Directional correlation characterization and classification of white matter tracts.

To study the architectural characteristics of white matter (WM) tracts, the directional correlation (DC), defined as the inner product of the major eigenvector of adjacent pixels, was used as a quantitative index to investigate directional similarity in WM tracts. A region-growing algorithm was employed to propagate an area from a seed point as a function of the DC threshold (DCt) to critically evaluate the directional properties of WM tracts. As the DCt was increased, more pixels were excluded from the propagated region as their DC fell below the DCt, and neighboring WM tracts could be distinguished as the area decreased. Taking the DC into account, a systematic classification routine for WM tracts was devised and tested on a mouse brain in vivo. The results show that individual WM tracts possess a high degree of directional similarity, and, by careful choice of the DCt value, the proposed classification algorithm can recognize all possible WM tracts in a given data set.     

In vivo waveguide elastography of white matter tracts in the human brain

White matter is composed primarily of myelinated axons which form fibrous, organized structures and can act as waveguides for the anisotropic propagation of sound. The evaluation of their elastic properties requires both knowledge of the orientation of these waveguides in space, as well as knowledge of the waves propagating along and through them. Here, we present waveguide elastography for the evaluation of the elastic properties of white matter tracts in the human brain, in vivo, using a fusion of diffusion tensor imaging, magnetic resonance elastography, spatial‐spectral filtering, a Helmholtz decomposition, and anisotropic inversions, and apply this method to evaluate the material parameters of the corticospinal tracts of five healthy human volunteers. We begin with an Orthotropic inversion model and demonstrate that redundancies in the solution for the nine elastic coefficients indicate that the corticospinal tracts can be approximated by a Hexagonal model (transverse isotropy) comprised of five elastic coefficients representative of a medium with fibers aligned parallel to a central axis, and provides longitudinal and transverse wave velocities on the order of 5.7 m/s and 2.1 m/s, respectively. This method is intended as a new modality to assess white matter structure and health by means of the evaluation of the anisotropic elasticity tensor of nerve fibers. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Study of the effect of CSF suppression on white matter diffusion anisotropy mapping of healthy human brain.

Healthy human brain diffusion anisotropy maps derived from standard spin echo diffusion tensor imaging (DTI) were compared with those using fluid-attenuated inversion recovery (FLAIR) preparation prior to DTI to null the signal from cerebrospinal fluid (CSF). Consistent comparisons entailed development of DTI postprocessing methods, image masking based on fitting quality, and an objective region-of-interest-based method for assessment of white matter extent. FLAIR DTI achieved an extended delineation of major white-matter tracts (genu, splenium, and body of the corpus callosum) close to large CSF-filled spaces (lateral ventricles), but did not affect representation of tracts remote from CSF (internal and external capsules and coronal radiation). This result, which was detectable qualitatively (visual inspection), was verified quantitatively by analyses of the relative anisotropy (RA) distribution over white matter structures for 11 subjects. FLAIR DTI thus suppresses the CSF signal that otherwise masks underlying anisotropic parenchymal tissue through partial volume averaging.     

The effect of Tai Chi practice on brain white matter structure: a diffusion tensor magnetic resonance imaging study

Diffusion tensor imaging (DTI) measures the displacement of water molecules across tissue components and thus provides information on the microstructure of brain white matter. This study examined the effect of Tai Chi and the relation of Tai Chi experiences and skills with brain white matter. Fractional anisotropy (FA) was obtained from the DTI magnetic resonance images of two group participants, namely, the long-term Tai Chi practitioners and sedentary counterparts. Whole-brain voxel-based analysis showed that the Tai Chi group had higher FA in the splenium of corpus callosum (p = 0.015) than the control group. Rank correlation analysis revealed that in the Tai Chi group, the FA value of the splenium of corpus callosum was moderately related with exercise duration (r = 0.45, p = 0.045) but highly related with skill level (r = 0.699, p = 0.001). Long-term Tai Chi practice could benefit to the brain white matter, and these impacts were correlated with exercise duration and skill level.     

Effects of echo time on diffusion quantification of brain white matter at 1.5T and 3.0T

The aim was to investigate the effects of echo time (TE) on diffusion quantification of brain white matter. Seven rhesus monkeys (all males; age, 4–6 years; weight, 5–7 kg) underwent diffusion tensor imaging (DTI) with a series of TEs in 1.5T and 3.0T MR scanners. The mean diffusivity (MD), fractional anisotropy (FA), primary (λ₁), and transverse eigenvalues (λ₂₃) were measured in a region of interest at the bilateral internal capsule. Pearson correlation showed that the FA and λ₁ increased and λ₂₃ decreased with TE both at 1.5T and 3.0T except for the MD. Repeated measurement analysis of variance (ANOVA) also showed significantly higher FA and lower MD and λ₂₃ at 3.0T than those at 1.5T (P < 0.01), but no statistical differences were found in λ₁ between these two field strengths (P = 0.709). These findings implied that TE and field strength might influence diffusion quantification in brain white matter. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Water diffusion anisotropy in white and gray matter of the human spinal cord

PURPOSE: To develop a reliable technique for diffusion imaging of the human spinal cord at 1.5 Tesla and to assess potential differences in diffusion anisotropy in cross-sectional images. MATERIALS AND METHODS: A single-shot echo-planar imaging sequence with double spin-echo diffusion preparation was optimized regarding cerebrospinal fluid artifacts, effective resolution, and contrast-to-noise ratios. Eleven healthy volunteers participated in the study for quantitative characterization of diffusion anisotropy in white matter (WM) and gray matter (GM) by means of two diffusion encoding schemes: octahedral-six-directions for fractional anisotropy (FA) evaluation and orthogonal-three-directions for anisotropy index (AI) calculation. RESULTS: Pulse-trigger gated sequences with optimal matrix size (read x phase = 64 x 32) and b-value (700 s/mm(2)) allowed the acquisition of high-resolved images (voxel size = 0.9 x 0.9 x 5.0 mm(3)). The GM butterfly shape was recognizable in both AI and FA maps. Both encoding schemes yielded high diffusion anisotropy in dorsal WM (FA = 0.79 +/- 0.07; AI = 0.39 +/- 0.04). Lateral WM showed slightly lower anisotropy (FA = 0.69 +/- 0.08; AI = 0.35 +/- 0.03) than dorsal WM. Clearly smaller anisotropy was found in regions containing GM (FA = 0.45 +/- 0.06; AI = 0.21 +/- 0.05). CONCLUSION: Diffusion anisotropy data of the spinal cord can be obtained in a clinical setting. Its application seems promising for the assessment of neurological disorders.     

Altered white matter diffusion anisotropy in normal and preterm infants at term‐equivalent age

To investigate white matter (WM) development, voxelwise analyses of diffusion tensor MRI (DTMRI) data, acquired from 12 very preterm and 11 preterm infants with gestational ages (GA) ranging from 25 to 29 and 29 to 32 weeks, respectively, and 10 newborn normal term infants were performed. T₂ relaxation measures were also generated to assess brain water content. Compared with newborn term infants, very preterm infants were found to possess reduced fractional anisotropy (FA) within the frontal lobe, and a number of anterior and posterior commissural pathways. Preterm infants possessed reduced FA mainly within the posterior regions of the corpus callosum. Unexpectedly, we observed significantly reduced FA and increased T₂ within a number of corticospinal projections in the newborn term infants compared to the preterm groups. This finding may reflect increased water concentration and/or a lowering of FA due to the presence of crossing interhemispheric WM projections. These findings indicate that care should be taken when interpreting FA indices without knowledge of the possible effects of water concentration in the newborn infant brain. Magn Reson Med 60:761–767, 2008. © 2008 Wiley‐Liss, Inc.     

Assessing atrophy of the major white matter fiber bundles of the brain from diffusion tensor MRI data.

Brain atrophy is a typical feature of many neurological conditions. Therefore, quantitative evaluation and spatial characterization of atrophy are potentially useful for monitoring the evolution of central nervous system (CNS) disorders. In this study, a method for measuring atrophy of the major white matter (WM) fiber bundles in the brain using diffusion tensor (DT) MRI data is developed. To this end, an atlas was created from sets of diffusion anisotropy images from normal subjects, and the deformations necessary to match single subject anisotropy images to this atlas were then computed. Because diffusion anisotropy images were used, this approach should be sensitive to fiber bundle volume changes in the same way that using T1-weighted images allows gray matter volume changes to be measured. The Jacobian determinant of the deformation field for each subject was then used as a measure of contraction or expansion of the tissue at each image voxel. An overview of the nonlinear registration problem is given; then an optimization of the parameters for the chosen algorithm is performed and the method for producing the atlas is described. The effectiveness of the method was then tested on data from five patients with multiple sclerosis (MS) and two patients with amyotrophic lateral sclerosis (ALS).     

Axial asymmetry of water diffusion in brain white matter.

The diffusion tensor (DT) is a three-dimensional (3D) model of diffusivity in biological tissues. In white matter (WM), the major eigenvector, which is the direction of greatest diffusivity, is generally assumed to align with the direction of the fiber bundles. The distribution of major eigenvectors in WM has been investigated using color-based maps and WM tractography (WMT). However, anatomical patterns in the medium and minor eigenvector directions have largely been ignored in DTI studies of the human brain. In this study, the patterns of medium and minor eigenvectors in the brain were investigated using both color-based maps and WMT. Specific WM structures, such as the corona radiata, internal and external capsules, sagittal stratum, cingulum, and superior longitudinal fasciculus, demonstrated coherent patterns in the medium and minor eigenvector directions. These patterns were consistent across subjects. The orthogonal or axial diffusion asymmetry may be explained by merging, diverging, or crossing fiber geometries. The effects of orthogonal diffusion asymmetry on WMT were also investigated. This study shows that WM axial asymmetry causes anisotropic dispersion patterns in the estimated tract trajectories. The medium and minor eigenvector patterns may be useful for elucidating the local dispersion distributions of WM tracts.     

In vivo mapping of the fast and slow diffusion tensors in human brain.

Recent studies have shown that the diffusional signal decay in human brain is non-monoexponential and may be described in terms of compartmentalized water fractions. Diffusion tensor imaging (DTI), which provides information about tissue structure and orientation, typically uses b values up to 1000 s x mm(-2) so that the signal is dominated by the fast diffusing fraction. In this study b factors up to 3500 s x mm(-2) are utilized, allowing the diffusion tensor properties of the more slowly diffusing fraction to be mapped for the first time. The mean diffusivity (MD) of the slow diffusion tensor was found to exhibit strong white/gray matter (WM/GM) contrast. Maps depicting the principal direction of the slow tensor indicated alignment with the fast tensor and the known orientation of the WM pathways.     

Reducing the impact of white matter lesions on automated measures of brain gray and white matter volumes

Purpose:

To develop an automated lesion‐filling technique (LEAP; LEsion Automated Preprocessing) that would reduce lesion‐associated brain tissue segmentation bias (which is known to affect automated brain gray [GM] and white matter [WM] tissue segmentations in people who have multiple sclerosis), and a WM lesion simulation tool with which to test it.

Materials and Methods:

Simulated lesions with differing volumes and signal intensities were added to volumetric brain images from three healthy subjects and then automatically filled with values approximating normal WM. We tested the effects of simulated lesions and lesion‐filling correction with LEAP on SPM‐derived tissue volume estimates.

Results:

GM and WM tissue volume estimates were affected by the presence of WM lesions. With simulated lesion volumes of 15 mL at 70% of normal WM intensity, the effect was to increase GM fractional (relative to intracranial) volumes by ≈2.3%, and reduce WM fractions by ≈3.6%. Lesion filling reduced these errors to ≈0.1%.

Conclusion:

The effect of WM lesions on automated GM and WM volume measures may be considerable and thereby obscure real disease‐mediated volume changes. Lesion filling with values approximating normal WM enables more accurate GM and WM volume measures and should be applicable to structural scans independently of the software used for the segmentation. J. Magn. Reson. Imaging 2010. © 2010 Wiley‐Liss, Inc.     

Diffusion time dependence of the apparent diffusion tensor in healthy human brain and white matter disease.

The diffusion time dependence of the brain water diffusion tensor provides information regarding diffusion restriction and hindrance but has received little attention, primarily due to limitations in gradient amplitude available on clinical MRI systems, required to achieve short diffusion times. Using new, more powerful gradient hardware, the diffusion time dependence of tensor-derived metrics were studied in human brain in the range 8-80 ms, which encompasses the shortest diffusion times studied to date. There was no evidence for a change in mean diffusivity, fractional anisotropy, or in the eigenvalues with diffusion time in healthy human brain. The findings are consistent with a model of unrestricted, but hindered water diffusion with semipermeable membranes, likely originating from the extracellular space in which the average extracellular separation is less than 7 microns. Similar findings in two multiple sclerosis plaques indicated that the size of the water diffusion space in the lesion did not exceed this dimension.     

Characterization of white matter alterations in phenylketonuria by magnetic resonance relaxometry and diffusion tensor imaging.

A multimodal MR study including relaxometry, diffusion tensor imaging (DTI), and MR spectroscopy was performed on patients with classical phenylketonuria (PKU) and matched controls, to improve our understanding of white matter (WM) lesions. Relaxometry yields information on myelin loss or malformation and may substantiate results from DTI attributed to myelin changes. Relaxometry was used to determine four brain compartments in normal-appearing brain tissue (NABT) and in lesions: water in myelin bilayers (myelin water, MW), water in gray matter (GM), water in WM, and water with long relaxation times (cerebrospinal fluid [CSF]-like signals). DTI yielded apparent diffusion coefficients (ADCs) and fractional anisotropies. MW and WM content were reduced in NABT and in lesions of PKU patients, while CSF-like signals were significantly increased. ADC values were reduced in PKU lesions, but also in the corpus callosum. Diffusion anisotropy was reduced in lesions because of a stronger decrease in the longitudinal than in the transverse diffusion. WM content and CSF-like components in lesions correlated with anisotropy and ADC. ADC values in lesions and in the corpus callosum correlated negatively with blood and brain phenylalanine (Phe) concentrations. Intramyelinic edema combined with vacuolization is a likely cause of the WM alterations. Correlations between diffusivity and Phe concentrations confirm vulnerability of WM to high Phe concentrations.     

How does DWI correlate with white matter structures?

Diffusion-weighted MRI (DWI) is widely used to characterize brain white matter (WM), particularly through the use of diffusion tensor imaging (DTI). In this study the spatial characteristics of DWI in WM of cat visual cortex were investigated at 9.4T at very high resolution. It is shown that the spatial extent of the WM tract as measured from the DWI images depends highly on the b-value. In particular, when the diffusion gradient is applied perpendicular to the main direction of the fiber tract, the estimated thickness of the tract at the commonly used b-value of 1000 s/mm2 exceeds by 50% the thickness as it appears on a T2-weighted image. Only at b-values greater than 6000 s/mm2 does the thickness of the tract approach the thickness characterized by the T2-weighted image and that observed on histological slices of the same area. Further analysis of these results indicates that the choice of b-value of 1000 s/mm2 may not be optimal for the demarcation of anisotropic WM structures. DWI at high b-value may contain spatial information that is more specific to WM tracts.     

Directional diffusion in relapsing-remitting multiple sclerosis: a possible in vivo signature of Wallerian degeneration

PURPOSE: To examine the role of directional dependence of the apparent diffusion coefficients in the evaluation of normal-appearing brain regions of patients with relapsing-remitting multiple sclerosis. MATERIALS AND METHODS: The role of diffusion tensor eigenvalues was investigated in the normal-appearing brain regions for 18 patients with relapsing-remitting multiple sclerosis and 15 age-matched normal controls. RESULTS: The isotropic apparent diffusion was increased in all regions. However, reduced anisotropy was significant only in regions with high anisotropy, including the corpus callosum and the internal capsule, and was due to increased diffusion tensor eigenvalues corresponding to diffusion transverse to the fibers without significant increase along the fibers. This characteristic pattern of changes in diffusion tensor eigenvalues has been observed previously in cases of Wallerian degeneration. Low-anisotropy regions corresponded to gray matter and gray/white interface regions. Since fiber tract orientations are not determined for regions of low anisotropy, this characteristic pattern of diffusion change is not detectable in these regions. CONCLUSION: Examination of diffusion tensor eigenvectors may provide insight into the changes observed in diffusion and a signature of Wallerian degeneration in the normal-appearing white matter of relapsing-remitting multiple sclerosis patients.     

创伤性脑白质损伤的扩散张量成像研究

目的:评价扩散张量成像(DTI)在创伤性脑白质损伤(WMI)中的应用价值。方法:16例创伤性脑外伤后经临床诊断有WMI的患者通过Philips 1.5TIntera Achieva MR扫描仪行常规MRI和DTI。后处理获得部分各向异性指数(FA)、表观弥散系数(ADC)和纤维示踪成像三维图。根据T2WI及T2快速场回波图像,分别于WMI区域、同侧同名或对侧同名纤维束正常区域取感兴趣区,测量FA值和ADC值并进行比较。结果:脑外伤患者损伤脑白质中挫伤和出血、仅挫伤和仅出血区域三者之间的FA值(F=0.68,P>0.05)和ADC值(F=0.53,P>0.05)均未见明显不同。除仅出血区域的ADC值与对照区域相比差异无统计学意义(t=1.36,P>0.05),挫伤和出血(t=9.72,P<0.05)、仅挫伤(t=8.28,P<0.05)和仅出血(t=5.44,P<0.05)区域的FA值较正常对照区域明显降低,挫伤和出血(t=4.71,P<0.05)、仅挫伤(t=4.81,P<0.05)的ADC值较正常对照明显增高,纤维示踪成像显示损伤区域脑白质较正常区域稀疏、分离、缺失。结论:DTI技术能够显示患者WMI区域的异常改变,但ADC值对出血的判断有局限性。     

Multiscale white matter fiber tract coregistration: a new feature-based approach to align diffusion tensor data.

In this paper an automatic multiscale feature-based rigid-body coregistration technique for diffusion tensor imaging (DTI) based on the local curvature kappa and torsion tau of the white matter (WM) fiber pathways is presented. As a similarity measure, the mean squared difference (MSD) of corresponding fiber pathways in (kappa, tau)-space is chosen. After the MSD is minimized along the arc length of the curve, principal component analysis is applied to calculate the transformation parameters. In addition, a scale-space representation of the space curves is incorporated, resulting in a multiscale robust coregistration technique. This fully automatic technique inherently allows one to apply region of interest (ROI) coregistration, and is adequate for performing both global and local transformations. Simulations were performed on synthetic DT data to evaluate the coregistration accuracy and precision. An in vivo coregistration example is presented and compared with a voxel-based coregistration approach, demonstrating the feasibility and advantages of the proposed technique to align DT data of the human brain.     

磁共振弥散张量成像对正常大脑白质纤维束构象的初步研究

目的利用磁共振弥散张量成像(DTI)研究正常成人脑内各部位各向异性程度及正常白质纤维束构象特征.方法对25名正常志愿者进行常规MR及DTI序列检查,重建FA图及三维彩色编码张量图.分别在半卵圆中心、基底节区和大脑脚层面测量主要白质束的FA值.结果DTI显示灰质与白质区各向异性存在显著差异,不同部位的白质纤维束各向异性程度亦不相同,且左右两侧基本对称,重建FA图和三维彩色编码张量图可显示白质内大部分主要的白质纤维束.结论DTI可清晰显示脑内白质纤维束的走行及分布,为了解脑功能与白质通路间关系提供了有力研究手段.     

High angular resolution diffusion imaging reveals intravoxel white matter fiber heterogeneity.

Magnetic resonance (MR) diffusion tensor imaging (DTI) can resolve the white matter fiber orientation within a voxel provided that the fibers are strongly aligned. However, a given voxel may contain a distribution of fiber orientations due to, for example, intravoxel fiber crossing. The present study sought to test whether a geodesic, high b-value diffusion gradient sampling scheme could resolve multiple fiber orientations within a single voxel. In regions of fiber crossing the diffusion signal exhibited multiple local maxima/minima as a function of diffusion gradient orientation, indicating the presence of multiple intravoxel fiber orientations. The multimodality of the observed diffusion signal precluded the standard tensor reconstruction, so instead the diffusion signal was modeled as arising from a discrete mixture of Gaussian diffusion processes in slow exchange, and the underlying mixture of tensors was solved for using a gradient descent scheme. The multitensor reconstruction resolved multiple intravoxel fiber populations corresponding to known fiber anatomy. Ma