Diffusion tensor MR imaging of the brain and white matter tractography

The Connectome Visualization Utility: software for visualization of human brain networks

In analysis of the human connectome, the connectivity of the human brain is collected from multiple imaging modalities and analyzed using graph theoretical techniques. The dimensionality of human connectivity data is high, and making sense of the complex networks in connectomics requires sophisticated visualization and analysis software. The current availability of software packages to analyze the human connectome is limited. The Connectome Visualization Utility (CVU) is a new software package designed for the visualization and network analysis of human brain networks. CVU complements existing software packages by offering expanded interactive analysis and advanced visualization features, including the automated visualization of networks in three different complementary styles and features the special visualization of scalar graph theoretical properties and modular structure. By decoupling the process of network creation from network visualization and analysis, we ensure that CVU can visualize networks from any imaging modality. CVU offers a graphical user interface, interactive scripting, and represents data uses transparent neuroimaging and matrix-based file types rather than opaque application-specific file formats.

Improving the accuracy of PGSE DTI experiments using the spatial distribution of b matrix

A novel method for improving the accuracy of diffusion tensor imaging (DTI) is proposed. It takes into account the b matrix spatial variations, which can be easily determined using a simple anisotropic diffusion phantom. In opposite to standard numerical procedure of the b matrix calculation that requires the exact knowledge of amplitudes, shapes and time dependencies of diffusion gradients, the new method, which we call BSD-DTI (B-matrix spatial distribution in DTI), relies on direct measurements of its space-dependent components. The proposed technique was demonstrated on the Bruker Biospec 94/20USR system, using the spin echo diffusion sequence to image an isotropic water phantom and an anisotropic capillary phantom. The accuracy of the diffusion tensor determination was improved by an overall factor of about 8 for the isotropic water phantom.

Independent contribution of individual white matter pathways to language function in pediatric epilepsy patients

BACKGROUND AND PURPOSE

Patients with epilepsy and malformations of cortical development (MCDs) are at high risk for language and other cognitive impairment. Specific impairments, however, are not well correlated with the extent and locale of dysplastic cortex; such findings highlight the relevance of aberrant cortico-cortical interactions, or connectivity, to the clinical phenotype. The goal of this study was to determine the independent contribution of well-described white matter pathways to language function in a cohort of pediatric patients with epilepsy.

MATERIALS AND METHODS

Patients were retrospectively identified from an existing database of pediatric epilepsy patients with the following inclusion criteria: 1. diagnosis of MCDs, 2. DTI performed at 3 T, and 3. language characterized by a pediatric neurologist. Diffusion Toolkit and Trackvis (http://www.trackvis.org) were used for segmentation and analysis of the following tracts: corpus callosum, corticospinal tracts, inferior longitudinal fasciculi (ILFs), inferior fronto-occipital fasciculi (IFOFs), uncinate fasciculi (UFs), and arcuate fasciculi (AFs). Mean diffusivity (MD) and fractional anisotropy (FA) were calculated for each tract. Wilcoxon rank sum test (corrected for multiple comparisons) was used to assess potential differences in tract parameters between language-impaired and language-intact patients. In a separate analysis, a machine learning algorithm (random forest approach) was applied to measure the independent contribution of the measured diffusion parameters for each tract to the clinical phenotype (language impairment). In other words, the importance of each tract parameter was measured after adjusting for the contribution of all other tracts.

RESULTS

Thirty-three MCD patients were included (age range: 3-18 years). Twenty-one patients had intact language, twelve had language impairment. All tracts were identified bilaterally in all patients except for the AF, which was not identified on the right in 10 subjects and not identified on the left in 11 subjects. MD and/or FA within the left AF, UF, ILF, and IFOF differed between language-intact and language-impaired groups. However, only parameters related to the left uncinate, inferior fronto-occipital, and arcuate fasciculi were independently associated with the clinical phenotype.

CONCLUSIONS

Scalar metrics derived from the left uncinate, inferior fronto-occipital, and arcuate fasciculi were independently associated with language function. These results support the importance of these pathways in human language function in patients with MCDs.

Magnetic resonance imaging characteristics of typical and atypical/anaplastic meningiomas - Case series and literature review

OBJECTIVE

The histologic grades of meningiomas have a significant impact on the risk of recurrence, prognosis, and the need for adjuvant treatment such as radiation therapy. The purpose of this study is to investigate the magnetic resonance imaging (MRI) characteristics of typical and atypical/anaplastic meningiomas.

METHODS

The medical records of 32 consecutive patients who underwent meningioma resections between April 2004 and November 2006 were retrospectively reviewed. Preoperative MR studies were reviewed by board-certified neuroradiologists. Both univariate and multivariate analyses were used to analyze the MR characteristics of the typical and atypical/anaplastic meningiomas. A review of pertinent literature was also conducted.

RESULTS

Thirty-two patients were identified during the study period. Histopathologic examination of the surgical specimens revealed 27 (84.4% - Group I) typical meningiomas and 5 (15.6% - Group 2) atypical/anaplastic meningiomas. The chi-square test showed that restricted diffusion was much more likely to be present in Group 2 (p < 0.01), and the choline-to-creatinine (Cho/Cr) ratio was significantly higher in Group 2 (8.8 vs. 5.1, p = 0.01). The multivariate analysis confirmed that the atypical/anaplastic group is much more likely to have restricted diffusion (p = 0.02) and higher Cho/Cr ratios (p = 0.03).

CONCLUSION

Meningiomas with restricted diffusion and higher Cho/Cr ratio on MR spectroscopy are more likely to be atypical/anaplastic types. Preoperative MRI utilizing these sequences can provide important information which can be valuable to counsel patients regarding prognosis, risk of recurrence and the need for adjuvant radiation in addition to surgical resection.

Changes in diffusion tensor tractographic findings associated with constraint-induced movement therapy in young children with cerebral palsy

OBJECTIVE

The objective of the study was to determine whether constraint-induced movement therapy (CIMT) could lead to changes in diffusion tensor tractography (DTT) associated with clinical improvement in young children with unilateral cerebral palsy (CP).

METHODS

A standardized pediatric CIMT protocol (4weeks, 120h of constraint) was used on 10 children with unilateral CP who were younger than 5years. DTT was performed in five participants before and after the intervention. Clinical outcome was measured by using the Pediatric Motor Activity Log (PMAL), Quality of Upper Extremity Skills Test (QUEST), and self-care domain of the Pediatric Evaluation of Disability Inventory.

RESULTS

In two patients, the affected corticospinal tract (CST) visible on pretreatment DTT became more prominent on posttreatment DTT. In one patient, the affected CST was not visible on pretreatment DTT, but was visible on posttreatment DTT. All the clinical outcomes significantly improved in the CIMT group compared with the control group. Changes in the PMAL how often scale (PMAL-HO) score significantly differed between the CIMT and control groups.

CONCLUSIONS

Changes in the properties of the affected CST on DTT were accompanied with improved arm function after CIMT in the children with CP.

SIGNIFICANCE

CIMT might lead to CST reorganization in young children with CP.

Diffusion-weighted imaging and demyelinating diseases: new aspects of an old advanced sequence

OBJECTIVE

The purpose of this article is to discuss classic applications in diffusion-weighted imaging (DWI) in demyelinating disease and progression of DWI in the near future.

CONCLUSION

DWI is an advanced technique used in the follow-up of demyelinating disease patients, focusing on the diagnosis of a new lesion before contrast enhancement. With technical advances, diffusion-tensor imaging; new postprocessing techniques, such as tract-based spatial statistics; new ways of calculating diffusion, such as kurtosis; and new applications for DWI and its spectrum are about to arise.

Repeatability of quantitative metrics derived from MR diffusion tractography in paediatric patients with epilepsy

OBJECTIVE

To quantify the test-retest repeatability of mean diffusivity (MD) and fractional anisotropy (FA) derived from diffusion tensor imaging (DTI) tractography in a cohort of paediatric patients with localization-related epilepsy.

METHODS

30 patients underwent 2 DTI acquisitions [repetition time/echo time (ms), 7000/90; flip, 90°; b-value, 1000 s mm(-2); voxel (mm), 2 × 2 × 2]. Two observers used Diffusion Toolkit and TrackVis ( www.trackvis.org ) to segment and analyse the following tracts: corpus callosum, corticospinal tracts, arcuate fasciculi, inferior longitudinal fasciculi and inferior fronto-occipital fasciculi. Mean MD and mean FA were calculated for each tract. Each observer independently analysed one of the DTI data sets for every patient.

RESULTS

Segmentation identified all tracts in all subjects, except the arcuate fasciculus. There was a highly consistent relationship between repeated observations of MD (r = 0.993; p < 0.0001) and FA (r = 0.990; p < 0.0001). For each tract, coefficients of variation ranged from 0.9% to 2.1% for MD and from 1.5% to 2.8% for FA. The 95% confidence limits (CLs) for change ranged from 2.8% to 6% for MD and from 4.3% to 8.6% for FA. For the arcuate fasciculus, Cohen's κ for agreement between the observers (identifiable vs not identifiable) was 1.0.

CONCLUSION

We quantified the repeatability of two commonly utilized scalar metrics derived from DTI tractography. For an individual patient, changes greater than the repeatability coefficient or 95% CLs for change are unlikely to be related to variability in their measurement.

ADVANCES IN KNOWLEDGE

Reproducibility of these metrics will aid in the design of future studies and might one day be used to guide management in patients with epilepsy.

Visualizing Meyer's loop: A comparison of deterministic and probabilistic tractography

BACKGROUND

Diffusion tensor tractography of the anterior extent of the optic radiation - Meyer's loop - prior to temporal lobe resection (TLR) may reduce the risk for postoperative visual field defect. Currently there is no standardized way to perform tractography.

OBJECTIVE

To visualize Meyer's loop using deterministic (DTG) and probabilistic tractography (PTG) at different probability levels, with the primary aim to explore possible differences between methods, and the secondary aim to explore anatomical accuracy.

METHODS

Twenty-three diffusion tensor imaging exams (11 controls and 7 TLR-patients, pre- and post-surgical) were analyzed using DTG and PTG thresholded at probability levels 0.2%, 0.5%, 1%, 5% and 10%. The distance from the tip of the temporal lobe to the anterior limit of Meyer's loop (TP-ML) was measured in 46 optic radiations. Differences in TP-ML between the methods were compared. Results of the control group were compared to dissection studies and to a histological atlas.

RESULTS

For controls and patients together, there were statistically significant differences (p<0.01) for TP-ML between all methods thresholded at PTG ≤1% compared to all methods thresholded at PTG ≥5% and DTG. There were no statistically significant differences between PTG 0.2%, 0.5% and 1% or between PTG 5%, 10% and DTG. For the control group, PTG ≤1% showed a closer match to dissection studies and PTG 1% showed the best match to histological tracings of Meyer's loop.

CONCLUSIONS

Choice of tractography method affected the visualized location of Meyer's loop significantly in a heterogeneous, clinically relevant study group. For the controls, PTG at probability levels ≤1% was a closer match to dissection studies. To determine the anterior extent of Meyer's loop, PTG is superior to DTG and the probability level of PTG matters.

Prolonged febrile seizures cause reversible reductions in white matter integrity

Prolonged febrile seizures (PFS) are the commonest cause of childhood status epilepticus and are believed to carry a risk of neuronal damage, in particular to the mesial temporal lobe. This study was designed to determine: i) the effect of prolonged febrile seizures on white matter and ii) the temporal evolution of any changes seen.

33 children were recruited 1 month following PFS and underwent diffusion tensor imaging (DTI) with repeat imaging at 6 and 12 months after the original episode of PFS. 18 age-matched healthy control subjects underwent similar investigations at a single time point. Tract-based spatial statistics (TBSS) was used to compare fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD) between patients and controls on a voxel-wise basis within the white matter skeleton.

Widespread reductions in FA along multiple white matter tracts were found at 1 and 6 months post-PFS, but these had resolved at 12 months. At one month post-PFS the main changes seen were reductions in AD but at 6 months these had predominantly changed to increases in RD.

These widespread white matter changes have not previously been noted following PFS. There are many possible explanations, but one plausible hypothesis is that this represents a temporary halting of normal white matter development caused by the seizure, that then resumes and normalises in the majority of children.

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Widespread reductions in FA occur in children after prolonged febrile seizures.

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These reductions persist up to 6 months post-PFS but resolve by 1 year.

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This may represent a seizure-related disruption of white matter development.

Decreased cerebellar fiber density in cortical myoclonic tremor but not in essential tremor

Pathophysiology of tremor generation remains uncertain in 'familial cortical myoclonic tremor with epilepsy' (FCMTE) and essential tremor (ET). In both disorders, imaging and pathological studies suggest involvement of the cerebellum and its projection areas. MR diffusion tensor imaging allows estimation of white matter tissue composition, and therefore is well suited to quantify structural changes in vivo. This study aimed to compare cerebellar fiber density between FCMTE and ET patients and healthy controls. Seven FCMTE patients, eight ET patients, and five healthy controls were studied. Cerebellum was annotated based on fractional anisotropy (FA) and mean diffusivity volumes. Mean cerebellar FA values were computed as well as mean cerebellar volume. Group statistics included one-way ANOVAs and post hoc independent t tests. Mean FA of the cerebellar region for FCMTE was 0.242 (SD = 0.012), for ET 0.259 (SD = 0.0115), and for controls 0.262 (SD = 0.0146). There was a significant group effect for FA (F(2) = 4.9, p = 0.02). No difference in mean cerebellar volume was found. Post hoc independent t tests revealed significantly decreased mean FA in FCMTE patients compared to controls (t[10] = 2.5, p = 0.03) and ET patients (t[13] = 2.9, p = 0.01), while there was no difference in mean FA between ET patients and controls (t[11] < 1.0). This study indicates for the first time microstructural damage of the cerebellar white matter in FCMTE in vivo. These results ascertain a role of the cerebellum in 'cortical tremor'.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

In vivo quantitative whole-brain diffusion tensor imaging analysis of APP/PS1 transgenic mice using voxel-based and atlas-based methods

INTRODUCTION

Diffusion tensor imaging (DTI) has been applied to characterize the pathological features of Alzheimer's disease (AD) in a mouse model, although little is known about whether these features are structure specific. Voxel-based analysis (VBA) and atlas-based analysis (ABA) are good complementary tools for whole-brain DTI analysis. The purpose of this study was to identify the spatial localization of disease-related pathology in an AD mouse model.

METHODS

VBA and ABA quantification were used for the whole-brain DTI analysis of nine APP/PS1 mice and wild-type (WT) controls. Multiple scalar measurements, including fractional anisotropy (FA), trace, axial diffusivity (DA), and radial diffusivity (DR), were investigated to capture the various types of pathology. The accuracy of the image transformation applied for VBA and ABA was evaluated by comparing manual and atlas-based structure delineation using kappa statistics. Following the MR examination, the brains of the animals were analyzed for microscopy.

RESULTS

Extensive anatomical alterations were identified in APP/PS1 mice, in both the gray matter areas (neocortex, hippocampus, caudate putamen, thalamus, hypothalamus, claustrum, amygdala, and piriform cortex) and the white matter areas (corpus callosum/external capsule, cingulum, septum, internal capsule, fimbria, and optic tract), evidenced by an increase in FA or DA, or both, compared to WT mice (p < 0.05, corrected). The average kappa value between manual and atlas-based structure delineation was approximately 0.8, and there was no significant difference between APP/PS1 and WT mice (p > 0.05). The histopathological changes in the gray matter areas were confirmed by microscopy studies. DTI did, however, demonstrate significant changes in white matter areas, where the difference was not apparent by qualitative observation of a single-slice histological specimen.

CONCLUSION

This study demonstrated the structure-specific nature of pathological changes in APP/PS1 mouse, and also showed the feasibility of applying whole-brain analysis methods to the investigation of an AD mouse model.

Axonal and glial microstructural information obtained with diffusion-weighted magnetic resonance spectroscopy at 7T

Diffusion-weighted magnetic resonance spectroscopy (DWS) offers unique access to compartment-specific microstructural information on tissue, and potentially sensitive detection of compartment-specific changes in disease. The specificity of DWS is, however, offset by its relative low sensitivity, intrinsic to all MRS-based methods, and further exacerbated by the signal loss due to the diffusion weighting and long echo times. In this work we first provide an experimental example for the type of compartment-specific information that can be obtained with DWS from a small volume of interest (VOI) in brain white matter. We then propose and discuss a strategy for the analysis of DWS data, which includes the use of models of diffusion in compartments with simple geometries. We conclude with a broader discussion of the potential role of DWS in the characterization of tissue microstructure and the complementarity of DWS with less-specific but more sensitive microstructural tools such as diffusion tensor imaging.

Serial MR analysis of early permanent and transient ischemia in rats: diffusion tensor imaging and high b value diffusion weighted imaging

Objective

To evaluate the temporal evolution and diagnostic values of the diffusion tensor imaging (DTI) and the high b value diffusion weighted imaging (DWI) in the early permanent and transient cerebral ischemia.

Materials and Methods

For permanent or 30-minute transient-ischemia induced 30 rats, DTI and DWIs at both high b (b = 3000 s/mm²) and standard b value (b = 1000 s/mm²) were obtained at the following conditions: at 15, 30, 45, 60 minutes after the occlusion of what for hyperacute permanent ischemia; at 1, 3, 5, 7, 9 hours after the occlusion for acute permanent ischemia; and at 15 minutes before reperfusion, 0.5, 2.5, and 24 hours after reperfusion for transient ischemia. The diffusion parameters and their ratios were obtained and compared between different b values, and among different time points and groups, respectively.

Results

For both b values, the apparent diffusion coefficient (ADC) ratio decreased for first three hours, and then slightly increased until 9 hours after the occlusion during a gradual continuous increase of DWI signal intensity (SI) ratio, with excellent correlation between ADC ratios and DWI SI ratios. The DWI showed a higher contrast ratio, but the ADC map showed a lower contrast ratio for permanent ischemia at high b value than at standard b value. Fractional anisotropy (FA) increased for 1 hour, then gradually decreased until 9 hours after the occlusion in permanent ischemia and showed transient normalization and secondary decay along with change in ADC in transient ischemia.

Conclusion

This study presents characteristic initial elevation and secondary decay of FA, higher contrast ratio of DWI, and lower contrast ratio of ADC map at high b value, in addition to the time evolutions of diffusion parameters in early permanent and transient ischemia.

Differentiating tumor recurrence from treatment necrosis: a review of neuro-oncologic imaging strategies

Differentiating treatment-induced necrosis from tumor recurrence is a central challenge in neuro-oncology. These 2 very different outcomes after brain tumor treatment often appear similarly on routine follow-up imaging studies. They may even manifest with similar clinical symptoms, further confounding an already difficult process for physicians attempting to characterize a new contrast-enhancing lesion appearing on a patient's follow-up imaging. Distinguishing treatment necrosis from tumor recurrence is crucial for diagnosis and treatment planning, and therefore, much effort has been put forth to develop noninvasive methods to differentiate between these disparate outcomes. In this article, we review the latest developments and key findings from research studies exploring the efficacy of structural and functional imaging modalities for differentiating treatment necrosis from tumor recurrence. We discuss the possibility of computational approaches to investigate the usefulness of fine-grained imaging characteristics that are difficult to observe through visual inspection of images. We also propose a flexible treatment-planning algorithm that incorporates advanced functional imaging techniques when indicated by the patient's routine follow-up images and clinical condition.

Stereoscopic Visualization of Diffusion Tensor Imaging Data: A Comparative Survey of Visualization Techniques

Diffusion tensor imaging (DTI) data has traditionally been displayed as a grayscale functional anisotropy map (GSFM) or color coded orientation map (CCOM). These methods use black and white or color with intensity values to map the complex multidimensional DTI data to a two-dimensional image. Alternative visualization techniques, such asVmaxmaps utilize enhanced graphical representation of the principal eigenvector by means of a headless arrow on regular nonstereoscopic (VM) or stereoscopic display (VMS). A survey of clinical utility of patients with intracranial neoplasms was carried out by 8 neuroradiologists using traditional and nontraditional methods of DTI display. Pairwise comparison studies of 5 intracranial neoplasms were performed with a structured questionnaire comparing GSFM, CCOM, VM, and VMS. Six of 8 neuroradiologists favoredVmaxmaps over traditional methods of display (GSFM and CCOM). When comparing the stereoscopic (VMS) and the non-stereoscopic (VM) modes, 4 favored VMS, 2 favored VM, and 2 had no preference. In conclusion, processing and visualizing DTI data stereoscopically is technically feasible. An initial survey of users indicated thatVmaxbased display methodology with or without stereoscopic visualization seems to be preferred over traditional methods to display DTI data.

Diffusion-weighted magnetic resonance imaging detection of basal forebrain cholinergic degeneration in a mouse model

Loss of basal forebrain cholinergic neurons is an early and key feature of Alzheimer's disease, and magnetic resonance imaging (MRI) volumetric measurement of the basal forebrain has recently gained attention as a potential diagnostic tool for this condition. The aim of this study was to determine whether loss of basal forebrain cholinergic neurons underpins changes which can be detected through diffusion MRI using diffusion tensor imaging (DTI) and probabilistic tractography in a mouse model. To cause selective basal forebrain cholinergic degeneration, the toxin saporin conjugated to a p75 neurotrophin receptor antibody (mu-p75-SAP) was used. This resulted in ~25% loss of the basal forebrain cholinergic neurons and significant loss of terminal cholinergic projections in the hippocampus, as determined by histology. To test whether lesion of cholinergic neurons caused basal forebrain, hippocampal, or whole brain atrophy, we performed manual segmentation analysis, which revealed no significant atrophy in lesioned animals compared to controls (Rb-IgG-SAP). However, analysis by DTI of the basal forebrain area revealed a significant increase in fractional anisotropy (FA; +7.7%), mean diffusivity (MD; +6.1%), axial diffusivity (AD; +8.5%) and radial diffusivity (RD; +4.0%) in lesioned mice compared to control animals. These parameters strongly inversely correlated with the number of choline acetyl transferase-positive neurons, with FA showing the greatest association (r(2)=0.72), followed by MD (r(2)=0.64), AD (r(2)=0.64) and RD (r(2)=0.61). Moreover, probabilistic tractography analysis of the septo-hippocampal tracts originating from the basal forebrain revealed an increase in streamline MD (+5.1%) and RD (+4.3%) in lesioned mice. This study illustrates that moderate loss of basal forebrain cholinergic neurons (representing only a minor proportion of all septo-hippocampal axons) can be detected by measuring either DTI parameters of the basal forebrain nuclei or tractography parameters of the basal forebrain tracts. These findings provide increased support for using DTI and probabilistic tractography as non-invasive tools for diagnosing and/or monitoring the progression of conditions affecting the integrity of the basal forebrain cholinergic system in humans, including Alzheimer's disease.

Diffusion tensor imaging in spinal cord compression

BACKGROUND

Although diffusion tensor imaging has been successfully applied in brain research for decades, several main difficulties have hindered its extended utilization in spinal cord imaging.

PURPOSE

To assess the feasibility and clinical value of diffusion tensor imaging and tractography for evaluating chronic spinal cord compression.

MATERIAL AND METHODS

Single-shot spin-echo echo-planar DT sequences were scanned in 42 spinal cord compression patients and 49 healthy volunteers. The mean values of the apparent diffusion coefficient and fractional anisotropy were measured in region of interest at the cervical and lower thoracic spinal cord. The patients were divided into two groups according to the high signal on T2WI (the SCC-HI group and the SCC-nHI group for with or without high signal). A one-way ANOVA was used. Diffusion tensor tractography was used to visualize the morphological features of normal and impaired white matter.

RESULTS

There were no statistically significant differences in the apparent diffusion coefficient and fractional anisotropy values between the different spinal cord segments of the normal subjects. All of the patients in the SCC-HI group had increased apparent diffusion coefficient values and decreased fractional anisotropy values at the lesion level compared to the normal controls. However, there were no statistically significant diffusion index differences between the SCC-nHI group and the normal controls. In the diffusion tensor imaging maps, the normal spinal cord sections were depicted as fiber tracts that were color-encoded to a cephalocaudal orientation. The diffusion tensor images were compressed to different degrees in all of the patients.

CONCLUSION

Diffusion tensor imaging and tractography are promising methods for visualizing spinal cord tracts and can provide additional information in clinical studies in spinal cord compresion.

Elevated mean diffusivity in the left hemisphere superior longitudinal fasciculus in autism spectrum disorders increases with more profound language impairment

BACKGROUND AND PURPOSE

Language impairments are observed in a subset of individuals with ASD. To examine microstructural brain white matter features associated with language ability in ASD, we measured the DTI parameters of language-related white matter tracts (SLF) as well as non-language-related white matter tracts (CST) in children with ASD/+LI and ASD/-LI) and in TD.

MATERIALS AND METHODS

Eighteen children with ASD/-LI (age range, 6.7-17.5 years), 17 with ASD/+LI (age range, 6.8-14.8 years), and 25 TD (age range, 6.5-18 years) were evaluated with DTI and tractography. Primary DTI parameters considered for analysis were MD and FA.

RESULTS

There was a main effect of diagnostic group on age-corrected MD (P < .05) with ASD/+LI significantly elevated compared with TD. This was most pronounced for left hemisphere SLF fiber tracts and for the temporal portion of the SLF. There was significant negative correlation between left hemisphere SLF MD values and the clinical assessment of language ability. There was no main effect of diagnostic group or diagnostic group X hemisphere interaction for FA. Although there was a main effect of diagnostic group on values of MD in the CST, this did not survive hemispheric subanalysis.

CONCLUSIONS

Abnormal DTI parameters (specifically significantly elevated MD values in ASD) of the SLF appear to be associated with language impairment in ASD. These elevations are particularly pronounced in the left cerebral hemisphere, in the temporal portion of the SLF, and in children with clinical language impairment.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

Diffusion tensor imaging for in vivo detection of degenerated optic radiation

Glaucomatous optic nerve atrophy may continue to the linked optic radiation by transneuronal degeneration, as described in animal models of glaucoma. In vivo visualization of the visual pathway represents a new challenge in the field of ophthalmology. We present a new approach for illustration of the optic radiation by diffusion tensor imaging (DTI) based on magnetic resonance imaging (MRI). The DTI was established by use of a 3T high-field scanner. The case of a patient with primary open-angle glaucoma is opposed to this one of a healthy subject to demonstrate the visible rarefication of the optic radiation. The goal was to introduce the technique of the DTI also in ophthalmology and to demonstrate that it may be useful to judge glaucoma-related differences.

White matter and cognition in adults who were born preterm

BACKGROUND AND PURPOSE

Individuals born very preterm (before 33 weeks of gestation, VPT) are at risk of damage to developing white matter, which may affect later cognition and behaviour.

METHODS

We used diffusion tensor MRI (DT-MRI) to assess white matter microstructure (fractional anisotropy; FA) in 80 VPT and 41 term-born individuals (mean age 19.1 years, range 17-22, and 18.5 years, range 17-22 years, respectively). VPT individuals were part of a 1982-1984 birth cohort which had been followed up since birth; term individuals were recruited by local press advertisement. General intellectual function, executive function and memory were assessed.

RESULTS

The VPT group had reduced FA in four clusters, and increased FA in four clusters relative to the Term group, involving several association tracts of both hemispheres. Clusters of increased FA were associated with more severe neonatal brain injury in the VPT group. Clusters of reduced FA were associated with lower birth weight and perinatal hypoxia, and with reduced adult cognitive performance in the VPT group only.

CONCLUSIONS

Alterations of white matter microstructure persist into adulthood in VPT individuals and are associated with cognitive function.

Evaluation of diffusion models of fiber tracts using diffusion tensor magnetic resonance imaging

Modeling of water diffusion in white matter is useful for revealing microstructure of the brain tissue and hence diagnosis and evaluation of white matter diseases. Researchers have modeled diffusion in white matter using mathematical and mechanical analysis at the cellular level. However, less work has been devoted to evaluate these models using macroscopic real data such as diffusion tensor magnetic resonance imaging (DTMRI) data. DTMRI is a noninvasive tool for evaluating white matter microstructure by measuring random motion of water molecules referred to as diffusion. It reflects directional information of microscopic structures such as fibers. Thus, it is applicable for evaluation and modification of mathematical models of white matter. Nevertheless, a realistic relation between a fiber model and imaging data does not exist. This work opens a promising avenue for relating DTMRI data to microstructural parameters of white matter. First, we propose a strategy for relating DTMRI and fiber model parameters to evaluate mathematical models in light of real data. The proposed strategy is then applied to evaluate and extend an existing model of white matter based on clinically available DTMRI data. Next, the proposed strategy is used to estimate microstructural characteristics of fiber tracts. We illustrate this approach through its application to approximation of myelin sheath thickness and fraction of volume occupied by fibers. Using sufficiently small imaging voxels, the proposed approach is capable of estimating model parameters with desirable precision.

Diffusion tensor imaging detects rarefaction of optic radiation in glaucoma patients

RATIONALE AND OBJECTIVES

Diffusion tensor imaging (DTI) can depict rarefaction of the optical fibres. Hence, we applied DTI to assess pathological changes of the optic radiation in glaucoma patients.

MATERIALS AND METHODS

Fifty glaucoma patients and 50 healthy age-matched controls were examined by a 3T high-field magnetic resonance scanner. Fiber tracts were volume rendered using a semiquantitative approach to assess rarefaction and results were correlated with the extent of optic nerve atrophy and reduced spatial-temporal contrast sensitivity of the retina using established ophthalmological examinations.

RESULTS

Twenty-two glaucoma patients (44%) showed significant rarefaction of the optic radiation: the volume was reduced to 67 ± 16% compared with controls. Hereby, the glaucomatous optic nerve atrophy stage correlated with the presence of DTI-derived rarefied optic radiation (Kendall tau-b 0.272, P = .016). Aside, cerebral microangiopathy affecting the optic radiation was significantly higher among glaucoma patients compared to controls (10 patients compared with 2 patients, P < .05).

CONCLUSION

In patients with glaucomatous optic nerve atrophy, there is anterograde and-most likely because of microangiopathic lesions within the optic radiation-retrograde transneuronal rarefaction of the optic radiation that can be assessed in vivo using DTI with good correlation to established ophthalmological examinations.

Effect of Regularization Parameter and Scan Time on Crossing Fibers with Constrained Compressed Sensing

Diffusion tensor imaging (DTI) is an MR imaging technique that uses a set of diffusion weighted measurements in order to determine the water diffusion tensor at each voxel. In DTI, a single dominant fiber orientation is calculated at each measured voxel, even if multiple populations of fibers are present within this voxel. A new approach called Crossing Fiber Angular Resolution of Intra-voxel structure (CFARI) for processing diffusion weighted magnetic resonance data has been recently introduced. Based on compressed sensing, CFARI is able to resolve intra-voxel structure from limited number of measurements, but its performance as a function of the scan and algorithm parameters is poorly understood at present. This paper describes simulation experiments to help understand CFARI performance tradeoffs as a function of the data signal-to-noise ratio and the algorithm regularization parameter. In the compressed sensing criterion, the choice of the regularization parameter beta is critical. If beta is too small, then the solution is the conventional least squares solution, while if beta is too large then the solution is identically zero. The correct selection of beta turns out to be data dependent, which means that it is also spatially varying. In this paper, simulations using two random tensors with different diffusivities having the same fractional anisotropy but with different principle eigenvalues are carried out. Results reveal that for a fixed scan time, acquisition of repeated measurements can improve CFARI performance and that a spatially variable, data adaptive regularization parameter is beneficial in stabilizing results.

ADC mapping of chronic cerebral hypoperfusion induced by carotid artery stenosis

BACKGROUND

Carotid artery stenosis is associated with the occurrence of acute and chronic ischemic lesions that increase with age in the elderly population. Diffusion Imaging and ADC mapping may be an appropriate method to investigate patients with chronic hypoperfusion consecutive to carotid stenosis. This non-invasive technique allows to investigate brain integrity and structure, in particular hypoperfusion induced by carotid stenosis diseases. The aim of this study was to evaluate the impact of a carotid stenosis on the parenchyma using ADC mapping.

METHODS

Fifty-nine patients with symptomatic (33) and asymptomatic (26) carotid stenosis were recruited from our multidisciplinary consultation. Both groups demonstrated a similar degree of stenosis. All patients underwent MRI of the brain including diffusion-weighted MR imaging with ADC mapping. Regions of interest were defined in the anterior and posterior paraventricular regions both ipsilateral and contralateral to the stenosis (anterior circulation). The same analysis was performed for the thalamic and occipital regions (posterior circulation).

RESULTS

ADC values of the affected vascular territory were significantly higher on the side of the stenosis in the periventricular anterior (P<0.001) and posterior (P<0.01) area. There was no difference between ipsilateral and contralateral ADC values in the thalamic and occipital regions.

CONCLUSIONS

We have shown that carotid stenosis is associated with significantly higher ADC values in the anterior circulation, probably reflecting an impact of chronic hypoperfusion on the brain parenchyma in symptomatic and asymptomatic patients. This is consistent with previous data in the literature.

Magnetic resonance imaging of vascular diseases of the white matter

Magnetic resonance imaging plays an important role in the diagnosis and management of cerebrovascular diseases. In addition to stroke, it can also demonstrate changes apparent in the white matter such as leukoaraiosis. These as well as other changes occurring with increasing age can be visualized. Among others, there is an increase in the number and size of perivascular spaces. Although many of these alterations may be clinically silent, with increasing load they may become symptomatic. Other vascular pathological findings such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy and the posterior reversible encephalopathy syndrome can also provoke changes in the cerebral white matter that are visible on magnetic resonance imaging and are discussed.

Characterization of the corpus callosum in very preterm and full-term infants utilizing MRI

The corpus callosum is the largest white matter tract, important for interhemispheric communication. The aim of this study was to investigate and compare corpus callosum size, shape and diffusion characteristics in 106 very preterm infants and 22 full-term infants. Structural and diffusion magnetic resonance images were obtained at term equivalent. The corpus callosum was segmented, cross-sectional areas were calculated, and shape was analyzed. Fractional anisotropy, mean, axial and radial diffusivity measures were obtained from within the corpus callosum, with additional probabilistic tractography analysis. Very preterm infants had significantly reduced callosal cross-sectional area compared with term infants (p=0.004), particularly for the mid-body and posterior sub-regions. Very preterm callosi were more circular (p=0.01). Fractional anisotropy was lower (p=0.007) and mean (p=0.006) and radial (p=0.001) diffusivity values were higher in very preterm infants' callosi, particularly at the anterior and posterior ends. The volume of tracts originating from the corpus callosum was reduced in very preterm infants (p=0.001), particularly for anterior mid-body (p=0.01) and isthmus tracts (p=0.04). This study characterizes callosal size, shape and diffusion in typically developing infants at term equivalent age, and reports macrostructural and microstructural abnormalities as a result of prematurity.

Uso do tensor de difusão na avaliação dos padrões de acometimento da substância branca em pacientes com tumores cerebrais: é uma ferramenta útil para o diagnóstico diferencial?

OBJETIVO: Propõe-se avaliar o papel do tensor de difusão na avaliação dos diferentes padrões de acometimento da substância branca em pacientes com tumores cerebrais e a utilidade desta técnica no diagnóstico diferencial dessas neoplasias. MATERIAIS E MÉTODOS: Quarenta e quatro pacientes com tumores cerebrais, incluindo gliomas de baixo grau, astrocitomas anaplásicos, glioblastomas multiformes e metástases, foram estudados com imagens de ressonância magnética convencional e tensor de difusão. Os tratos de substância branca próximos aos tumores foram caracterizados como deslocados, interrompidos, infiltrados e edematosos. RESULTADOS: Houve significativa sobreposição entre os padrões de acometimento da substância branca pelo tensor de difusão, uma vez que o padrão deslocado foi observado em todos os tipos de tumor. Os padrões interrompido e infiltrado foram encontrados em glioblastomas multiformes e astrocitomas anaplásicos e o padrão edematoso foi observado em metástases. CONCLUSÃO: Os padrões de envolvimento dos tratos de substância branca cerebral avaliados pelo tensor de difusão auxiliam no mapeamento dos tratos adjacentes aos tumores e fornecem informações importantes sobre a extensão do tumor, no entanto, eles não possibilitam fazer a distinção entre gliomas de baixo e alto graus e metástases

Diffusion tensor imaging of peripheral nerves

Magnetic resonance diffusion tensor imaging (DTI) allows the directional dependence of water diffusion to be studied. Analysis of the resulting image data allows for the determination of fractional anisotropy (FA), apparent diffusion coefficient (ADC), as well as allowing three-dimensional visualization of the fiber tract (tractography). We visualized the ulnar nerve of ten healthy volunteers with DTI. We found FA to be 0.752 ± 0.067 and the ADC to be 0.96 ± 0.13 × 10(-3) mm(2)/s. A nuts-and-bolts description of the physical aspects of DTI is provided as an educational process for readers.

Usefulness of diffusion tensor imaging and fiber tractography in neurological and neurosurgical pediatric diseases

INTRODUCTION

Diffusion tensor imaging (DTI) with fiber tractography (FT) is a recently introduced imaging technique that is unique in providing detailed imaging of white matter (WM) tracts and connectivity between different regions of the brain not easily appreciated with other imaging methods.

DISCUSSION

DTI has been used in recent years to investigate several disease conditions involving WM, including brain malformations, cerebral ischemia, multiple sclerosis, neurocutaneous syndromes, and brain tumors.

CONCLUSION

In this paper, we focus our attention on the main applications of DTI-FT in the field of pediatric neurology, adding our personal experience.

White matter microstructural differences linked to left perisylvian language network in children with dyslexia

Studies of dyslexia using diffusion tensor imaging (DTI) have reported fractional anisotropy (FA) differences in left inferior frontal gyrus (LIFG) and left temporo-parietal white matter, suggesting that impaired reading is associated with atypical white matter microstructure in these regions. These anomalies might reflect abnormalities in the left perisylvian language network, long implicated in dyslexia. While DTI investigations frequently report analyses on multiple tensor-derived measures (e.g., FA, orientation, tractography), it is uncommon to integrate analyses to examine the relationships between atypical findings. For the present study, semi-automated techniques were applied to DTI data in an integrated fashion to examine white matter microstructure in 14 children with dyslexia and 17 typically developing readers (ages 7-16 years). Correlations of DTI metrics (FA and fiber orientation) to reading skill (accuracy and speed) and to probabilistic tractography maps of the left perisylvian language tracts were examined. Consistent with previous reports, our findings suggest FA decreases in dyslexia in LIFG and left temporo-parietal white matter. The LIFG FA finding overlaps an area showing differences in fiber orientation in an anterior left perisylvian language pathway. Additionally, a positive correlation of FA to reading speed was found in a posterior circuit previously associated with activation on functional imaging during reading tasks. Overall, integrating results from several complementary semi-automated analyses reveals evidence linking atypical white matter microstructure in dyslexia to atypical fiber orientation in circuits implicated in reading including the left perisylvian language network.

Integration of functional MRI and white matter tractography in stereotactic radiosurgery clinical practice

PURPOSE

To study the efficacy of the integration of functional magnetic resonance imaging (fMRI) and diffusion tensor imaging tractography data into stereotactic radiosurgery clinical practice.

METHODS AND MATERIALS

fMRI and tractography data sets were acquired and fused with corresponding anatomical MR and computed tomography images of patients with arteriovenous malformation (AVM), astrocytoma, brain metastasis, or hemangioma and referred for stereotactic radiosurgery. The acquired data sets were imported into a CyberKnife stereotactic radiosurgery system and used to delineate the target, organs at risk, and nearby functional structures and fiber tracts. Treatment plans with and without the incorporation of the functional structures and the fiber tracts into the optimization process were developed and compared.

RESULTS

The nearby functional structures and fiber tracts could receive doses of >50% of the maximum dose if they were excluded from the planning process. In the AVM case, the doses received by the Broadmann-17 structure and the optic tract were reduced to 700 cGy from 1,400 cGy and to 1,200 cGy from 2,000 cGy, respectively, upon inclusion into the optimization process. In the metastasis case, the motor cortex received 850 cGy instead of 1,400 cGy; and in the hemangioma case, the pyramidal tracts received 780 cGy instead of 990 cGy. In the astrocytoma case, the dose to the motor cortex bordering the lesion was reduced to 1,900 cGy from 2,100 cGy, and therefore, the biologically equivalent dose in three fractions was delivered instead.

CONCLUSIONS

Functional structures and fiber tracts could receive high doses if they were not considered during treatment planning. With the aid of fMRI and tractography images, they can be delineated and spared.

Diffusion imaging for therapy response assessment of brain tumor

Advanced imaging provides insight into biophysical, physiologic, metabolic, or functional properties of tissues. Because water mobility is sensitive to cellular homeostasis, cellular density, and microstructural organization, it is considered a valuable tool in the advanced imaging arsenal. This article summarizes diffusion imaging concepts and highlights clinical applications of diffusion MR imaging for oncologic imaging. Diffusion tensor imaging and its derivative maps of diffusion anisotropy allow assessment of tumor compression or destruction of adjacent normal tissue anisotropy and may aid to assess tumor infiltration and aid presurgical planning.

Neuroanatomic analysis of diffusion tensor imaging of white matter tracts with dejerine sections and neuroimaging

We present a neuroanatomical comparison with stereotaxic precision correlating modern diffusion tensor imaging (DTI), MRI data and classical neuroanatomical textbooks. The neuroatlas compiled by J. and A. Dejerine deserves emphasis for its role of a bridge between precise classical neuroanatomy techniques with modern neuroimaging methods. We present its utility in identifying major white matter tracts of the brain in concert with DTI in the horizontal-plane axis to elucidate axonal directionality. The axonal directionality of DTI is necessary for understanding white matter connectivity in the brain. Many DTI studies publish findings in selective regions of the brain, and our study is mainly devoted to comparing our images with data from Dejerine for the entire brain.

Diffusion tensor MR imaging evaluation of the corpus callosum of patients with multiple sclerosis

OBJECTIVE

To evaluate the fractional anisotropy (FA) values of the normal-appearing white matter of the corpus callosum (CC) in patients with relapsing-remitting multiple sclerosis (MS).

METHOD

Fifty-seven patients with diagnosis of relapsing-remitting MS and 47 age- and gender-matched controls were studied. A conventional MR imaging protocol and a DTI sequence were performed. One neuroradiologist placed the regions of interest (ROIs) in the FA maps in five different portions of the normal-appearing CC (rostrum, genu, anterior and posterior portion of the body and splenium) in all cases. The statistical analysis was performed with the Mann-Whitney U test and p<0.05 was considered statistically significant.

RESULTS

The FA values were lower in the MS patients compared with the controls (p<0.05) in the following CC regions: rostrum (0.720 vs 0.819), anterior body (0.698 vs 0.752), posterior body (0.711 vs 0.759) and splenium (0.720 vs 0.880).

CONCLUSION

In this series, there was a robust decrease in the FA in all regions of the normal-appearing CC, being significant in the rostrum, body and splenium. This finding suggests that there is a subtle and diffuse abnormality in the CC, which could be probably related to myelin content loss, axonal damage and gliosis.