Dynamic corticospinal white matter connectivity changes during stroke recovery: A diffusion tensor probabilistic tractography study

Purpose

To investigate corticospinal tract connectivity changes at the cortical surface using diffusion tensor imaging (DTI) tractography during recovery from stroke.

Materials and Methods

Using data from 10 stroke patients (four subcortical) and six elderly controls, we developed an automated method to quantify altered motor connectivity that involves the use of a simplified cortical surface model as a seed mask with target regions defined within the corticospinal tracts to initiate a probabilistic tractography algorithm.

Results

We found no change in volume overlap of the generated corticospinal tracts in the stroke patients compared to controls, but significant connectivity changes at the boundary of the simplified cortical surface mask, especially within the ipsilesional hemisphere of stroke patients over time. Using the cortical regions with significantly enhanced connectivity as a seed mask on the patient data, tracts that are directly associated with stroke recovery can be delineated. Measures of uncertainty in fiber orientation within these fiber tracts significantly correlated with functional outcome.

Conclusion

The novel findings from this study highlight the usefulness of this methodology to study white matter repair/reorganization during stroke recovery. J. Magn. Reson. Imaging 2009;29:529–536. © 2009 Wiley‐Liss, Inc.     

MR neurography of the median nerve at 3.0T: optimization of diffusion tensor imaging and fiber tractography

Objectives

The purpose of this study was to systematically assess the optimal b-value and reconstruction parameters for DTI and fiber tractography of the median nerve at 3.0 T.

Methods

Local ethical board approved study with 45 healthy volunteers (15 men, 30 women; mean age, 41 ± 3.4 years) who underwent DTI of the right wrist at 3.0 T. A single-shot echo-planar-imaging sequence (TR/TE 10123/40 ms) was acquired at four different b-values (800, 1000, 1200, and 1400 s/mm²). Two independent readers performed post processing and fiber-tractography. Fractional anisotropy (FA) maps were calculated. Fiber tracts of the median nerve were generated using four different algorithms containing different FA thresholds and different angulation tolerances. Data were evaluated quantitatively and qualitatively.

Results

Tracking algorithms using a minimum FA threshold of 0.2 and a maximum angulation of 10° were significantly better than other algorithms. Fiber tractography generated significantly longer fibers in DTI acquisitions with higher b-values (1200 and 1400 s/mm² versus 800 s/mm²; p < 0.001). The overall quality of fiber tractography was best at a b-value of 1200 s/mm² (p < 0.001).

Conclusions

In conclusion, our results indicate use of b-values between 1000 and 1400 s/mm² for DTI of the median nerve at 3.0 T. Optimal reconstruction parameters for fiber tractography should encompass a minimum FA threshold of 0.2 and a maximum angulation tolerance of 10.     

Limitations and requirements of diffusion tensor fiber tracking: an assessment using simulations.

Diffusion tensor fiber tracking potentially can give information about in vivo brain connectivity. However, this technique is difficult to validate due to the lack of a gold standard. Fiber tracking reliability will depend on the quality of the data and on the robustness of the algorithms used. Information about the effects of various anatomical and image acquisition parameters on fiber tracking reliability may be used in the design of imaging sequences and of tracking algorithms. In this study, tracking was performed on two different simulated models to study the effects on tracking quality of SNR, anisotropy, curvature, fiber cross-section, background anisotropy, step size, and interpolation. Tracking was also performed on volunteer data to assess the relevance of the simulations to real data. Our results show that, in general, tracking with high SNR and high anisotropy using interpolation and a low step size gives the most reliable results. Partial volume effects are shown to have a detrimental effect when the background is anisotropic and when tracking narrow fibers. The results derived from real data show similar trends and thus support the findings of the simulations. These simulations may therefore help to determine which structures can be tracked for a given image quality.     

MR tractography based on directional diffusion function validation in somatotopic organization of the pyramidal tract

RATIONALE AND OBJECTIVES: Conventional tractography based on the "streamline" method only partially visualizes the pyramidal tract because of fiber crossing with other white matter tracts. Recently a new tractography method based on directional diffusion function (DDF) has been proposed. This method was reported to visualize the pyramidal tract to a larger extent than conventional techniques do. To validate the DDF-based tractography method, we studied the somatotopic organization of the pyramidal tract in the posterior limb of the internal capsule (PLIC). MATERIALS AND METHODS: Pyramidal tracts in the intact hemispheres of 14 brain tumor patients were drawn using the directional diffusion function-based tractography method. Each pyramidal tract was divided into four fiber bundles according to the cephalocaudal positions of their termination in the precentral gyrus. The cephalocaudal positions in the precentral gyrus of the four fiber bundles were correlated with their positional relationships in the PLIC along the mediolateral and anteroposterior axes. RESULTS: Fiber bundles terminating more caudally in the precentral gyrus were located significantly more anteriorly in the PLIC (r = 0.59, Spearman's correlation coefficient, P < .0001). On the other hand, no significant correlation was shown between the cephalocaudal positions in the precentral gyrus of the four fiber bundles and their relative positions in the PLIC along the mediolateral axis. CONCLUSIONS: Estimated organization of the fiber bundles of the pyramidal tract in the PLIC was consistent with anatomically known somatotopic organization, which supported the validity of the DDF-based tractography method.     

Diffusion tensor imaging and fiber tractography of the median nerve at 1.5T: optimization of <Emphasis Type="Italic">b</Emphasis> value

Objective  The objective of this study was to systematically assess the optimal b value for diffusion tensor imaging and fiber tractography of the median nerve at 1.5 T. Materials and methods  This is a prospective study which was carried out with institutional review board approval and written informed consent from the study subjects. Fifteen healthy volunteers (seven men, eight women; mean age, 31.2 years) underwent diffusion tensor imaging of the wrist. A single-shot spin-echo-based echo-planar imaging sequence (TR/TE, 7000/103 ms) was performed in each subject at eight different b values ranging from 325 to 1,550 s/mm². Number and length of reconstructed fiber tracts, fiber density index (FDi), fractional anisotropy (FA), and apparent diffusion coefficient (ADC) were calculated for the median nerve. Signal-to-noise ratio (SNR) was also calculated for each acquisition. The overall image quality was assessed by two readers in consensus by ranking representative fiber tract images for each subject using a scale range from 1 to 8 (1 = best to 8 = worst image quality). Results  Longest fibers were observed for b values between 675 and 1,025 s/mm². Maximum FDi was found at b values of 1,025 s/mm². FA was between 0.5 and 0.6 for all b values. ADC gradually decreased from 1.44 × 10⁻³ to 0.92 × 10⁻³ mm²/s with increasing b values. Maximum SNR ± standard deviation (175.4 ± 72.6) was observed at the lowest b value and decreased with increasing b values. SNR at b values of 1,025 s/mm² was 48.5% of the maximum SNR. Optimal fiber tract image quality was found for b values of 1,025 s/mm². Conclusions  The optimal b value for diffusion tensor imaging and fiber tractography of the median nerve at 1.5 T was 1,025 s/mm².     

Fast diffusion tensor imaging and tractography of the whole cervical spinal cord using point spread function corrected echo planar imaging

Diffusion tensor imaging has been used in a number of spinal cord studies, but severe distortions caused by susceptibility induced field inhomogeneities limit its applicability to investigate small volumes within acceptable acquisition times. A way to evaluate image distortions is to map the point spread function of the voxel intensity in a reference scan. In this study, the point spread function was mapped for an echo‐planar imaging sequence in the human cervical spinal cord with isotropic resolution and large field of view. Correction with the point spread function map improved anatomical consistency, and full cervical tractography was thereby possible from a C1 seed region in healthy controls and one individual with spinal cord injury. It is suggested that point spread function mapping of the spinal cord can be used in combination with sequence‐based methods for reduction of susceptibility artifacts or in high‐field imaging settings where off‐resonance effects are pronounced. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Ex vivo diffusion tensor imaging and quantitative tractography of the rat spinal cord during long-term recovery from moderate spinal contusion

Purpose

To characterize DTI metric changes throughout the length of the entire spinal cord from the acute through chronic stages of spinal cord injury (SCI).

Materials and Methods

Ex vivo DTI was performed at 9.4 Tesla to examine changes in water diffusion throughout the entire spinal cord (7‐cm) up to 25 weeks after injury in a rat model of contusive SCI. Animals were grouped according to recovery times after injury (2, 5, 15, 20, or 25 weeks), and various DTI metrics were evaluated including transverse and longitudinal apparent diffusion coefficient (tADC and lADC), mean diffusivity (MD), and fractional anisotropy (FA).

Results

An overall decrease in lADC throughout the cord and decreases in MD remote from the lesion site were observed, along with an increase in tADC within fiber tracts throughout the recovery period. These trends were statistically significant at P < 0.05 and were found in both white and gray matter regions. tADC and lADC distributions in fiber bundles extracted using DTI tractography were well fit by an exponential model (R = 0.998) with time constants of 4.6 and 3.3 days, respectively.

Conclusion

Results from the current study support the hypothesis that the spinal cord undergoes continual changes during recovery from SCI. J. Magn. Reson. Imaging 2008;28:1068–1079. © 2008 Wiley‐Liss, Inc.     

肘部尺神经扩散张量成像技术参数优化的研究

目的：优化肘部尺神经扩散张量成像(DTI)参数。方法使用5组不同 b 值和扩散梯度方向数量(NDGDs)DTI 序列采集13名志愿者肘部尺神经图像并建立扩散示踪图(DTT)。比较不同成像参数条件下,尺神经各向异性分数(FA)、表观扩散系数(ADC)、神经纤维束长度和 DTI 图像质量的差异性。结果18个正常尺神经 DTI 结果纳入研究。不同成像条件下,尺神经 FA 值无明显差异。当 NDGDs 一定时,b 值升高,图像质量下降,尺神经 ADC 值减低;而 NDGDs 对 ADC 值和图像质量无显著影响。b=1000 s/mm2,NDGDs=20时,测得尺神经纤维束长度最长,且 DTT 的主观评分最高。结论以 b=1000 s/mm2,NDGDs=20用于肘部尺神经 DTI,可获得良好的图像质量和稳定的观测指标。     

扩散张量成像在周围神经变性及再生中的研究进展

周围神经损伤后,其远端轴突和髓鞘发生沃勒变性,如何评估轴索再生、残端修复和神经支配区功能重建是临床研究的难题。目前,临床上主要通过神经电生理检查判断神经损伤程度,但对评估神经完全损伤及近端病变缺乏敏感性及特异性。MR扩散张量成像(DTI)能够无创性定量评估周围神经变性及再生过程,联合扩散张量纤维示踪技术(DTT)可追踪神经纤维束的方向、排列、髓鞘脱失等信息。就DTI技术评估周围神经损伤变性及再生的研究进展予以综述。     

Arrangement of fiber tracts forming Probst bundle in complete callosal agenesis: report of two cases with an evaluation by diffusion tensor tractography

We report two patients with complete callosal agenesis in whom Probst bundles in both hemispheres could be depicted by diffusion tensor tractography (DTT). While one patient had no associated telencephalic anomaly other than callosal agenesis, the other had cortical dysplasia in the right frontal lobe. Although Probst bundles in the three normal hemispheres were well developed, that in the hemisphere which was affected by cortical dysplasia was small and poorly developed. DTT also showed that the fibers from the frontal pole ran more on the inner side of the Probst bundle than those from a more caudal region of the frontal lobe. Furthermore, fibers from the orbital gyri ran along the outermost side of Probst bundle. The arrangement of these fiber tracts in Probst bundle may reflect the developmental process of callosal fibers in their normal formation.     

Diffusion tensor imaging and fiber tractography of C6 rat glioma

PURPOSE: To apply diffusion tensor images using 30 noncollinear directions for diffusion-weighted gradient schemes to characterize diffusion tensor imaging (DTI) features associated with C6 glioma-bearing rat brains, and ideally visualize fiber tractography datasets. MATERIALS AND METHODS: Fiber tractographies of normal male Fischer 344 rat brains were constructed from DTI datasets acquired with a 30 noncollinear diffusion gradient scheme. Cultured C6 cell were intracranially injected into the cortex of male Fischer 344 rats. The time course of the tumor growth was monitored with DTI and fiber tractography using diffusion-weighting gradients in 30 noncollinear directions. RESULTS: Fiber tractographies through the corpus callosum (CC) were easily visualized with the 30-direction gradient scheme, and the fiber trajectories of the motor cortex and striatum were well represented in normal rats. Fiber tractography indicated that the neuronal fibers of the CC were compressed or disappeared by growing C6 glioma, which affected surrounding brain tissue. CONCLUSION: We have demonstrated in this study that fiber tractography with the 30 noncollinear diffusion gradient scheme method can be used to help provide a better understanding regarding the influence of a tumor on the surrounding regions of normal brain tissue in vivo.     

Orthogonal tensor invariants and the analysis of diffusion tensor magnetic resonance images.

This paper outlines the mathematical development and application of two analytically orthogonal tensor invariants sets. Diffusion tensors can be mathematically decomposed into shape and orientation information, determined by the eigenvalues and eigenvectors, respectively. The developments herein orthogonally decompose the tensor shape using a set of three orthogonal invariants that characterize the magnitude of isotropy, the magnitude of anisotropy, and the mode of anisotropy. The mode of anisotropy is useful for resolving whether a region of anisotropy is linear anisotropic, orthotropic, or planar anisotropic. Both tensor trace and fractional anisotropy are members of an orthogonal invariant set, but they do not belong to the same set. It is proven that tensor trace and fractional anisotropy are not mutually orthogonal measures of the diffusive process. The results are applied to the analysis and visualization of diffusion tensor magnetic resonance images of the brain in a healthy volunteer. The theoretical developments provide a method for generating scalar maps of the diffusion tensor data, including novel fractional anisotropy maps that are color encoded for the mode of anisotropy and directionally encoded colormaps of only linearly anisotropic structures, rather than of high fractional anisotropy structures.     

Model‐based analysis of multishell diffusion MR data for tractography: How to get over fitting problems

In this article, we highlight an issue that arises when using multiple b‐values in a model‐based analysis of diffusion MR data for tractography. The non‐monoexponential decay, commonly observed in experimental data, is shown to induce overfitting in the distribution of fiber orientations when not considered in the model. Extra fiber orientations perpendicular to the main orientation arise to compensate for the slower apparent signal decay at higher b‐values. We propose a simple extension to the ball and stick model based on a continuous gamma distribution of diffusivities, which significantly improves the fitting and reduces the overfitting. Using in vivo experimental data, we show that this model outperforms a simpler, noise floor model, especially at the interfaces between brain tissues, suggesting that partial volume effects are a major cause of the observed non‐monoexponential decay. This model may be helpful for future data acquisition strategies that may attempt to combine multiple shells to improve estimates of fiber orientations in white matter and near the cortex. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

White matter microstructural abnormalities in children with spina bifida myelomeningocele and hydrocephalus: a diffusion tensor tractography study of the association pathways

PURPOSE: To quantify microstructural abnormalities in the major association pathways of children affected by spina bifida myelomeningocele (SBM) and shunted hydrocephalus using whole-brain diffusion tensor imaging (DTI). MATERIALS AND METHODS: The institutional review board approved this Health Insurance Portability and Accountability Act (HIPAA)-compliant study and written informed consent/assent were obtained prior to the study. The 69 participants included 38 children with SBM and shunted hydrocephalus (age mean +/- SD = 12.30 +/- 2.10 years; 22 boys; 10 left-handed) and 31 age- and sex-matched normally-developing children (11.56 +/- 2.72 years; 15 boys, four left-handed). Diffusion tensor tractography (DTT) was performed to delineate and quantify bilaterally four major association pathways (arcuate, inferior longitudinal, inferior fronto-occipital, and uncinate fasciculi). RESULTS: The group with SBM did not exhibit the pattern of age-related decreases in the diffusivities observed in the controls. The transverse and axial diffusivities were significantly elevated in most of the white matter pathways of the participants with SBM. The fractional anisotropy (FA) was significantly lower in most of the association pathways. Many of the association pathways were not traceable in some participants with SBM compared to the controls at the selected FA thresholds. CONCLUSION: DTT revealed diffusion tensor characteristics of abnormal development (nonvisualization/poor visualization of tracts, downward arrow FA, upward arrow diffusivities), impairment in myelination (upward arrow transverse diffusivity) as well as abnormalities in intrinsic axonal characteristics and extraaxonal/extracellular space (upward arrow axial diffusivity) in the association pathways of the SBM children. The differences in the diffusion metrics observed in the children with SBM are suggestive of abnormal white matter development and persistent degeneration with increased age.     

Diffusion tensor tractography of the human brain cortico-ponto-cerebellar pathways: a quantitative preliminary study

Purpose

To investigate the utility of diffusion tensor tractography at 1mm slice thickness to map and quantify the whole trajectory of different cortico‐ponto‐cerebellar pathways of the healthy adult human brain.

Materials and Methods

This work was approved by the local Institutional Review Board, and was Health Insurance Portability and Accountability Act (HIPAA) compliant. Five healthy right‐handed men (age range, 24–37 years) were studied and written informed consent was obtained. Diffusion tensor imaging data acquired with 1‐mm slice thickness at a 3.0 Tesla (T) clinical MRI scanner were prepared and analyzed using tractography methods to reconstruct the cortico‐ponto‐cerebellar pathways which included the fronto‐ponto‐cerebellar, parieto‐ponto‐cerebellar, occipito‐ponto‐cerebellar, and temporo‐ponto‐cerebellar tracts.

Results

We demonstrate the feasibility of tractographic mapping and quantification of the four cortico‐ponto‐cerebellar system components based on their cortical connections in the healthy human brain using DTI data with thin 1‐mm sections.

Conclusion

In vivo quantification of different cortico‐ponto‐cerebellar pathways based on cortical connection is feasible, using 1‐mm slices at 3.0T. J. Magn. Reson. Imaging 2010;32:809–817. © 2010 Wiley‐Liss, Inc.