Morphometry of in vivo human white matter association pathways with diffusion-weighted magnetic resonance imaging

Intraoperative diffusion-tensor MR imaging: shifting of white matter tracts during neurosurgical procedures--initial experience

PURPOSE

To prospectively evaluate the location of white matter tracts with diffusion-tensor imaging (DTI) during neurosurgical procedures.

MATERIALS AND METHODS

Ethical committee approval and signed informed consent were obtained. A 1.5-T magnetic resonance imager with an adapted rotating surgical table that is placed in a radiofrequency-shielded operating theater was used for pre- and intraoperative imaging. DTI was performed by applying an echo-planar imaging sequence with six diffusion directions in 38 patients (20 female patients, 18 male patients; age range, 7-77 years; mean age, 45.6 years) who were undergoing surgery (35 craniotomy and three burr hole procedures). Color-encoded maps of fractional anisotropy were generated by depicting white matter tracts. A rigid registration algorithm was used to compare pre- and intraoperative images.

RESULTS

Intraoperative DTI was technically feasible in all patients, and no major image distortions occurred in the areas of interest. Pre- and intraoperative color-encoded maps of fractional anisotropy could be registered; these maps depicted marked and highly variable shifting of white matter tracts during neurosurgical procedures. In the 27 patients who underwent brain tumor resection, white matter tract shifting ranged from an inward shift of 8 mm to an outward shift of 15 mm (mean shift +/- standard deviation, outward shift of 2.5 mm +/- 5.8). In 16 (59%) of 27 patients, outward shifting was detected; in eight (30%), inward shifting was detected. In eight patients who underwent temporal lobe resections for drug-resistant epilepsy, shifting was only inward and ranged from 2 to 14 mm (9 mm +/- 3.3). In two of the three patients who underwent burr hole procedures, outward shifting occurred.

CONCLUSION

Intraoperative DTI can depict shifting of major white matter tracts that is caused by surgical intervention.

Novel MR contrasts to reveal more about the brain

Twenty percent of patients with refractory focal epilepsy have an undetermined etiologic basis for their epilepsy despite extensive investigation, including optimal MR imaging. Surgical treatment of this group is associated with a less favorable postoperative outcome. Even with improvements in imaging techniques, a proportion of these patients will remain "MR imaging-negative." It is likely, however, that some of the discrete macroscopic focal lesions that are currently occult will be identified by imaging techniques interrogating different microstructural characteristics. Furthermore, these methods may provide pathologic specificity when used in combination. The description and application of these techniques in epilepsy are the focus of this article.

Functional connectivity in the human language system: a cortico-cortical evoked potential study

A better understanding of the mechanisms involved in human higher cortical functions requires a detailed knowledge of neuronal connectivity between functional cortical regions. Currently no good method for tracking in vivo neuronal connectivity exists. We investigated the inter-areal connections in vivo in the human language system using a new method, which we termed 'cortico-cortical evoked potentials' (CCEPs). Eight patients with epilepsy (age 13-42 years) underwent invasive monitoring with subdural electrodes for epilepsy surgery. Six patients had language dominance on the side of grid implantation and two had bilateral language representation by the intracarotid amobarbital test. Conventional cortical electrical stimulation was performed to identify the anterior and posterior language areas. Single pulse electrical stimuli were delivered to the anterior language (eight patients), posterior language (four patients) or face motor (two patients) area, and CCEPs were obtained by averaging electrocorticograms (ECoGs) recorded from the perisylvian and extrasylvian basal temporal language areas time-locked to the stimulus. The subjects were not asked to perform any tasks during the study. Stimulation at the anterior language area elicited CCEPs in the lateral temporo-parietal area (seven of eight patients) in the middle and posterior part of the superior temporal gyrus, the adjacent part of the middle temporal gyrus and the supramarginal gyrus. CCEPs were recorded in 3-21 electrodes per patient. CCEPs occurred at or around the particular electrodes in the posterior language area which, when stimulated, produced speech arrest. Similar early and late CCEPs were obtained from the basal temporal area by stimulating the anterior language area (three of three patients). In contrast, stimulation of the adjacent face motor area did not elicit CCEPs in language areas but rather in the postcentral gyrus. Stimulation of the posterior language area produced CCEPs in the anterior language (three of four patients) as well as in the basal temporal area (one of two patients). These CCEPs were less well defined. These findings suggest that perisylvian and extrasylvian language areas participate in the language system as components of a network by means of feed-forward and feed-back projections. Different from the classical Wernicke-Geschwind model, the present study revealed a bidirectional connection between Broca's and Wernicke's areas probably through the arcuate fasciculus and/or the cortico-subcortico-cortical pathway. CCEPs were recorded from a larger area than the posterior language area identified by electrical stimulation. This suggests the existence of a rather broad neuronal network surrounding the previously recognized core region of this area.

DT-MRI denoising and neuronal fiber tracking

Diffusion tensor imaging can provide the fundamental information required for viewing structural connectivity. However, robust and accurate acquisition and processing algorithms are needed to accurately map the nerve connectivity. In this paper, we present a novel algorithm for extracting and visualizing the fiber tracts in the CNS, specifically in the brain. The automatic fiber tract mapping problem will be solved in two phases, namely a data smoothing phase and a fiber tract mapping phase. In the former, smoothing of the diffusion-weighted data (prior to tensor calculation) is achieved via a weighted TV-norm minimization, which strives to smooth while retaining all relevant detail. For the fiber tract mapping, a smooth 3D vector field indicating the dominant anisotropic direction at each spatial location is computed from the smoothed data. Neuronal fibers are then traced by calculating the integral curves of this vector field. Results are expressed using three modes of visualization: (1) Line integral convolution produces an oriented texture which shows fiber pathways in a planar slice of the data. (2) A streamtube map is generated to present a 3D view of fiber tracts. Additional information, such as degree of anisotropy, can be encoded in the tube radius, or by using color. (3) A particle system form of visualization is also presented. This mode of display allows for interactive exploration of fiber connectivity with no additional preprocessing.

Imaging of postthalamic visual fiber tracts by anisotropic diffusion weighted MRI and diffusion tensor imaging: principles and applications

Diffusion weighted MRI offers the possibility to study the course of the cerebral white matter tracts. In the present manuscript, the basics, the technique and the limitations of diffusion tensor imaging and anisotropic diffusion weighted MRI are presented and their applications in various neurological and neurosurgical diseases are discussed with special emphasis on the visual system. A special focus is laid on the combination of fiber tract imaging, anatomical imaging and functional MRI for presurgical planning and intraoperative neuronavigation of lesions near the visual system.

Combined analysis of DTI and fMRI data reveals a joint maturation of white and grey matter in a fronto-parietal network

The aim of this study was to explore whether there are networks of regions where maturation of white matter and changes in brain activity show similar developmental trends during childhood. In a previous study, we showed that during childhood, grey matter activity increases in frontal and parietal regions. We hypothesized that this would be mediated by maturation of white matter. Twenty-three healthy children aged 8-18 years were investigated. Brain activity was measured using the blood oxygen level-dependent (BOLD) contrast with functional magnetic resonance imaging (fMRI) during performance of a working memory (WM) task. White matter microstructure was investigated using diffusion tensor imaging (DTI). Based on the DTI data, we calculated fractional anisotropy (FA), an indicator of myelination and axon thickness. Prior to scanning, WM score was evaluated. WM score correlated independently with FA values and BOLD response in several regions. FA values and BOLD response were extracted for each subject from the peak voxels of these regions. The FA values were used as covariates in an additional BOLD analysis to find brain regions where FA values and BOLD response correlated. Conversely, the BOLD response values were used as covariates in an additional FA analysis. In several cortical and sub-cortical regions, there were positive correlations between maturation of white matter and increased brain activity. Specifically, and consistent with our hypothesis, we found that FA values in fronto-parietal white matter correlated with BOLD response in closely located grey matter in the superior frontal sulcus and inferior parietal lobe, areas that could form a functional network underlying working memory function.

Diffusion tensor MRI correlates with executive dysfunction in patients with ischaemic leukoaraiosis

BACKGROUND

Cerebral small vessel disease is a common cause of vascular dementia. Both discrete lacunar infarcts and more diffuse ischaemic changes, seen as confluent high signal (leukoaraiosis) on T2 weighted magnetic resonance imaging (MRI), occur. However, there is a weak correlation between T2 lesion load and cognitive impairment. Diffusion tensor MRI (DTI) is a new technique that may provide a better index of white matter damage.

OBJECTIVES

To determine whether DTI measures are correlated more strongly with cognitive performance than lesion load on T2 weighted images, and whether these correlations are independent of conventional MRI parameters.

METHODS

36 patients with ischaemic leukoaraiosis (leukoaraiosis plus a previous lacunar stroke) and 19 healthy volunteers underwent DTI, conventional MRI, and neuropsychological assessment.

RESULTS

On DTI, diffusivity was increased both within lesions and in normal appearing white matter. Mean diffusivity of normal appearing white matter correlated with full scale IQ (r = -0.46, p = 0.009) and tests of executive function. These correlations remained significant after controlling for age, sex, brain volume, and T1/T2 lesion volumes. No significant correlation was identified between T2 lesion load and IQ or neuropsychological scores. Of conventional measures, brain volume correlated best with cognitive function.

CONCLUSIONS

Diffusion tensor measurements correlate better with cognition than conventional MRI measures. They may be useful in monitoring disease progression and as a surrogate marker for treatment trials. The findings support the role of white matter damage and disruption of white matter connections in the pathogenesis of cognitive impairment in cerebral small vessel disease.

MRI-based morphometric of typical and atypical brain development

The neuroinformatics landscape in which human brain morphometry occurs has advanced dramatically over the past few years. Rapid advancement in image acquisition methods, image analysis tools and interpretation of morphometric results make the study of in vivo anatomic analysis both challenging and rewarding. This has revolutionized our expectations for current and future diagnostic and investigative work with the developing brain. This paper will briefly cover the methods of morphometric analysis available for neuroanatomic analysis, and tour some sample results from a prototype retrospective database of neuroanatomic volumetric information. From these observations, issues regarding the anatomic variability of developmental maturation of neuroanatomic structures in both typically and atypically developing populations can be discussed.

Fiber Tract–based Atlas of Human White Matter Anatomy

Two- and three-dimensional (3D) white matter atlases were created on the basis of high-spatial-resolution diffusion tensor magnetic resonance (MR) imaging and 3D tract reconstruction. The 3D trajectories of 17 prominent white matter tracts could be reconstructed and depicted. Tracts were superimposed on coregistered anatomic MR images to parcel the white matter. These parcellation maps were then compared with coregistered diffusion tensor imaging color maps to assign visible structures. The results showed (a). which anatomic structures can be identified on diffusion tensor images and (b). where these anatomic units are located at each section level and orientation. The atlas may prove useful for educational and clinical purposes.

Stroke patients' evolving symptoms assessed by tractography

PURPOSE

To test the hypothesis that an interval evolution in the location of the depicted sensorimotor tract relative to the infarct (the "tract-infarct relationship") may be related to stroke victims' symptom progression.

MATERIALS AND METHODS

Patients (N = 7) who underwent multiple diffusion-tensor imaging (DTI) studies during symptomatic progression were included in this study. DTI was performed using a single-shot echo-planar imaging (EPI) technique with a motion-probing gradient in six orientations, a b-value of 800 seconds/mm2, and six image averages. The total scan time was four minutes and 24 seconds. Fiber-tracking of the sensorimotor pathways was performed, and the locations of these tracts were retrospectively assessed in relation to the evolution of the symptoms.

RESULTS

Five of the seven patients showed an interval enlargement of the infarct on diffusion-weighted (DW) images. In two of these cases the lesion enlarged to involve the sensorimotor tracts, while in three cases the lesion enlarged only so far to come into close proximity to the sensorimotor tract. In the remaining two cases there was no interval enlargement of the infarct, and therefore the tract-infarct relationship could not account for the evolution of the symptoms in these cases.

CONCLUSION

A tract-infarct relationship can be observed with the use of a fiber-tracking technique, and the results may improve our understanding of the symptom progression seen in stroke victims.

Combination of event-related fMRI and diffusion tensor imaging in an infant with perinatal stroke

Focal ischemic brain injury, or stroke, is an important cause of later handicap in children. Early assessment of structure-function relationships after such injury will provide insight into clinico-anatomic correlation and potentially guide early intervention strategies. We used combined functional MRI (fMRI) with diffusion tensor imaging (DTI) in a 3-month-old infant to explore the structure-function relationship after unilateral perinatal stroke that involved the visual pathways. With visual stimuli, fMRI showed a negative BOLD activation in the visual cortex of the intact right hemisphere, principally in the anterior part, and no activation in the injured hemisphere. The functional activation in the intact hemisphere correlated clearly with the fiber tract of the optic radiation visualized with DTI. DTI confirmed the absence of the optic radiation in the damaged left hemisphere. In addition, event-related fMRI (ER-fMRI) experiments were performed to define the characteristics of the BOLD response. The shape is that of an inverted gamma function (similar to a negative mirror image of the known positive adult BOLD response). The maximum decrease was reached at 5-7 s with signal changes of -1.7 +/- 0.4%.Thus, this report describes for the first time the combined use of DTI and event-related fMRI in an infant and provides insight into the localization of the fMRI visual response in the young infant and the characteristics of the BOLD response.

Analysis of noise effects on DTI-based tractography using the brute-force and multi-ROI approach

Diffusion tensor tractography based on line propagation is a promising and widely used technique, but it is known to be sensitive to noise and the size and location of the seed regions of interest (ROIs). The effects of these parameters on the tractography results were analyzed quantitatively using high-resolution diffusion tensor imaging (DTI) with a high signal-to-noise ratio (SNR) on a fixed mouse brain. The anterior commissure (AC), as judged from a T2-weighted image, was used as an anatomical reference within which the tracts could be located. Monte Carlo simulation was performed by adding Gaussian noise to the time domain data and repeating the tractography. Deviations of the tracking results were measured as a function of SNR. Such noise effects were evaluated for a simple one-ROI approach and a combined two-ROI and brute-force (BF) approach. The influence of ROI size and location for the two-ROI + BF approach was also analyzed. The results confirmed the hypothesis that one can increase the validity of DTI-based tractography by adopting the BF and multi-ROI approach, with respect to the simple one-ROI approach.

Intraoperative three-dimensional visualization of the pyramidal tract in a neuronavigation system (PTV) reliably predicts true position of principal motor pathways

BACKGROUND

This prospective study employs anisotropic diffusion-weighted (ADW) magnetic resonance imaging for the integration of individual spatial information concerning the principal motor pathways into the operating room during microneurosurgery in the central region. We hypothesize that the three-dimensional (3-D) visualization of the pyramidal tract position (PTV) in a neuronavigation system based on ADW provides valid information concerning the position and extension of the principal motor pathways.

METHODS

A total of 13 consecutive patients with lesions adjacent to the pyramidal tracts and the central region underwent microneurosurgery with the help of pyramidal tract visualization (PTV). An ADW sequence obtained preoperatively was fused to an anatomic navigation sequence. The 3-D reconstructions of the precentral gyrus (PG), the pyramidal tract, and the tumor were available in a customized neuronavigation system during surgery. Intraoperatively the PG was identified on the basis of the aforementioned data. Electric motorcortex stimulation (CS) was used to directly verify the PG location and indirectly the fiber tract position.

RESULTS

In 11 cases (92%) the prediction of the principal motor pathways' position was correct. In one case of a meningioma, according to PTV, the tumor was falsely localized postcentrally. In the case of a precentral cavernoma, no motor response could be elicited by cortical stimulation.

CONCLUSION

Intraoperative PTV on the basis of ADW provides the neurosurgeon with reliable information concerning the position of the principal motor pathways during intracranial procedures as proved with intraoperative electrophysiological testing. The technique has the potential to reduce operative morbidity. PTV is straightforward and can be adapted to other customized neuronavigation devices.

Automatic segmentation of thalamic nuclei from diffusion tensor magnetic resonance imaging

The nuclei of the thalamus have traditionally been delineated by their distinct cyto/myeloarchitectural appearance on histology. Here, we show that diffusion tensor magnetic resonance imaging (DTI) can noninvasively resolve the major thalamic nuclei based on the characteristic fiber orientation of the corticothalamic/thalamocortical striations within each nucleus. Using an automatic clustering algorithm, we extracted the Talairach coordinates for the individual thalamic nuclei. The center-of-mass coordinates for the segmented nuclei were found to agree strongly with those obtained from a histological atlas. The ability to resolve thalamic nuclei with DTI will allow for morphometric analysis of specific nuclei and improved anatomical localization of functional activation in the thalamus.

Brain fiber tracking with clinically feasible diffusion-tensor MR imaging: initial experience

Two technical challenges must be overcome before brain fiber tracking with diffusion-tensor magnetic resonance (MR) imaging can be applied to clinical practice: Imaging time must be shortened, and image distortion must be minimized. Single-shot echo-planar MR imaging with parallel imaging technique enabled both objectives to be accomplished. Twenty-three consecutive patients with brain tumors underwent MR imaging with a 1.5-T whole-body MR system. Fiber tracts on the lesion side in the brain had varying degrees of displacement or disruption as a result of the tumor. Tract disruption resulted from direct tumor involvement, compression on the tract, and vasogenic edema surrounding the tumor. This diffusion-tensor MR imaging method with the parallel imaging technique allows clinically feasible brain fiber tracking.

MR diffusion tensor imaging: recent advance and new techniques for diffusion tensor visualization

Recently, diffusion tensor imaging is attracting the biomedical researchers for its application in depiction of fiber tracts based on diffusion anisotropy. In this paper, we briefly describe the basic theory of diffusion tensor MR imaging, the determination process of diffusion tensor, and the basic concepts of diffusion tensor visualization techniques. Several results of clinical application in our institute are also introduced. Finally, the limitations, advantages and disadvantages of the techniques are discussed for further application of diffusion tensor visualization.

White matter tractography using diffusion tensor deflection

Diffusion tensor MRI provides unique directional diffusion information that can be used to estimate the patterns of white matter connectivity in the human brain. In this study, the behavior of an algorithm for white matter tractography is examined. The algorithm, called TEND, uses the entire diffusion tensor to deflect the estimated fiber trajectory. Simulations and imaging experiments on in vivo human brains were performed to investigate the behavior of the tractography algorithm. The simulations show that the deflection term is less sensitive than the major eigenvector to image noise. In the human brain imaging experiments, estimated tracts were generated in corpus callosum, corticospinal tract, internal capsule, corona radiata, superior longitudinal fasciculus, inferior longitudinal fasciculus, fronto-occipital fasciculus, and uncinate fasciculus. This approach is promising for mapping the organizational patterns of white matter in the human brain as well as mapping the relationship between major fiber trajectories and the location and extent of brain lesions.

Diffusion Tensor Imaging in Patients With Major Cerebral Artery Occlusive Disease

Diffusion tensor (DT) imaging provides quantitative information about the magnitude and the directionality (anisotropy) of water diffusion in vivo and can detect pathologic changes in brain ischemia. This study tried to detect ischemic brain damage using DT imaging in patients with symptomatic chronic major cerebral artery occlusive disease. DT imaging was performed using a 3.0 Tesla magnetic resonance (MR) scanner in 50 patients with unilateral internal carotid artery or middle cerebral artery stenosis or occlusion, who had no obvious infarct lesions on conventional MR imaging. Thirty-three patients underwent DT imaging before and after vascular reconstruction surgery. Fractional anisotropy (FA) was calculated in the middle cerebral artery territory. Preoperative FA values in the ipsilateral side were significantly lower than those in the contralateral side. After surgery, the FA value was significantly increased. DT imaging may indicate ischemic brain damage, not visualized by conventional MR imaging, in patients with major cerebral artery occlusive disease.

Magnetic resonance microimaging of intraaxonal water diffusion in live excised lamprey spinal cord

Anisotropy of water diffusion in axon tracts, as determined by diffusion-weighted MRI, has been assumed to reflect the restriction of water diffusion across axon membranes. Reduction in this anisotropy has been interpreted as degeneration of axons. These interpretations are based primarily on a priori reasoning that has had little empirical validation. We used the experimental advantages of the sea lamprey spinal cord, which contains several very large axons, to determine whether intraaxonal diffusion is isotropic and whether anisotropy is attributable to restriction of water mobility by axon surface membranes. Through the application of magnetic resonance microimaging, we were able to measure the purely intraaxonal diffusion characteristics of the giant reticulospinal axons (20-40 microm in diameter). The intraaxonal apparent diffusion coefficients of water parallel (longitudinal ADC, l-ADC) and perpendicular (transverse ADC, t-ADC) to the long axis were 0.98 +/- 0.06 (10(-3) mm2 sec) and 0.97 +/- 0.11 (10(-3) mm2 sec), respectively. In white matter regions that included multiple axons, l-ADCs were almost identical regardless of axon density in the sampled axon tract. By comparison, t-ADCs were reduced and varied inversely with the number of axons (and thus axolemmas) in a fixed cross-sectional area. Thus, diffusion was found to be isotropic when measured entirely within a single axon and anisotropic when measured in regions that included multiple axons. These findings support the hypothesis that the cell membrane is the primary source of diffusion anisotropy in fiber tracts of the central nervous system.

Diffusion-tensor imaging of cognitive performance

MR methods have for some years been used to assess cognitive performance. Recently, studies have shown that diffusion-tensor imaging (DTI), which provides noninvasive maps of microscopic structural information of oriented tissue in vivo, is finding utility in studies of cognition in the normal and abnormal aging population. These studies suggest that water proton nonrandom, anisotropic diffusion measured by DTI is highly sensitive to otherwise subtle disease processes not easily seen with conventional MRI tissue contrast mechanisms and raises new issues of the role of MR in assessing cognitive potential.

Advances in postnatal neuroimaging: relevance to pathogenesis and treatment of brain injury

The human brain is susceptible to a wide variety of insults. The permanent residua of these abnormalities are represented in dysfunction of one or more areas of neurodevelopment. A full understanding of normal brain development, mechanisms of brain injury, and consequences for subsequent brain development is required to determine which infants are at risk for neurodevelopmental handicap, and to monitor the effects of new treatments and management regimens designed to prevent these disabilities. Advanced magnetic resonance techniques, such as quantitative morphometric magnetic resonance techniques, diffusion-weighted magnetic resonance techniques, and magnetic resonance spectroscopy applied to the study of early human brain development have given us a better understanding of the pathophysiologic mechanisms of brain injury and its effects on subsequent brain development. Magnetic resonance imaging has provided an invaluable tool for the study of the fetal and newborn brain in vivo.

Fiber tracking: principles and strategies - a technical review

The state of the art of reconstruction of the axonal tracts in the central nervous system (CNS) using diffusion tensor imaging (DTI) is reviewed. This relatively new technique has generated much enthusiasm and high expectations because it presently is the only approach available to non-invasively study the three-dimensional architecture of white matter tracts. While there is no doubt that DTI fiber tracking is providing exciting new opportunities to study CNS anatomy, it is very important to understand its limitations. In this review we therefore assess the basic principles and the assumptions that need to be made for each step of the study, including both data acquisition and the elaborate fiber reconstruction algorithms. Special attention is paid to situations where complications may arise, and possible solutions are reviewed. Validation issues and potential future directions and improvements are also discussed.

A 3D fiber model of the human brainstem

A new neuroanatomic approach to evaluate the fiber orientation in gross histological sections of the human brain was developed. Serial sections of a human brainstem were used to derive fiber orientation maps by analysis of polarized light sequences of these sections. Fiber inclination maps visualize angles of inclination, and fiber direction maps show angles of direction. These angles define vectors which can be visualized as RGB-colors. The serial sections were aligned to each other using the minimized Euclidian distance as fit criterion. In the 3D data set of the human brainstem the major fiber tracts were segmented, and three-dimensional models of these fiber tracts were generated. The presented results demonstrate that two kinds of fiber atlases are feasible: a fiber orientation atlas representing a vector in each voxel, which shows the nerve fiber orientation, and a volume-based atlas representing the major fiber tracts. These models can be used for the evaluation of diffusion tensor data as well as for neurosurgical planning.

Preoperative assessment of motor cortex and pyramidal tracts in central cavernoma employing functional and diffusion-weighted magnetic resonance imaging

BACKGROUND

Functional MRI (fMRI) combines anatomic with functional information and has therefore been widely used for preoperative planning of patients with mass lesions affecting functionally important brain regions. However, the courses of functionally important fiber tracts are not visualized. We therefore propose to combine fMRI with diffusion-weighted MRI (DWI) that allows visualization of large fiber tracts and to implement this data in a neuronavigation system.

METHODS

DWI was successfully performed at a field strength of 1.5 Tesla, employing a spin-echo sequence with gradient sensitivity in six noncollinear directions to visualize the course of the pyramidal tracts, and was combined with echo-planar T2* fMRI during a hand motor task in a patient with central cavernoma.

RESULTS

Fusion of both data sets allowed visualization of the displacement of both the primary sensorimotor area (M1) and its large descending fiber tracts. Intraoperatively, these data were used to aid in neuronavigation. Confirmation was obtained by intraoperative electrical stimulation. Postoperative MRI revealed an undisrupted pyramidal tract in the neurologically intact patient.

CONCLUSION

The combination of fMRI with DWI allows for assessment of functionally important cortical areas and additional visualization of large fiber tracts. Information about the orientation of fiber tracts in normal appearing white matter in patients with tumors within the cortical motor system cannot be obtained by other functional or conventional imaging methods and is vital for reducing operative morbidity as the information about functional cortex. This technique might, therefore, have the prospect of guiding neurosurgical interventions, especially when linked to a neuronavigation system.

Diffusion-tensor MRI: theory, experimental design and data analysis - a technical review

This article treats the theoretical underpinnings of diffusion-tensor magnetic resonance imaging (DT-MRI), as well as experimental design and data analysis issues. We review the mathematical model underlying DT-MRI, discuss the quantitative parameters that are derived from the measured effective diffusion tensor, and describe artifacts that arise in typical DT-MRI acquisitions. We also discuss difficulties in identifying appropriate models to describe water diffusion in heterogeneous tissues, as well as in interpreting experimental data obtained in such issues. Finally, we describe new statistical methods that have been developed to analyse DT-MRI data, and their potential uses in clinical and multi-site studies.

The basis of anisotropic water diffusion in the nervous system - a technical review

Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

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

Holoprosencephaly in children: diffusion tensor MR imaging of white matter tracts of the brainstem--initial experience

PURPOSE

To evaluate the dimensions of specific white matter tracts in the brainstems (region of brain thought to be least affected) of children with holoprosencephaly by using diffusion tensor magnetic resonance (MR) imaging and to correlate these abnormalities with forebrain malformation severity and neurologic deficit severity.

MATERIALS AND METHODS

Thirteen patients with holoprosencephaly underwent diffusion tensor MR imaging, with which white matter color maps were generated. Type of holoprosencephaly was correlated with presence or absence of specific brainstem white matter tracts. Furthermore, patient rank based on cortico-ponto-spinal tract (CPST) and middle cerebellar peduncle (MCP) dimensions was correlated with holoprosencephaly type and neurodevelopmental score by using Spearman rank correlation analysis.

RESULTS

Two patients had alobar holoprosencephaly, five had the semilobar type, one had the lobar type, and one had the middle-hemisphere-variant type. Four patients were excluded from analysis. In the two patients with alobar holoprosencephaly, the CPSTs were absent bilaterally. In all of the remaining patients except one, who had semilobar holoprosencephaly in which the CPSTs could not be identified at the level of the medulla oblongata, all tracts were present bilaterally. Holoprosencephaly type and neurodevelopmental score correlated strongly with CPST and MCP dimensions (P <.01) over and above the effect of age.

CONCLUSION

In vivo identification of brainstem white matter tract abnormalities in patients with holoprosencephaly can be achieved by performing diffusion tensor MR imaging.

Three-dimensional diffusion tensor magnetic resonance microimaging of adult mouse brain and hippocampus

The use of diffusion tensor information as an additional contrast in MR microimaging was investigated in ex vivo mouse brain and isolated hippocampus. Diffusion Tensor Imaging (DTI) provided unique contrast to identify many internal structures of the gray matter such as hippocampus, thalamus, and cortex. In hippocampus, stratum granulosum and stratum pyramidale could be identified using the isotropic water diffusion constant. Stratum moleculare and stratum radiatum were identified from their characteristic fiber architecture revealed by color-coded DTI. Identification of these structures allowed reconstruction of their 3-D volume. Thus, high-resolution DTI has excellent potential as a tool for 3-D characterization of murine brains.

Brain white matter anatomy of tumor patients evaluated with diffusion tensor imaging

We applied multislice, whole-brain diffusion tensor imaging (DTI) to two patients with anaplastic astrocytoma. Data were analyzed using DTI-based, color-coded images and a 3-D tract reconstruction technique for the study of altered white matter anatomy. Each tumor was near two major white matter tracts, namely, the superior longitudinal fasciculus and the corona radiata. Those tracts were identified using the color-coded maps, and spatial relationships with the tumors were characterized. In one patient the tumor displaced adjacent white matter tracts, whereas in the other it infiltrated the superior longitudinal fasciclus without displacement of white matter. DTI provides new information regarding the detailed relationship between tumor growth and nearby white matter tracts, which may be useful for preoperative planning.

Diffusion imaging in brain tumors

Diffusion-weighted imaging is of limited value in the MR imaging diagnosis of various tumor pathologies, except in differentiating between dermoids/epidermoids and arachnoid cysts. Diffusion tractography, on the other hand, allows accurate depiction of important white-matter tracts adjacent to brain tumors. This technique uses data derived from diffusion tensor imaging.

Imaging cortical association tracts in the human brain using diffusion-tensor-based axonal tracking

Diffusion-tensor fiber tracking was used to identify the cores of several long-association fibers, including the anterior (ATR) and posterior (PTR) thalamic radiations, and the uncinate (UNC), superior longitudinal (SLF), inferior longitudinal (ILF), and inferior fronto-occipital (IFO) fasciculi. Tracking results were compared to existing anatomical knowledge, and showed good qualitative agreement. Guidelines were developed to reproducibly track these fibers in vivo. The interindividual variability of these reconstructions was assessed in a common spatial reference frame (Talairach space) using probabilistic mapping. As a first illustration of this technical capability, a reduction in brain connectivity in a patient with a childhood neurodegenerative disease (X-linked adrenoleukodystrophy) was demonstrated.

Whole brain segmentation: automated labeling of neuroanatomical structures in the human brain

We present a technique for automatically assigning a neuroanatomical label to each voxel in an MRI volume based on probabilistic information automatically estimated from a manually labeled training set. In contrast to existing segmentation procedures that only label a small number of tissue classes, the current method assigns one of 37 labels to each voxel, including left and right caudate, putamen, pallidum, thalamus, lateral ventricles, hippocampus, and amygdala. The classification technique employs a registration procedure that is robust to anatomical variability, including the ventricular enlargement typically associated with neurological diseases and aging. The technique is shown to be comparable in accuracy to manual labeling, and of sufficient sensitivity to robustly detect changes in the volume of noncortical structures that presage the onset of probable Alzheimer's disease.

Intraoperative mapping of the subcortical language pathways using direct stimulations. An anatomo-functional study

Functional neuroimaging has improved pre-planning of surgery in eloquent cortical areas, but remains unable to map white matter. Thus, tumour resection in functional subcortical regions still presents a high risk of sequelae. The authors successfully used intraoperative electrical stimulations to perform subcortical language pathway mapping in order to avoid postoperative definitive deficit, and correlated these functional findings with the anatomical location of the eloquent bundles detected using postoperative MRI. At the same time, this also improved knowledge of fibre connectivity. Thirty patients harbouring a cortico-subcortical low-grade glioma in the left dominant hemisphere were operated on whilst awake using intraoperative electrical functional mapping during surgical resection. Language cortical sites and subcortical pathways were clearly identified and preserved in the 30 cases. The anatomo-functional correlations between data obtained using intraoperative subcortical mapping and postoperative MRI revealed the existence in all patients of common pathways which seem essential to language. This was shown by inducing reproducible speech disturbances during stimulations as follows: the subcallosal fasciculus (initiation disorders), the periventricular white matter (dysarthria), the arcuate fasciculus and the insular connections (anomia). Clinically, all patients except three presented a transient postoperative dysphasia, which resolved within 3 months. On control MRI, 14 resections were total and 16 subtotal due to infiltration of functional bundles described above. It is recommended that the combination of the techniques as described could prove ideal for future non-invasive reliable subcortical mapping both in healthy volunteers and in patients harbouring a (cortico)subcortical lesion in order to optimize surgical pre-planning.

Diffusion tensor imaging and its application to neuropsychiatric disorders

Magnetic resonance diffusion tensor imaging (DTI) is a new technique that can be used to visualize and measure the diffusion of water in brain tissue; it is particularly useful for evaluating white matter abnormalities. In this paper, we review research studies that have applied DTI for the purpose of understanding neuropsychiatric disorders. We begin with a discussion of the principles involved in DTI, followed by a historical overview of magnetic resonance diffusion-weighted imaging and DTI and a brief description of several different methods of image acquisition and quantitative analysis. We then review the application of this technique to clinical populations. We include all studies published in English from January 1996 through March 2002 on this topic, located by searching PubMed and Medline on the key words "diffusion tensor imaging" and "MRI." Finally, we consider potential future uses of DTI, including fiber tracking and surgical planning and follow-up.

Validation of diffusion tensor magnetic resonance axonal fiber imaging with registered manganese-enhanced optic tracts

Noninvasive mapping of white matter tracts using diffusion tensor magnetic resonance imaging (DTMRI) is potentially useful in revealing anatomical connectivity in the human brain. However, a gold standard for validating DTMRI in defining axonal fiber orientation is still lacking. This study presents the first validation of the principal eigenvector of the diffusion tensor in defining axonal fiber orientation by superimposing DTMRI with manganese-enhanced MRI of optic tracts. A rat model was developed in which optic tracts were enhanced by manganese ions. Manganese ion (Mn(2+)) is a potent T1-shortening agent and can be uptaken and transported actively along the axon. Based on this property, we obtained enhanced optic tracts with a T1-weighted spin-echo sequence 10 h after intravitreal injection of Mn(2+). The images were compared with DTMRI acquired with exact spatial registration. Deviation angles between tangential vectors of the enhanced tracts and the principal eigenvectors of the diffusion tensor were then computed pixel by pixel. We found that under signal-to-noise (SNR) of 30, the variance of deviation angles was (13.27 degrees). In addition, the dependence of this variance on SNR obeys stochastic behavior if SNR is greater than 10. Based on this relation, we estimated that an rms deviation of less than 10 degrees could be achieved with DTMRI when SNR is 40 or greater. In conclusion, our method bypasses technical difficulties in conventional histological approach and provides an in vivo gold standard for validating DTMRI in mapping white matter tracts.

Functional and diffusion-weighted magnetic resonance images of space-occupying lesions affecting the motor system: imaging the motor cortex and pyramidal tracts

OBJECT

During neurosurgical interventions, preservation of subcortical axons is as important as preservation of cortical neurons. The goal of this study was to assess the combined use of functional (f) and diffusion-weighted (DW) magnetic resonance (MR) imaging to assist in the preservation of the structure and function of the motor system.

METHODS

The authors evaluated the combination of fMR imaging and DW MR imaging to detect cortical motor areas with their corresponding pyramidal tracts in 12 healthy volunteers and in 10 consecutive patients with various space-occupying lesions affecting the central motor system. Activation within the primary motor cortex (M1) and white matter bundles originating from this cortical region was demonstrated in 21 of the 22 individuals examined. Additional activation was exhibited along the course of white matter tracts at the level of the pons and. in the contralateral hemisphere, in the M1. Fiber tract displacement was visualized in all patients in white matter that had appeared normal on routine T1- and T2-weighted MR images.

CONCLUSIONS

The combination of DW MR and fMR imaging allows visualization of the origin, direction, and functionality of large white matter tracts. This will prove helpful for imaging structural connectivity within the brain during functional imaging. Moreover, local relationships of cerebral tumors that encroach upon M1 and subcortical fiber tracts can be defined. This promises to decrease patient morbidity and to broaden the clinical applications of functional imaging.

Diffusion tensor imaging and axonal tracking in the human brainstem

Diffusion tensor MRI was used to demonstrate in vivo anatomical mapping of brainstem axonal connections. It was possible to identify the corticospinal tract (CST), medial lemniscus, and the superior, medial, and inferior cerebellar peduncles. In addition, the cerebral peduncle could be subparcellated into component tracts, namely, the frontopontine tract, the CST, and the temporo-/parieto-/occipitopontine tract. Anatomical landmarks and tracking thresholds were established for each fiber and, using these standards, reproducibility of automated tracking as assessed by intra- and interrater reliability was found to be high (kappa > 0.82). Reconstructed fibers corresponded well to existing anatomical knowledge, validating the tracking. Information on the location of individual tracts was coregistered with quantitative MRI maps to automatically measure MRI parameters on a tract-by-tract basis. The results reveal that each tract has a unique spatial signature in terms of water relaxation and diffusion anisotropy.

Water diffusion changes in Wallerian degeneration and their dependence on white matter architecture

This study investigates water diffusion changes in Wallerian degeneration. We measured indices derived from the diffusion tensor (DT) and T2-weighted signal intensities in the descending motor pathways of patients with small chronic lacunar infarcts of the posterior limb of the internal capsule on one side. We compared these measurements in the healthy and lesioned sides at different levels in the brainstem caudal to the primary lesion. We found that secondary white matter degeneration is revealed by a large reduction in diffusion anisotropy only in regions where fibers are arranged in isolated bundles of parallel fibers, such as in the cerebral peduncle. In regions where the degenerated pathway crosses other tracts, such as in the rostral pons, paradoxically there is almost no change in diffusion anisotropy, but a significant change in the measured orientation of fibers. The trace of the diffusion tensor is moderately increased in all affected regions. This allows one to differentiate secondary and primary fiber loss where the increase in trace is considerably higher. We show that DT-MRI is more sensitive than T2-weighted MRI in detecting Wallerian degeneration. Significant diffusion abnormalities are observed over the entire trajectory of the affected pathway in each patient. This finding suggests that mapping degenerated pathways noninvasively with DT-MRI is feasible. However, the interpretation of water diffusion data is complex and requires a priori information about anatomy and architecture of the pathway under investigation. In particular, our study shows that in regions where fibers cross, existing DT-MRI-based fiber tractography algorithms may lead to erroneous conclusion about brain connectivity.

Efficient cardiac diffusion tensor MRI by three-dimensional reconstruction of solenoidal tensor fields

Tensor tomography is being investigated as a technique for reconstruction of in vivo diffusion tensor fields that can potentially be used to reduce the number of magnetic resonance imaging (MRI) measurements. Specifically, assessments are being made of the reconstruction of cardiac diffusion tensor fields from 3D Radon planar projections using a filtered backprojection algorithm in order to specify the helical fiber structure of myocardial tissue. Helmholtz type decomposition is proposed for 3D second order tensor fields. Using this decomposition a Fourier projection theorem is formulated in terms of the solenoidal and irrotational components of the tensor field. From the Fourier projection theorem, two sets of Radon directional measurements, one that reconstructs the solenoidal component and one that reconstructs the irrotational component of the tensor field, are prescribed. Based on these observations filtered backprojection reconstruction formulae are given for the reconstruction of a 3D second order tensor field and its solenoidal and irrotational components from Radon projection measurements. Computer simulations demonstrate the validity of the mathematical formulations and demonstrate that a realistic model of the helical fiber structure of the myocardial tissue specifies a diffusion tensor field for which the first principal vector (the vector associated with the maximum eigenvalue) of the solenoidal component accurately approximates the first principal vector of the diffusion tensor. A priori knowledge of this allows the orientation of the myocardial fiber structure to be specified utilizing one half of the number of MRI measurements of a normal diffusion tensor field study.

Abnormal brain white matter in schizophrenia: a diffusion tensor imaging study

Fractional anisotropy and the mean diffusion coefficient were measured in the cerebral volume in 20 schizophrenic and 24 healthy subjects, men and women, using diffusion tensor imaging. In addition, 3D SPGR was used for segmentation of brain tissue into grey and white matter and cerebrospinal fluid. In schizophrenic patients, fractional anisotropy was reduced in the splenium of the corpus callosum and in adjacent occipital white matter. The segmentation revealed no tissue deficits in the volume of reduced fractional anisotropy. The mean diffusion was increased in the total white and grey matter volume of the schizophrenic patients compared with the healthy subjects. The findings support the view that global and regional white matter abnormalities occur in chronic schizophrenia.

In vivo 3D visualization of normal pyramidal tracts in human subjects using diffusion weighted magnetic resonance imaging and a neuronavigation system

We describe the potential of anisotropic diffusion weighted imaging to visualize the course of large cerebral fiber tracts. Five healthy volunteers were investigated at a field strength of 1.5 Tesla, employing a spin-echo diffusion weighted sequence with gradient sensitivity in six non-collinear directions to visualize the course of the pyramidal tracts. The pyramidal tracts were segmented and reconstructed for three-dimensional visualization. Reconstruction results together with a fusioned high resolution 3D T1 weighted image data set were available in a customized neuronavigation system. Origination in the primary motor cortex, convergence in the centrum semiovale, the posterior limb of the internal capsule, the cerebral peduncles, the splitting at the level of the pons, and the pyramidal decussation were identified in all subjects. Fiber tract maps might have the prospect of guiding neurosurgical interventions, especially when being linked to a neuronavigation system. Other potential applications include the demonstration of the anatomical substrate of functional connectivity in the human brain.

Three-dimensional visualization of the pyramidal tract in a neuronavigation system during brain tumor surgery: first experiences and technical note

OBJECTIVE

To integrate spatial three-dimensional information concerning the pyramidal tracts into a customized system for frameless neuronavigation during brain tumor surgery.

METHODS

Four consecutive patients with intracranial tumors in eloquent areas underwent diffusion-weighted and anatomic magnetic resonance imaging studies within 48 hours before surgery. Diffusion-weighted datasets were merged with anatomic data for navigation purposes. The pyramidal tracts were segmented and reconstructed for three-dimensional visualization. The reconstruction results, together with the fused-image dataset, were available during surgery in the environment of a customized neuronavigation system.

RESULTS

In all four patients, the combination of reconstructed data and fused images was a helpful additional source of information concerning the tumor seat and topographical interaction with the pyramidal tract. In two patients, intraoperative motor cortex stimulation verified the tumor seat with regard to the precentral gyrus.

CONCLUSION

Diffusion-weighted magnetic resonance imaging allows individual estimation of large fiber tracts applicable as important information in intraoperative neuronavigation and in planning brain tumor resection. A three-dimensional representation of fibers associated with the pyramidal tract during brain tumor surgery is feasible with the presented technique and is a helpful adjunct for the neurosurgeon. The main drawbacks include the length of time required for the segmentation procedure, the lack of direct intraoperative control of the pyramidal tract position, and brain shift. However, mapping of large fiber tracts and its intraoperative use for neuronavigation have the potential to increase the safety of neurosurgical procedures and to reduce surgical morbidity.

MR diffusion tensor imaging of white matter tract disruption in stroke at 3 T

Recent advances in MR diffusion weighted imaging (DWI) enable the identification of anisotropic white matter tracts with diffusion tensor imaging (DTI). We aimed to use a novel DTI technique to safely study patients with recent stroke in a high field (3 T) MR machine with its intrinsically higher spatial resolution and signal-to-noise ratio. Of ten patients studied, six had disruption of white matter tracts as determined by DTI. A further patient had distortion of white matter tracts around an infarct rather than actual disruption of the tracts themselves. The lack of tract destruction may imply a beneficial prognosis, information that is not available with conventional DWI.