Microscopic and compartment shape anisotropies in gray and white matter revealed by angular bipolar double‐PFG MR

Diffusion MR has become one of the most important tools for studying neuronal tissues. Conventional single‐pulsed‐field‐gradient methodologies are capable of faithfully depicting diffusion anisotropy in coherently ordered structures, providing important microstructural information; however, it is extremely difficult to characterize randomly oriented compartments using conventional single‐pulsed‐field‐gradient MR. The angular double‐pulsed‐field‐gradient methodology can potentially overcome the limitations of conventional single‐pulsed‐field‐gradient MR, and offer microstructural information on microscopic anisotropy and compartment shape anisotropy even when anisotropic compartments are completely randomly oriented. Here, we used angular double‐pulsed‐field‐gradient MR at different mixing times to study isolated gray matter and white matter, respectively, and the results are compared with phantoms in which compartments are randomly oriented and coherently organized, respectively. We find that angular bipolar double‐pulsed‐field‐gradient MR offers novel microstructural information, especially in the gray matter, depicting the local microscopic and compartment shape anisotropies present. Furthermore, direct comparison between the angular dependencies arising from white and gray matter at different mixing times reveals signatures for these tissues that are based on compartment shape anisotropy. These findings demonstrate that microstructural information can indeed be obtained from gray matter, and therefore, angular double‐pulsed‐field‐gradient MR is promising for future application in MRI of the central‐nervous‐system. Magn Reson Med, 2011. © 2011 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.     

Highly diffusion-sensitized MRI of brain: dissociation of gray and white matter.

The brains of six healthy volunteers were scanned with a full tensor diffusion MRI technique to study the effect of a high b value on diffusion-weighted images (DWIs). The b values ranged from 500 to 5000 s/mm(2). Isotropic DWIs, trace apparent diffusion coefficient (ADC) maps, and fractional anisotropy (FA) maps were created for each b value. As the b value increased, ADC decreased in both the gray and white matter. Furthermore, ADC of the white matter became lower than that of the gray matter, and, as a result, the white matter became brighter than the gray matter in the isotropic DWIs. Quantitative analysis showed that these changes were due to nonmonoexponential diffusion signal decay of the brain tissue, which was more prominent in white matter than in gray matter. There was no significant change in relation to the b value in the FA maps. High b value appears to have a dissociating effect on gray and white matter in DWIs.     

Enhancing measured diffusion anisotropy in gray matter by eliminating CSF contamination with FLAIR.

In this work, the effect of fluid-attenuated inversion recovery (FLAIR) on measured diffusion anisotropy was investigated in gray matter. DTI data were obtained with and without FLAIR in six normal volunteers. The application of FLAIR was experimentally demonstrated to lead to a consistent increase in fractional anisotropy (FA) in gray-matter regions, which was attributed to suppressed partial volume effects from CSF. In addition to these experimental results, Monte Carlo simulations were performed to ascertain the effect of noise on the measured FA under the experimental conditions of this study. The experimentally observed effect of noise was corroborated by the simulation, indicating that the increase in the measured FA was not due to a noise-related bias but to an actual increase in diffusion anisotropy. This enhanced measurement of diffusion anisotropy can be potentially used to differentiate directionally dependent structure and tracking fibers in gray matter.     

Observation of microscopic diffusion anisotropy in the spinal cord using double-pulsed gradient spin echo MRI.

A double-pulsed gradient spin echo (d-PGSE) filtered MRI sequence is proposed to detect microscopic diffusion anisotropy in heterogeneous specimen. The technique was developed, in particular, to characterize local microscopic anisotropy in specimens that are macroscopically isotropic, such as gray matter. In such samples, diffusion tensor MRI (DTI) produces an isotropic or nearly isotropic diffusion tensor despite the fact that the medium may be anisotropic at a microscopic length scale. Using d-PGSE filtered MRI, microscopic anisotropy was observed in a "gray matter" phantom consisting of randomly oriented tubes filled with water, as well as in fixed pig spinal cord, within a range of b-values that can be readily achieved on clinical and small animal MR scanners. These findings suggest a potential use for this new contrast mechanism in clinical studies and biological research applications.     

Characterization of anisotropy in high angular resolution diffusion-weighted MRI.

The methods of group theory are applied to the problem of characterizing the diffusion measured in high angular resolution MR experiments. This leads to a natural representation of the local diffusion in terms of spherical harmonics. In this representation, it is shown that isotropic diffusion, anisotropic diffusion from a single fiber, and anisotropic diffusion from multiple fiber directions fall into distinct and separable channels. This decomposition can be determined for any voxel without any prior information by a spherical harmonic transform, and for special cases the magnitude and orientation of the local diffusion may be determined. Moreover, non-diffusion-related asymmetries produced by experimental artifacts fall into channels distinct from the fiber channels, thereby allowing their separation and a subsequent reduction in noise from the reconstructed fibers. In the case of a single fiber, the method reduces identically to the standard diffusion tensor method. The method is applied to normal volunteer brain data collected with a stimulated echo spiral high angular resolution diffusion-weighted (HARD) acquisition.     

脑灰质异位的CT及MRI诊断

目的:分析脑灰质异位(HGM)的CT及MRI表现,加深对本病的认识,减少漏诊及误诊的分生。方法:搜集我院经临床、CT和/或MRI证实的HGM17例,CT检查9例,MRI检合5例,CT MRI检查3例,回顾性分析其影像学表现。结果:17例中,单侧12例(左侧8例,右侧4例),其中室管膜下结节型3例,混合型桥带型5例,非室管膜下结节型4例。非室管膜下结节型均为单发,额顶叶、颞顶叶、顶枕叶及半卵圆中心各1例:双侧5例,室管膜下结节型2例,其中1例合并胼胝体发育不全;室管膜下弥漫型2例,其中1例合并四叠体池及枕大池蛛网膜囊肿;非室管膜下弥漫性1例。上述病灶位于脑白质内、侧脑室室管膜下或两省均有,呈结节、团块或条带状,在CT及MRI各序列上均与脑灰质密度或信号相同。结论:HGM在CT及MRI上具有特征性的影像学表现,因MRI为多参数成像,在对不典型及小HGM的检出优于CT。     

Double‐wave‐vector diffusion‐weighted imaging reveals microscopic diffusion anisotropy in the living human brain

Diffusion‐tensor imaging is widely used to characterize diffusion in biological tissue, however, the derived anisotropy information, e.g., the fractional anisotropy, is ambiguous. For instance, low values of the diffusion anisotropy in brain white matter voxels may reflect a reduced axon density, i.e., a loss of fibers, or a lower fiber coherence within the voxel, e.g., more crossing fibers. This ambiguity can be avoided with experiments involving two diffusion‐weighting periods applied successively in a single acquisition, so‐called double‐wave‐vector or double‐pulsed‐field‐gradient experiments. For a long mixing time between the two periods such experiments are sensitive to the cells' eccentricity, i.e., the diffusion anisotropy present on a microscopic scale. In this study, it is shown that this microscopic diffusion anisotropy can be detected in white matter in the living human brain, even in a macroscopically isotropic region‐of‐interest (fractional anisotropy = 0). The underlying signal difference between parallel and orthogonal wave vector orientations does not show up in standard diffusion‐weighting experiments but is specific to the double‐wave‐vector experiment. Furthermore, the modulation amplitude observed is very similar for regions‐of‐interest with different fractional anisotrpy values. Thus, double‐wave‐vector experiments may provide a direct and reliable access to white matter integrity independent of the actual fiber orientation distribution within the voxel. Magn Reson Med 69:1072–1082, 2013. © 2012 Wiley Periodicals, Inc.     

Measurement of GABA and contaminants in gray and white matter in human brain in vivo.

A preliminary study of discrimination between GABA and macromolecules (MMs) in human brain by proton double quantum filtering (DQF) at 3.0 T in vivo is presented. GABA-tuned and MM-tuned DQ filters were designed with dual-band 180 degrees radiofrequency (RF) pulses that were tuned for selective refocusing of GABA (3.0 and 1.9 ppm) and putative MM resonances (3.0 and 1.7 ppm), respectively. GABA and putative MM signals were extracted from a combined analysis of the filtered mixture signals and the calculated editing yields. Unexpectedly, the GABA and putative MM signals exhibited a similar doublet linewidth at the optimized TE = 82 ms. Furthermore, substantial MM-tuned DQF signal remained at TE = 148 ms, indicating the presence of a component other than MM. With water segmentation data, the GABA-tuned and MM-tuned DQF measures from the medial prefrontal and left frontal lobes were combined to give the concentrations of GABA and the additional component as 1.1 +/- 0.1 and 0.8 +/- 0.1 mM (mean +/- SD, N=3) for gray matter (GM) and 0.4 +/- 0.1 and 0.7+/-0.1 mM (N=3) for white matter (WM), respectively.     

Microstructural changes in ischemic cortical gray matter predicted by a model of diffusion-weighted MRI

PURPOSE: To understand the diffusion attenuated MR signal from normal and ischemic brain tissue in order to extract structural and physiological information using mathematical modeling, taking into account the transverse relaxation rates in gray matter. MATERIALS AND METHODS: We fit our diffusion model to the diffusion-weighted MR signal obtained from cortical gray matter in healthy subjects. Our model includes variable volume fractions, intracellular restriction effects, and exchange between compartments in addition to individual diffusion coefficients and transverse relaxation rates for each compartment. A global optimum was found from a wide range of parameter permutations using cluster computing. We also present simulations of cell swelling and changes of exchange rate and intracellular diffusion as possible cellular mechanisms in ischemia. RESULTS: Our model estimates an extracellular volume fraction of 0.19 in accordance with the accepted value from histology. The absolute apparent diffusion coefficient obtained from the model was similar to that of experiments. The model and the experimental results indicate significant differences in diffusion and transverse relaxation between the tissue compartments and slow water exchange. Our model reproduces the signal changes observed in ischemia via physiologically credible mechanisms. CONCLUSION: Our modeling suggests that transverse relaxation has a profound influence on the diffusion attenuated MR signal. Our simulations indicate cell swelling as the primary cause of the diffusion changes seen in the acute phase of brain ischemia.     

MRI in the diagnosis of heterotopic gray matter

Summary This report illustrates the MR appearance of three cases of heterotopic gray matter first discovered in adulthood.     

磁共振脑灰白质成像在脑灰质异位症中的临床应用

目的：探讨磁共振脑灰白质成像在脑灰质异位症中的诊断价值。方法：回顾性分析5例经临床及MRI检查确诊的脑灰质异位症患者，全部患者均行脑MRI常规T1WI、T2WI序列、FLAIR序列和脑灰白质成像检查。结果：5例脑灰质异位症患者在MRI各扫描序列上病灶信号与脑灰质信号完全相同，特别是在脑灰白质成像上对异位的灰质灶观察更为满意，更有利于判断病变的性质。结论：脑灰白质成像在观察灰质异位方面有独到价值，在分析癫痫病因方面可提供更直接的影像学信息。对于脑灰质异位症患者，脑灰白质成像对于帮助发现病灶并判断其性质甚为必要，是对常规序列的必要补充，具有较高的临床应用价值。     

Anisotropy in high angular resolution diffusion-weighted MRI.

The diffusion in voxels with multidirectional fibers can be quite complicated and not necessarily well characterized by the standard diffusion tensor model. High angular resolution diffusion-weighted acquisitions have recently been proposed as a method to investigate such voxels, but the reconstruction methods proposed require sophisticated estimation schemes. We present here a simple algorithm for the identification of diffusion anisotropy based upon the variance of the estimated apparent diffusion coefficient (ADC) as a function of measurement direction. The rationale for this method is discussed, and results in normal human subjects acquired with a novel diffusion-weighted stimulated-echo spiral acquisition are presented which distinguish areas of anisotropy that are not apparent in the relative anisotropy maps derived from the standard diffusion tensor model. Published 2001 Wiley-Liss, Inc.     

Diffusion anisotropy in subcortical white matter and cortical gray matter: changes with aging and the role of CSF-suppression

PURPOSE: To determine the relevance of cerebrospinal fluid (CSF)-suppression for the measurement of diffusion anisotropy in well-localized areas of the brain, particularly the subcortical white matter (WM) within the gyri and cortical gray matter (GM), in young and elderly subjects, and to assess the changes of water diffusivity in the brain with normal aging. MATERIALS AND METHODS: Quantitative measures of anisotropy in 26 regions, including subcortical WM (i.e., in the gyri), cortical GM, major deep WM, and deep GM regions of young (21-25 years, N = 8) and elderly (61-74 years, N = 10) normal volunteers, were assessed with CSF-suppressed diffusion tensor imaging (DTI) relative to standard DTI. RESULTS: CSF-suppressed DTI demonstrated significant increases in fractional anisotropy (FA) of 3-12% in the young and 2-14% in the elderly groups with the largest changes being in the subcortical WM of the gyri. Furthermore, FA decreased by 10-19% in the subcortical WM of the gyri of the elderly subjects relative to the young, primarily due to increases in the perpendicular diffusivity, lambda(3), with age. CONCLUSION: CSF-suppressed DTI yields more accurate measures of quantitative anisotropy in cortical and subcortical brain regions. Reductions of anisotropy with aging were predominantly observed in subcortical WM of the gyri.