Diffusion tensor MR imaging of the brain and white matter tractography

MR tractography with diffusion tensor imaging in clinical routine

Using MRI, we demonstrated that the depiction of the cerebral white matter fiber tracts has become a routine procedure. Diffusion tensor (DT) sequences may be analyzed with combined volume analysis and tractography extraction software, giving indirect visualization of white matter connections. We obtained DT data from 20 subjects with normal MR imaging and five patients presenting cerebral diseases such as brain tumors, multiple sclerosis and stroke, with five patients explored on two different MR scanners. Data were transferred to dedicated workstations for anatomical realignment, determination of voxel eigenvectors and calculation of fiber tract orientations in a region of interest. In all subjects, axonal directions underlying the main neuronal pathways could be delineated. Comparisons between diseased regions and contralateral areas demonstrated changes in voxel anisotropy in injured regions, revealing possible preferential fiber orientations within diffuse T2 hyperintensities. Rapid data processing allows imaging of the normal and diseased fiber pathways as part of the routine MRI examination. Therefore, it appears that whenever white matter disease is suspected a tractography can be performed with this fast and simple method that we proved to be reliable and reproducible.

Fiber tracking by diffusion tensor imaging in corticospinal tract stroke: Topographical correlation with clinical symptoms

Fiber tracking of the white matter using diffusion tensor imaging is a new imaging technique to visualize the integrity of the white matter. This study investigated the capability of this technique to localize the lacunar infarctions, particularly with respect to the body parts affected, by correlating the location of the lesion with the clinical symptoms topographically. Twenty-seven patients with capsular and pericapsular small acute infarctions underwent diffusion tensor imaging and subsequent fiber tracking of the corticospinal tract (CST). According to the lesion topography with regard to the CST, the infarctions were classified into four types: (1) the anterior type (n = 9) involving the anterior part of the CST, (2) the central type (n = 9) involving the middle or whole part of the CST, (3) the posterior type (n = 5) involving the posterior part of the CST and (4) the intact type (n = 4) not involving the CST. Motor weakness of the face, upper extremities and lower extremities was found at 100%, 67% and 44%, respectively in the anterior type, at 89%, 100% and 89%, respectively in the central type and at 20%, 80% and 100%, respectively in the posterior type. The intact type was not associated with motor weakness. In conclusion, the fiber tracking technique of the CST enables the specific localization of capsular and pericapsular infarctions with regard to the body parts affected. These results also confirm the topographical accuracy of the fiber tracking of the CST.

Prefrontal white matter volume is disproportionately larger in humans than in other primates

Determining how the human brain differs from nonhuman primate brains is central to understanding human behavioral evolution. There is currently dispute over whether the prefrontal cortex, which mediates evolutionarily interesting behaviors, has increased disproportionately. Using magnetic resonance imaging brain scans from 11 primate species, we measured gray, white and total volumes for both prefrontal and the entire cerebrum on each specimen (n = 46). In relative terms, prefrontal white matter shows the largest difference between human and nonhuman, whereas gray matter shows no significant difference. This suggests that connectional elaboration (as gauged by white matter volume) played a key role in human brain evolution.

Diffusion Tensor Tractography of Gliomatosis Cerebri

In previous reports, tracts obtained by diffusion tensor (DT) fiber tracking were terminated or deviated by the brain tumors or surrounding edema. There has been no report showing diffusion tensor tractography penetrating through the tumor. A case of glioma is reported, whose DT fiber tract passing through the tumor was observed by changing the threshold of fractional anisotropy.

Diffusion tensor magnetic resonance imaging of brain tumors

DTI seems to offer the possibility of adding important information to presurgical planning. Although experience is limited, DTI seems to provide useful local information about the structures near the tumor, and this seems to be useful in planning. In the future, DTI may provide an improved way to monitor intraoperative surgical procedures as well as their complications. Furthermore, evaluation of the response to treatment with chemotherapy and radiation therapy might also be possible. Although DTI has some limitations, its active investigation and further study are clearly warranted.

Bilateral limbic diffusion abnormalities in unilateral temporal lobe epilepsy

Diffusion tensor magnetic resonance imaging can acquire quantitative information on the microstructural integrity of white matter structures and depict brain connectivity in vivo based on the behavior of water diffusion. Diffusion tensor imaging-derived tractography has been used for virtual dissection of the fornix and cingulum in healthy subjects, but not in patients with temporal lobe epilepsy (TLE). Eight patients with medically intractable TLE and unilateral mesial temporal sclerosis and nine healthy control subjects were imaged using diffusion tensor imaging. Fiber tracking was performed to delineate the fornix and cingulum, which were quantitatively analyzed. Bilateral symmetrical reduction in fractional anisotropy was observed in the fornix of patients with TLE, together with an increase in water mobility perpendicular to the axis of the fibers. The findings in the cingulum are similar to those of the fornix with the exception of significantly increased bulk diffusivity in the latter. We observed strikingly symmetrical bilateral abnormalities of axonal integrity in the fornix and cingulum in a series of patients with unilateral mesial temporal sclerosis. Our findings suggest that TLE with unilateral mesial temporal sclerosis is associated with bilateral limbic system pathology.

Advances in neurosurgical technique in the current management of brain tumors

Despite significant advances in anatomical and functional neuroimaging modalities (eg, magnetic resonance [MR] imaging [MRI], MR spectroscopy [MRS], diffusion and perfusion MR, functional MRI [fMRI], magnetic-source imaging [MSI], diffusion tensor imaging [DTI]) and neuronavigation techniques, intraoperatively obtained functional information remains of crucial importance to the neurosurgeon, especially when operating on tumors that are located in or adjacent to functional cortical sites and subcortical pathways. This article focuses on recent advances in the surgical management of of intracerebral tumors with special emphasis on intraoperative cortical and subcortical stimulation mapping methods, and the prognostic significance of surgery on patient outcome.

White matter imaging in holoprosencephaly in children

PURPOSE OF REVIEW

Holoprosencephaly is a disorder of forebrain development characterized by a failure of the brain to separate into two hemispheres during early development. It is now clear that many cases of holoprosencephaly are caused by alterations in the genetic programmes that pattern the nervous system. Less is known about how a holoprosencephalic brain either forms or fails to form connections between various brain structures.

RECENT FINDINGS

Abnormalities in the corpus callosum, corticospinal tract, medial lemniscus and cerebellar peduncles can be seen in holoprosencephaly. Diffusion tensor imaging has been and will continue to be an important tool for imaging white matter in the brain, and will be reviewed here. Furthermore, recent evidence suggests that holoprosencephaly can be associated with delays or abnormalities in myelination. The functional implications of white matter abnormalities in children with holoprosencephaly is only beginning to be understood.

SUMMARY

Modern neuroimaging has led to a better appreciation of the variability seen in holoprosencephaly, an anomaly known to have multiple etiologies. Recent reviews of the biology of holoprosencephaly identify the condition as a defect in dorsoventral patterning. More detailed white and grey matter structure-function studies are likely to shed light on how a brain with drastically altered composition and connectivity does or does not organize itself to accomplish increasingly complex developmental functions.

Diffusion tensor MRI visualizes decreased subcortical fiber connectivity in focal cortical dysplasia

Diffusion tensor imaging (DTI) was applied to 12 patients with focal cortical dysplasia (FCD) in frontal or occipital cortex. Fiber tractography was obtained from seeding points in superior longitudinal fasciculus or posterior corona radiata. Mean fractional anisotropy of fiber bundles around the affected cortex was decreased in comparison to the contralateral hemisphere with statistical significance (paired t test, P = 0.0274). On visual analysis, tractography depicted decreased volume of fiber bundles connected to the dysplastic cortex invariably even in those with a normal T2 signal intensity of underlying white matter adjacent to FCD. DTI has high potential to be applied to localize the FCD and to provide a better understanding of the pathological changes in the white matter.

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.

ADC mapping of neurodegeneration in the brainstem and cerebellum of patients with progressive ataxias

Analysis of the apparent diffusion coefficient (ADC) maps derived from diffusion-weighted MR imaging is emerging as a reproducible, sensitive, and quantitative tool to evaluate brain damage in diseases of the white and gray matter. To explore the potentials of ADC maps analysis in degenerative ataxias, we examined 28 patients and 26 age-matched controls with T1, T2, and diffusion (b values 0-1000 along the three main body axes)-weighted MR images. Twenty-four patients had inherited genetically proven diseases including spinocerebellar ataxia type 1 (SCA1) (n = 9), spinocerebellar ataxia type 2 (SCA2) (n = 8), and Friedreich's ataxia (FA) (n = 7), whereas four patients had sporadic adult onset pure cerebellar ataxia (three idiopathic, one gluten intolerance). Area and linear measurements of the CNS structures contained in the posterior cranial fossa (PCF) preliminary enabled classification of the patients in the three morphological categories reflecting the gross pathology findings, namely olivopontocerebellar atrophy (OPCA) (n = 10: six SCA2 and four SCA1), spinal atrophy (SA) (n = 7: all FA), and cortical cerebellar atrophy (CCA) (n = 4: three idiopathic and one gluten intolerance). Seven patients with SCA1 (n = 5) or SCA2 (n = 2) had morphologic changes reminiscent of OPCA, but their values were still in the lower normal range and were classified as undefined. Mean diffusivity (D) maps of the entire brain were generated and D was measured with regions of interest (ROI) in the medulla, pons, middle cerebellar peduncles, and the peridentate white matter. Moreover, after exclusion of the skull with manual segmentation and of the CSF with application of a threshold value, histograms were obtained for D of the brainstem and cerebellum and for D of the cerebral hemispheres. As compared to controls, a (P < 0.001) increase of D was observed in the medulla, middle cerebellar peduncles, and peridentate white matter in OPCA and undefined patients groups who had also significantly increased values of the 25th and 50th percentiles in the brainstem and cerebellum D histogram. In CCA (P = 0.01), an increase of the 25th and 50th percentile of the D value was observed in the brainstem and cerebellum histograms. The SA group showed (P < 0.001) an increased D in the medulla only. A correlation between clinical severity as assessed with the Inherited Ataxias Clinical Rating Scale (IACRS) and the 50th percentile of the D value in the brainstem and cerebellum histogram (r = 0.69) was observed in patients with SCA1 or SCA2. Diffusion MR imaging reveals variable patterns of increase of D in the brainstem, cerebellum, and cerebral hemispheres in degenerative ataxias that match the known distribution of the neuropathological changes.

Intraoperative subcortical stimulation mapping for hemispherical perirolandic gliomas located within or adjacent to the descending motor pathways: evaluation of morbidity and assessment of functional outcome in 294 patients

OBJECT

Intraoperative stimulation mapping of subcortical white matter tracts during the resection of gliomas has become a valuable surgical adjunct that is used to reduce morbidity associated with tumor removal. The purpose of this retrospective analysis was to assess the morbidity and functional outcome associated with this method, thus allowing the surgeon to predict the likelihood of causing a temporary or permanent motor deficit.

METHODS

In this study, the authors report their experience with intraoperative stimulation mapping to locate subcortical motor pathways in 294 patients who underwent surgery for hemispheric gliomas within or adjacent to the rolandic cortex. Data were collected regarding intraoperative cortical and subcortical stimulation mapping results, along with the patient's neurological status pre- and postoperatively. For patients in whom an additional motor deficit occurred postoperatively, its evolution was examined. Of 294 patients, an additional postoperative motor deficit occurred in 60 (20.4%). Of those 60, 23 (38%) recovered to their preoperative baseline status within the 1st postoperative week. Another 12 (20%) recovered from their postoperative motor deficit by the end of the 4th postoperative week, and 11 more recovered to their baseline status by the end of the 3rd postoperative month. Thus, 46 (76.7%) of 60 patients with postoperative motor deficits regained their baseline function within the first 90 days after surgery. The remaining 14 patients (4.8% of the entire study population of 294) had a persistent motor deficit after 3 months. Patients whose subcortical pathways were identified with stimulation mapping were more prone to develop an additional (temporary or permanent) motor deficit than those in whom subcortical pathways could not be identified (27.5% compared with 13.1%, p = 0.003). This was also true when additional (permanent) motor deficits lasted more than 3 months (7.4% when subcortical pathways were found, compared with 2.1% when they were not found; p = 0.041).

CONCLUSIONS

In patients with gliomas that are located within or adjacent to the rolandic cortex and, thus, the descending motor tracts, stimulation mapping of subcortical pathways enables the surgeon to identify these descending motor pathways during tumor removal and to achieve an acceptable rate of permanent morbidity in these high-risk functional areas.

The effects of toluene on the central nervous system

In recent decades the organic solvent toluene (methylbenzene) has emerged as one of the best-studied neurotoxins. Long-term and intense exposure to toluene vapors in humans who abuse spray paint and related substances has led to the recognition that toluene has a severe impact on central nervous system myelin. Chronic toluene abuse produces a devastating neurological disorder, of which dementia is the most disabling component. The clinical syndrome, toluene leukoencephalopathy, can be detected by a combination of characteristic symptoms and signs, detailed neurobehavioral evaluation, and brain magnetic resonance imaging. In this paper, we consider the impact of toluene abuse on our society, describe the specific neurobehavioral deficits in toluene leukoencephalopathy, review the spectrum of neuroimaging findings in patients with this disorder, summarize the teratogenic effects of toluene in both humans and animal models, and offer possible explanations for the range of neuropathological damage seen in brains of individuals who chronically abuse toluene.

Combined Magnetic Resonance Tractography and Functional Magnetic Resonance Imaging in Evaluation of Brain Tumors Involving the Motor System

The goal of neuro-oncologic surgery is to maximize tumor resection while preserving vital brain functions. Identification of the relation between the tumor and adjacent functional cortical areas as well as efferent subcortical white matter tracts is important for preservation of function. Combined diffusion tensor imaging with magnetic resonance (MR) tractography and functional blood oxygen level-dependent MR imaging were applied successfully for preoperative planning and guidance in 2 patients with tumors near the motor cortex. The combination of these novel functional imaging techniques can provide new information for presurgical planning.

Diffusion-weighted MRI and the evaluation of spinal cord axonal integrity following injury and treatment

Diffusion-based magnetic resonance imaging (MRI) (DWI) has been shown experimentally to detect both injury and functionally significant neuroprotection of injured spinal cord white matter that would otherwise go undetected with conventional MRI techniques. The diffusion of water in the central nervous system (CNS) is thought to be affected by both its location (intracellular or extracellular), and by diffusion barriers formed by cell membranes and myelin sheaths. There is, however, controversy concerning how to obtain, interpret, and present DWI data. Computer simulations and MR microscopy have been helpful in resolving some of these issues, as well as determining exact histologic correlates to DWI findings.

Comparisons of Regional White Matter Diffusion in Healthy Neonates and Adults Performed with a 3.0-T Head-only MR Imaging Unit

PURPOSE

To evaluate the normal brains of adults and neonates for regional and age-related differences in apparent diffusion coefficient (ADC) and fractional anisotropy (FA).

MATERIALS AND METHODS

Eight healthy adults and 20 healthy neonates were examined with a 3.0-T head-only magnetic resonance (MR) imaging unit by using a single-shot diffusion-tensor sequence. Trace ADC maps, FA maps, directional maps of the putative directions of white matter (WM) tracts, and fiber-tracking maps were obtained. Regions of interest-eight in WM and one in gray matter (GM)-were predefined for the ADC and FA measurements. The Student t test was used to compare FA and ADC between adults and neonates, whereas the Tukey multiple-comparison test was used to compare FA and ADC in different brain regions in the adult and neonate groups.

RESULTS

A global elevation in ADC (P <.001) in both GM and WM and a reduction in FA (P <.001) in WM were observed in neonates as compared with these values in adults. In addition, significant regional variations in FA and ADC were observed in both groups. Regional variations in FA and ADC were less remarkable in adults, whereas neonates had consistently higher FA values and lower ADC values in the central WM as compared with these values in the peripheral WM. Fiber tracking revealed only major WM tracts in the neonates but fibers extending to the peripheral WM in the adults.

CONCLUSION

There were regional differences in FA and ADC values in the neonates; such variations were less remarkable in the adults.

Computation of the fractional anisotropy and mean diffusivity maps without tensor decoding and diagonalization: Theoretical analysis and validation

Diffusion tensor MRI (DT-MRI) is a promising modality for in vivo mapping of the organization of deep tissues. The most commonly used DT-MRI invariant maps are the mean diffusivity, mu(D), relative anisotropy (RA), and fractional anisotropy (FA). Because of the computational burden, anisotropy maps are generally computed offline. The availability of a simple procedure to compute RA, FA, and mu(D) online would make DT-MRI more useful in clinical applications that require immediate feedback. In this study, analytical expressions that relate the commonly used tensor anisotropy measures obtained from the decoded and diagonalized DT with those obtained from the first and second moments of the measured diffusion-weighted (DW) data are derived. Specifically, it is shown that for the principal icosahedron encoding scheme, RA is related to the mean and standard deviation (SD) of the DW measurements that can be computed online. Since FA is commonly used as an anisotropy measure, an analytical expression relating RA and FA was derived from the tensor invariants. These results were validated using both Monte Carlo simulations and high-resolution, normal whole-brain DT-MRI measurements acquired with different b-factors, encoding schemes, and signal-to-noise ratio (SNR) levels. The bias introduced by the rotationally variant encoding schemes into the diffusion measures is also investigated.

Three-dimensional white matter tractography by diffusion tensor imaging in ischaemic stroke involving the corticospinal tract

Diffusion tensor MR imaging (DTI) provides information on diffusion anisotropy, which can be expressed with three-dimensional (3D) white matter tractography. We used 3D white matter tractography to show the corticospinal tract in eight patients with acute or early subacute ischaemic stroke involving the posterior limb of the internal capsule or corona radiata and to assess involvement of the tract. Infarcts and the tract were shown simultaneously, providing information on their spatial relationships. In five of the eight patients, 3D fibre tract maps showed the corticospinal tract in close proximity to the infarct but not to pass through it. All these patients recovered well, with maximum improvement from the lowest score on manual muscle testing (MMT) up to the full score through rehabilitation. In the other three patients the corticospinal tract was shown running through the infarct; reduction in MMT did not necessarily improve favourably or last longer, other than in one patient. As 3D white matter tractography can show spatial relationships between the corticospinal tract and an infarct, it might be helpful in prognosis of gross motor function.

[Radiologic diagnostics of dementia]

Dementia is one of the most common diseases in the elderly population and is getting more and more important with the ageing of the population. A radiologic structural examination with CT or MRI is meanwhile a standard procedure in the diagnostic work up of patients with dementia syndrome. Radiology enables an early diagnosis and a differential diagnosis between different causes of dementia. Because structural changes occur only late in the disease process, a more detailed structural analysis using volumetric techniques or the use of functional imaging techniques is mandatory. These days, structural imaging uses MRI which enables to detect early atrophic changes at the medial temporal lobe with focus on the amygdala hippocampal complex. These changes are also present in the normal ageing process. In patients with Alzheimer's disease, however, they are more rapid and more pronounced. The use of functional imaging methods such as perfusion MRI, diffusion MRI or fMRI allow new insights into the pathophysiologic changes of dementia. The article gives an overview of the current status of structural imaging and an outlook into the potential of functional imaging methods. Detailed results of structural and functional imaging are presented in other articles of this issue.

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.

Neuroimaging and early diagnosis of Alzheimer disease: a look to the future

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the most common cause of dementia in the elderly. Current consensus statements have emphasized the need for early recognition and the fact that a diagnosis of AD can be made with high accuracy by using clinical, neuropsychologic, and imaging assessments. Magnetic resonance (MR) or computed tomographic (CT) imaging is recommended for the routine evaluation of AD. Coronal MR images can be useful to document or quantify atrophy of the hippocampus and entorhinal cortex, both of which occur early in the disease process. Both volumetric and subtraction MR techniques can be used to quantify and monitor dementia progression and rates of regional atrophy. MR measures are also increasingly being used to monitor treatment effects in clinical trials of cognitive enhancers and antidementia agents. Positron emission tomography (PET) and single photon emission CT offer value in the differential diagnosis of AD from other cortical and subcortical dementias and may also offer prognostic value. In addition, PET studies have demonstrated that subtle abnormalities may be apparent in the prodromal stages of AD and in subjects who carry susceptibility genes. PET ligands are in late-stage development for demonstration of amyloid plaques, and human studies have already begun. Functional MR-based memory challenge tests are in development as well.

Diffusion tensor imaging and aging - a review

Diffusion-tensor imaging (DTI) non-invasively provides maps of microscopic structural information of oriented tissue in vivo, which is finding utility in studies of the aging population. In contrast to the white matter maturation process, investigators have observed significant declines in the white matter ordering in normal as well as in abnormal aging. These studies suggest that water proton non-random, anisotropic diffusion measured by DTI is highly sensitive to otherwise subtle disease processes not normally seen with conventional MRI tissue contrast mechanisms.