Basic concepts of MR imaging, diffusion MR imaging, and diffusion tensor imaging

MR image contrast is based on intrinsic tissue properties and specific pulse sequences and parameter adjustments. A growing number of MRI imaging applications are based on diffusion properties of water. To better understand MRI diffusion-weighted imaging, a brief overview of MR physics is presented in this article followed by physics of the evolving techniques of diffusion MR imaging and diffusion tensor imaging.     

非酮症性高血糖偏侧舞蹈症脑部CT及MRI表现

目的分析非酮症性高血糖偏侧舞蹈症患者脑部CT及MRI影像学特点。方法回顾性分析11例经临床确诊的非酮症性高血糖偏身舞蹈症患者的脑部CT及MRI表现。11例患者均接受至少一次CT及MRI平扫,其中2例行MRI增强检查,4例加做DWI序列,6例加做MRS序列,2例加做DTI序列,3例复查MRI平扫,1例复查CT平扫。结果 9例CT见豆状核和/或尾状核头部片状高密度影,2例CT阴性;T_1WI 9例高信号,2例阴性;DWI 2例高低混杂信号,1例低信号,1例阴性;MRS检查:NAA峰6例均降低,Cho峰6例均升高,NAA/Cr比值6例均降低,Cho/Cr比值6例均升高;DTI 1例显示左侧内囊前肢纤维束受损,FA值减低,1例阴性。结论非酮症性高血糖偏侧舞蹈症脑部CT及MRI均具有特征性影像表现,DWI、MRS及DTI等序列对确定病变性质、评估神经元丢失或损伤、纤维束受损等有价值。     

足月儿缺氧缺血性脑病的常规MRI、DWI、DTI及MRS表现

近年,先进的磁共振(MR)技术和3特斯拉磁体(3T)已越来越多地被应用于新生儿,拓宽了围产期缺氧缺血性脑病(HIE)的知识.本文介绍了足月儿缺氧缺血性肺病在3T磁共振的表现,其中包括常规和先进的成像技术,如扩散加权成像(DWI),扩散张量成像(DTI)和磁共振波谱(MRS);重点强调了每种技术发现病灶的时间过程,以及与...     

Direct estimation of the fiber orientation density function from diffusion-weighted MRI data using spherical deconvolution

Diffusion-weighted magnetic resonance imaging can provide information related to the arrangement of white matter fibers. The diffusion tensor is the model most commonly used to derive the orientation of the fibers within a voxel. However, this model has been shown to fail in regions containing several fiber populations with distinct orientations. A number of alternative models have been suggested, such as multiple tensor fitting, q-space, and Q-ball imaging. However, each of these has inherent limitations. In this study, we propose a novel method for estimating the fiber orientation distribution directly from high angular resolution diffusion-weighted MR data without the need for prior assumptions regarding the number of fiber populations present. We assume that all white matter fiber bundles in the brain share identical diffusion characteristics, thus implicitly assigning any differences in diffusion anisotropy to partial volume effects. The diffusion-weighted signal attenuation measured over the surface of a sphere can then be expressed as the convolution over the sphere of a response function (the diffusion-weighted attenuation profile for a typical fiber bundle) with the fiber orientation density function (ODF). The fiber ODF (the distribution of fiber orientations within the voxel) can therefore be obtained using spherical deconvolution. The properties of the technique are demonstrated using simulations and on data acquired from a volunteer using a standard 1.5-T clinical scanner. The technique can recover the fiber ODF in regions of multiple fiber crossing and holds promise for applications such as tractography.     

Contributions of diffusion-weighted imaging to fetal and neonatal imaging

Diffusion-weighted imaging has contributed to our understanding of fetal and neonatal brain development. This article will review the role of diffusion-weighted imaging and diffusion tensor imaging in fetuses and neonates, focusing primarily on normal brain developmental processes and then on pathologic processes that can be characterized using diffusion tensor imaging.     

磁共振扩散成像在脑胶质瘤预后预测的研究进展

脑胶质瘤是中枢神经系统最常见的恶性肿瘤,不同病理级别及基因型的脑胶质瘤预后差别很大。MR扩散成像通过检测水分子的微观运动反映组织结构的变化,对于脑胶质瘤预后预测有着重要的临床意义。扩散张量成像、扩散峰度成像、体素内非相干运动成像、拉伸指数模型扩散加权成像、超高b值DWI成像、神经突起方向离散度与密度成像等可以定量检测组织内水分子的扩散信息,反映肿瘤的异质性及细胞增殖情况,这为准确预测脑胶质瘤预后提供了新的思路。作者对MR扩散成像在脑胶质瘤预后预测的研究进展进行综述。     

扩散张量磁共振成像原理及其在颅内肿瘤中的应用

扩散张量磁共振成像是一种非创性显示和分析白质束的新技术,在临床上越来越多地应用于对颅内肿瘤的研究,它可以观察白质束受肿瘤破坏的异常表现,预测肿瘤的恶性程度, 指导手术计划的制定.本文就扩散张量成像的基本原理、图像显示技术及其在颅内肿瘤的应用进行综述.     

Advances in magnetic resonance neuroimaging techniques in the evaluation of neonatal encephalopathy

Magnetic resonance (MR) imaging has become an essential tool in the evaluation of neonatal encephalopathy. Magnetic resonance-compatible neonatal incubators allow sick neonates to be transported to the MR scanner, and neonatal head coils can improve signal-to-noise ratio, critical for advanced MR imaging techniques. Refinement of conventional imaging techniques include the use of PROPELLER techniques for motion correction. Magnetic resonance spectroscopic imaging and diffusion tensor imaging provide quantitative assessment of both brain development and brain injury in the newborn with respect to metabolite abnormalities and hypoxic-ischemic injury. Knowledge of normal developmental changes in MR spectroscopy metabolite concentration and diffusion tensor metrics is essential to interpret pathological cases. Perfusion MR and functional MR can provide additional physiological information. Both MR spectroscopy and diffusion tensor imaging can provide additional information in the differential of neonatal encephalopathy, including perinatal white matter injury, hypoxic-ischemic brain injury, metabolic disease, infection, and birth injury.     

Mild traumatic brain injury: is diffusion imaging ready for primetime in forensic medicine?

ABSTRACT: Mild traumatic brain injury (MTBI) is difficult to accurately assess with conventional imaging because such approaches usually fail to detect any evidence of brain damage. Recent studies of MTBI patients using diffusion-weighted imaging and diffusion tensor imaging suggest that these techniques have the potential to help grade tissue damage severity, track its development, and provide prognostic markers for clinical outcome. Although these results are promising and indicate that the forensic diagnosis of MTBI might eventually benefit from the use of diffusion-weighted imaging and diffusion tensor imaging, healthy skepticism and caution should be exercised with regard to interpreting their meaning because there is no consensus about which methods of data analysis to use and very few investigations have been conducted, of which most have been small in sample size and examined patients at only one time point after injury.     

Hepatic epithelioid hemangioendothelioma: findings at CT and MRI including preliminary observations at diffusion-weighted echo-planar imaging

Objective  

Hepatic epithelioid hemangioendothelioma (EHE) is a rare vascular tumor with low- to intermediate-grade malignant potential. We describe cross-sectional imaging findings of pathologically confirmed EHE including preliminary observations on lesion characteristics and apparent diffusion coefficients (ADCs) at diffusion-weighted MR imaging (DWI).     

扩散张量成像在不典型脑膜瘤与星形细胞肿瘤鉴别诊断中的价值

目的探讨扩散张量成像(DTI)鉴别不典型脑膜瘤与星形细胞肿瘤的价值。材料与方法搜集45例经病理证实的脑肿瘤患者(不典型脑膜瘤17例;不同级别星形细胞肿瘤28例),均做了MR DTI,获得FA图和ADC图,分别测量肿瘤实质区、瘤周区与健侧正常脑白质区的平均FA值和ADC值。用统计学软件SPSS19.0分别对不典型脑膜瘤与星形细胞肿瘤两组肿瘤实质区、瘤周区和健侧正常脑白质区FA值和ADC值进行统计学分析,统计结果均以x±s表示,P<0.05为差异有统计学意义。结果不典型脑膜瘤肿瘤实质区、瘤周区、健侧正常脑白质区FA值分别为0.23±0.05、0.20±0.04、0.28±0.04;星形细胞肿瘤上述3个区FA值分别为0.05±0.01、0.14±0.03、0.26±0.04,上述两组肿瘤实质区、瘤周区FA值比较均有显著统计学差异(P值均<0.01),不典型脑膜瘤肿瘤实质区和瘤周区FA值高于星形细胞肿瘤(P<0.05)。不典型脑膜瘤肿瘤实质区、瘤周区、健侧正常脑白质区ADC值分别为(0.82±0.13)、(1.22±0.32)、(0.73±0.10)×10-9mm2/s;星形细胞肿瘤上述3个区ADC值分别为(1.82±0.17)、(1.71±0.10)、(0.70±0.06)×10-9mm2/s。上述两组肿瘤实质区、周区ADC值比较均有显著统计学差异(P值均<0.01)。结论测量肿瘤实质区和瘤周区FA值和ADC值可鉴别不典型脑膜瘤和星形细胞肿瘤。     

How to correct susceptibility distortions in spin-echo echo-planar images: application to diffusion tensor imaging

Diffusion tensor imaging is often performed by acquiring a series of diffusion-weighted spin-echo echo-planar images with different direction diffusion gradients. A problem of echo-planar images is the geometrical distortions that obtain near junctions between tissues of differing magnetic susceptibility. This results in distorted diffusion-tensor maps. To resolve this we suggest acquiring two images for each diffusion gradient; one with bottom-up and one with top-down traversal of k-space in the phase-encode direction. This achieves the simultaneous goals of providing information on the underlying displacement field and intensity maps with adequate spatial sampling density even in distorted areas. The resulting DT maps exhibit considerably higher geometric fidelity, as assessed by comparison to an image volume acquired using a conventional 3D MR technique.     

全身磁共振弥散加权成像对骨转移性病变的诊断价值

目的探讨磁共振全身弥散成像对骨转移性病变的诊断价值。方法32例恶性肿瘤患者行全身磁共振弥散成像检查。使用反转恢复平面回波弥散序列分5段进行全身扫描。结果反转恢复平面回波弥散序列成功进行了全身磁共振弥散成像。重建出的最大密度投影像图像均一,清晰。32例恶性肿瘤患者中19例患者检出了多发骨转移灶;7例患者为单发转移灶;2例淋巴瘤表现为骨髓信号增高;1例为多发骨髓瘤;3例假阴性。结论全身磁共振弥散成像对骨转移性病变是一种有效的检出方法。     

Diffusion tensor imaging in cerebral tumor diagnosis and therapy

Diffusion-weighted MR images and their analysis using the tensor model open up many new possibilities for tissue characterization, surgical planning, and treatment follow-up in patients with cerebral neoplasms. These possibilities are only just beginning to be fully explored. This article reviews the physical principles underlying diffusion tensor imaging (DTI); the various postprocessing methods available for DTI data in the context of tumor imaging; the commonly encountered patterns of tumor-related alteration to cerebral white matter, as depicted by directionally encoded color maps; and the current state of the art in DTI-based tumor diagnosis and treatment planning.     

磁共振扩散加权成像在肾盂积水及结核性脓肾鉴别中的应用

目的探讨磁共振扩散加权成像技术在结核性脓肾与肾积水鉴别诊断中的价值.方法回顾性分析了经手术病理证实的7例结核性脓肾及10例肾积水病人的磁共振扩散加权成像(DWI)的图像特点.结果首先利用扩散加权图像拟合出表观扩散系数(ADC)图,随后对肾脏集合系统进行ADC的测量.在扩散图中肾盂积水病人肾盂内信号的强度低于结核性脓肾,而结核性脓肾肾盂内所测的ADC低于肾盂积水.结论磁共振扩散加权成像是一种可信度较高且无创的结核性脓肾与肾积水的鉴别诊断方法.     

磁共振功能性成像用于胶质瘤术前术中的研究进展

脑胶质瘤是最常见原发脑内肿瘤,起源于神经胶质细胞,以局部侵袭性生长为特征。MRI是诊断脑胶质瘤的主要影像手段。随着MR波谱成像(MR spectroscopy,MRS)、扩散张量成像(diffusion tensor imaging,DTI)和血氧水平依赖性功能磁共振成像(blood oxygen level-dependent functional magnetic resonance imaging,BOLD-f MRI)等功能MRI的发展,对于以手术为主的胶质瘤,能进行个性化的术前规划和术中监测,从而降低术后损伤,改善预后。作者就MRI及MRS、DTI、BOLD-f MRI这三种功能成像对胶质瘤术前规划和术中监测及存在的问题进行综述。     

High b-value and diffusion tensor imaging in a canine model of dysmyelination and brain maturation

Recent studies in rodents have demonstrated that diffusion imaging is highly sensitive to differences in myelination. These studies suggest that demyelination/dysmyelination cause increases in the radial diffusivity from diffusion tensor imaging (DTI) measurements and decreases in the restricted diffusion component from high b-value diffusion-weighted imaging experiments. In this study, the shaking pup (sh pup), a canine model of dysmyelination, was studied on a clinical MRI scanner using a combination of conventional diffusion tensor imaging and high b-value diffusion-weighted imaging methods. Diffusion measurements were compared between control dogs and sh pups in the age range 3 months to 16 months, which is similar to the period of early childhood through adolescence in humans. The study revealed significant group differences in nearly all diffusion measures with the largest differences in the zero-displacement probability (Po) from high b-value DWI and the radial diffusivity from DTI, which are consistent with the observations from the published rodent studies. Age-related changes in Po, FA, mean diffusivity, radial diffusivity and axial diffusivity were observed in whole brain white matter for the control dogs, but not the sh pups. Regionally, age-related changes in the sh pup white matter were observed for Po, mean diffusivity and radial diffusivity in the internal capsule, which may be indicative of mild myelination. These studies demonstrate that DWI may be used to study myelin abnormalities and brain development in large animal models on clinical MRI scanners, which are more amenable to translation to human studies.     

兔VX2肝癌模型动态磁共振扩散张量成像的量化研究

目的探讨对兔VX2种植型肝癌模型的成瘤过程进行动态磁共振扩散张量成像(DTI)量化研究的可行性及价值。方法在肿瘤植入后第14、18、22、26天分别对4组共16只新西兰大白兔VX2种植型肝癌模型和4组共4只正常新西兰大白兔进行磁共振成像及DTI;动态量化分析平均扩散系数(DCavg)、部分各向异性值(FA)的变化规律并与病理结果对照。结果兔VX2种植型肝癌模型的DCavg呈先递增后递减趋势;FA呈先递减后递增趋势;与对照组间差异有显著性;病理结果的多项分析中,瘤内凝固性坏死和纤维增生呈明显递增趋势。结论DTI的量化指标能够动态观测兔VX2种植型肝癌模型的部分生长特性;为DTI对肝脏病变的临床应用提供一定的理论指导。     

A novel tensor distribution model for the diffusion-weighted MR signal

Diffusion MRI is a non-invasive imaging technique that allows the measurement of water molecule diffusion through tissue in vivo. The directional features of water diffusion allow one to infer the connectivity patterns prevalent in tissue and possibly track changes in this connectivity over time for various clinical applications. In this paper, we present a novel statistical model for diffusion-weighted MR signal attenuation which postulates that the water molecule diffusion can be characterized by a continuous mixture of diffusion tensors. An interesting observation is that this continuous mixture and the MR signal attenuation are related through the Laplace transform of a probability distribution over symmetric positive definite matrices. We then show that when the mixing distribution is a Wishart distribution, the resulting closed form of the Laplace transform leads to a Rigaut-type asymptotic fractal expression, which has been phenomenologically used in the past to explain the MR signal decay but never with a rigorous mathematical justification until now. Our model not only includes the traditional diffusion tensor model as a special instance in the limiting case, but also can be adjusted to describe complex tissue structure involving multiple fiber populations. Using this new model in conjunction with a spherical deconvolution approach, we present an efficient scheme for estimating the water molecule displacement probability functions on a voxel-by-voxel basis. Experimental results on both simulations and real data are presented to demonstrate the robustness and accuracy of the proposed algorithms.     

Diffusion- and perfusion-weighted MR imaging in persistent migrainous visual disturbances

Pathological changes on diffusion-weighted MR scans had been described in hemiplegic migraine and perfusion changes had been demonstrated in typical migraine aura with radio-isotope studies and, more recently, MR perfusion imaging. However, there is relatively little knowledge of the pathophysiology of long-lasting migraine aura and its possibly variant phenotype, visual snow. Our aim was to investigate with advanced MR techniques whether patients with long-lasting visual disturbance showed regional alterations in cerebral water diffusion and perfusion. We have studied four patients using MR perfusion and MR diffusion imaging. Two patients had typical visual aura and two had a primary persistent visual disturbance (visual snow phenomenon). All patients had normal conventional structural MR imaging. MR diffusion-weighted images were acquired with a b-value of up to 1000 s/mm2. From the diffusion weighted images we generated maps of apparent diffusion coefficient (ADC), which were inspected visually and used for ADC measurements of predefined regions of interest, which included the visual, frontal, insular and temporal cortices. MR perfusion imaging was performed using a bolus tracking technique with dynamic susceptibility-weighted images. Colour coded maps of relative cerebral blood volume, mean transit time and bolus arrival time were generated, as well as time-signal intensity curves over the anterior, middle and posterior cerebral artery territories. The maps of the ADC and above perfusion parameters appeared symmetrical in all patients with no evidence of decreased water diffusion or cerebral perfusion in the occipital regions, or elsewhere. There was no statistically significant difference between the ADC measurements of the primary visual cortices and other cortical regions. Our findings suggest that regional changes in cerebral water diffusion and perfusion do not play an important part in the pathophysiology of persistent migraine aura or primary persistent visual disturbance.