiation. Using diffusion tensor imaging (DTI), the non-invasive evaluation of large fiber tracts including the optic radiation has become possible. This method is sensitive to changes of the axonal state such as wallerian degeneration. We have compared DTI data from 6 acquired blind patients with those of a group of 11 healthy control subjects. Neither the relative anisotropy quotient of the visual fiber tract and the pyramidal tract showed a statistically significant difference between the blind patients and the control group nor did the absolute values of the relative anisotropy in the pyramidal tract and the visual fiber tract. There was no axonal degeneration of the optic radiation in late onset acquired blindness. With the optic pathways remaining intact, transmitting electric signals of retinal implants to the visual regions of the human brain seems to be possible even after decades of acquired blindness.     

Probing brain connectivity by combined analysis of diffusion MRI tractography and electrocorticography

Electrocorticography (ECoG) allows for measurement of task-related local field potentials directly from cortex in neurosurgical patients. Diffusion tensor imaging (DTI) tractography is an MRI technique that allows for reconstruction of brain white matter tracts, which can be used to infer structural connectivity. This paper reports a novel merger of these two modalities. A processing stream is described in which fiber tracts near intracranial macroelectrodes showing task-related functional responses are isolated to explore structural networks related to working memory maintenance. Results show that ECoG-constrained tractography is useful for revealing structural connectivity patterns related to spatially- and temporally-specific functional responses.     

立体定向电极规划中的神经纤维束示踪

目的基于患者的弥散张量成像数据(diffusion tensor imaging,DTI),在立体定向电极手术置入规划过程中提供电极触点周围神经纤维束信息,以帮助医生在更全面掌握脑解剖结构和神经连接情况的基础上制定电极植入规划方案。方法基于3D Slicer的软件框架和模块化的设计思想,设计了一个可用于计算和呈现电极周围神经束的处理模块。在模块中根据手动规划的立体定向电极路径计算各触点位置的空间坐标,并呈现电极规划方案中电极触点周围的神经纤维束分布情况。结果在T1加权像上手动选择电极路径后,模块可在影像结构上呈现多支电极路径周围神经纤维束的空间分布和连接情况;并依据电极路径在二维影像上的投影位置及斜切面模式下的显示结果,呈现电极与神经纤维束和影像结构之间的相对位置关系。结论本文的处理模块可在结构像基础上进一步提供手动选择路径中电极周围纤维束的空间位置及分布范围的信息,辅助医生规划与调整电极置入路径。