Fractional Anisotropy Threshold Research Articles

Conditions for quantitative evaluation of injured spinal cord by in vivo diffusion tensor imaging and tractography: Preclinical longitudinal study in common marmosets

Conventional magnetic resonance imaging (MRI) can detect hemorrhage, edema, syrinx, and spinal cord atrophy, but not axonal disruption after spinal cord injury (SCI). We previously demonstrated that diffusion tensor tractography (DTT) could depict axonal disruption after hemisection SCI in common marmosets. In the present study, to determine the relationship between DTT results and functional recovery after contusive SCI, we performed longitudinal DTT, behavioral, and histological analyses before and after contusive SCI in common marmosets. By comparing the tract fiber estimate depicted by DTT with neuronal fibers labeled with RT97 and SMI-31, anti-neurofilament antibodies, we determined the optimal fractional anisotropy (FA) threshold for fiber tracking to be 0.40. The ratio of the number of tract fiber estimates at the lesion site to the number before SCI, determined by DTT, was significantly correlated with the functional recovery after SCI. Moreover, comparison of the longitudinal pre- and post-SCI FA and axial diffusivity (λ(‖)) values revealed that they decreased after injury at the sites caudal to the lesion epicenter in the corticospinal tract and rostral to the lesion epicenter in the dorsal column. The FA values, then, showed partial recovery in the dorsal column. FA-value-oriented color DTT was used to represent axonal sparing or regeneration of the different tracts. These findings indicated that DTT analysis might be a versatile non-invasive tool for evaluating the axonal disruption after SCI.

Assessment of Median Nerve with MR Neurography by Using Diffusion-Tensor Imaging: Normative and Pathologic Diffusion Values

To determine normative diffusion values of the median nerve at several anatomic locations in healthy men and women of variable age and to compare these normative values with those in patients with carpal tunnel syndrome. After ethics board approval and written informed consent were obtained, 45 healthy volunteers (30 women, 15 men) and 15 patients (10 women, five men) were studied. Volunteers were divided into three age groups. Magnetic resonance (MR) neurography with diffusion-tensor imaging (DTI) was performed in all study participants at 3.0 T by using a single-shot echo-planar imaging sequence (repetition time msec/echo time msec, 10 123/40; b=1200 sec/mm2). Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) of the median nerve were determined by two readers at three locations: the levels of the distal radioulnar joint, pisiform bone, and hamate bone. Normative FA and ADC values were calculated for men and women, different age groups, and different anatomic locations. FA and ADC did not differ between men and women (P=.28 and P=.38, respectively). FA decreased and ADC increased when moving from proximal to distal locations (P<.001). FA decreased and ADC increased significantly with age (P<.001). There was a significant difference between healthy volunteers and patients with carpal tunnel syndrome (P<.001 for both FA and ADC). An FA threshold of 0.47 and an ADC threshold of 1.054×10(-3) mm2/sec might be used in the diagnosis of carpal tunnel syndrome. Normative diffusion values for MR neurography of the median nerve with DTI depend on the anatomic location and age but not on sex. Age-specific FA and ADC threshold values might be used to diagnose carpal tunnel syndrome.

A new approach for corticospinal tract reconstruction based on navigated transcranial stimulation and standardized fractional anisotropy values

To establish a novel approach for fiber tracking based on navigated transcranial magnetic stimulation (nTMS) mapping of the primary motor cortex and to propose a new algorithm for determination of an individualized fractional anisotropy value for reliable and objective fiber tracking. 50 patients (22 females, 28 males, median age 58 years (20-80)) with brain tumors compromising the primary motor cortex and the corticospinal tract underwent preoperative MR imaging and nTMS mapping. Stimulation spots evoking muscle potentials (MEP) closest to the tumor were imported into the fiber tracking software and set as seed points for tractography. Next the individual FA threshold, i.e. the highest FA value leading to visualization of tracts at a predefined minimum fiber length of 110 mm, was determined. Fiber tracking was then performed at a fractional anisotropy value of 75% and 50% of the individual FA threshold. In addition, fiber tracking according to the conventional knowledge-based approach was performed. Results of tractography of either method were presented to the surgeon for preoperative planning and integrated into the navigation system and its impact was rated using a questionnaire. Mapping of the motor cortex was successful in all patients. A fractional anisotropy threshold for corticospinal tract reconstruction could be obtained in every case. TMS-based results changed or modified surgical strategy in 23 of 50 patients (46%), whereas knowledge-based results would have changed surgical strategy in 11 of 50 patients (22%). Tractography results facilitated intraoperative orientation and electrical stimulation in 28 of 50 (56%) patients. Tracking at 75% of the individual FA thresholds was considered most beneficial by the respective surgeons. Fiber tracking based on nTMS by the proposed standardized algorithm represents an objective visualization method based on functional data and provides a valuable instrument for preoperative planning and intraoperative orientation and monitoring.

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

The purpose of this study was to systematically assess the optimal b-value and reconstruction parameters for DTI and fiber tractography of the median nerve at 3.0T. Local ethical board approved study with 45 healthy volunteers (15 men, 30 women; mean age, 41 ± 3.4 years) who underwent DTI of the right wrist at 3.0T. A single-shot echo-planar-imaging sequence (TR/TE 10123/40 ms) was acquired at four different b-values (800, 1000, 1200, and 1400 s/mm(2)). Two independent readers performed post processing and fiber-tractography. Fractional anisotropy (FA) maps were calculated. Fiber tracts of the median nerve were generated using four different algorithms containing different FA thresholds and different angulation tolerances. Data were evaluated quantitatively and qualitatively. Tracking algorithms using a minimum FA threshold of 0.2 and a maximum angulation of 10° were significantly better than other algorithms. Fiber tractography generated significantly longer fibers in DTI acquisitions with higher b-values (1200 and 1400 s/mm(2) versus 800 s/mm(2); p<0.001). The overall quality of fiber tractography was best at a b-value of 1200 s/mm(2) (p<0.001). In conclusion, our results indicate use of b-values between 1000 and 1400 s/mm(2) for DTI of the median nerve at 3.0T. Optimal reconstruction parameters for fiber tractography should encompass a minimum FA threshold of 0.2 and a maximum angulation tolerance of 10.

Histological Validation of DW-MRI Tractography in Human Postmortem Tissue

Despite several previous attempts, histological validation of diffusion-weighted magnetic resonance imaging (DW-MRI)-based tractography as true axonal fiber pathways remains difficult. In the present study, we establish a method to compare histological and tractography data precisely enough for statements on the level of single tractography pathways. To this end, we used carbocyanine dyes to trace connections in human postmortem tissue and aligned them to high-resolution DW-MRI of the same tissue processed within the diffusion tensor imaging (DTI) formalism. We provide robust definitions of sensitivity (true positives) and specificity (true negatives) for DTI tractography and characterize tractography paths in terms of receiver operating characteristics. With sensitivity and specificity rates of approximately 80%, we could show a clear correspondence between histological and inferred tracts. Furthermore, we investigated the effect of fractional anisotropy (FA) thresholds for the tractography and identified FA values between 0.02 and 0.08 as optimal in our study. Last, we validated the course of entire tractography curves to move beyond correctness determination based on pairs of single points on a tract. Thus, histological techniques, in conjunction with alignment and processing tools, may serve as an important validation method of DW-MRI on the level of inferred tractography projections between brain areas.

MRI in the assessment of motor function restoration in patients with chronic supratentorial infarction

To analyze potential of different MRI methods in the quantitative assessment of brain lesions after ischemic stroke, 19 patients (mean age 38.9±6.2 years) with hemiparesis of various severity resulted from supratentorial ischemic stroke (6–12 months prior the examination) were studied. A relationship was established between such parameters as fractional anisotropy (FA), apparent diffusion coefficient (ADC), size of the brain lesion, and severity of motor deficit. The FA and ADC values obtained in the corticospinal tract (CST) projection allow them to be considered as indicators of the degree of the CST post-ischemic damage predicting motor deficit. FA was found to be the most reproducible indicator of the CST structural integrity. FA threshold values (index, %) for unfavorable outcome of the motor function restoration were determined as follows: 50% for posterior limb of internal capsule, 42% for cerebral peduncle, and 65% for pons varolii. High sensitivity and specificity of the obtained parameters provides ground for their use in identifying patients with poor prognosis for the motor function restoration.

Three-Dimensional Interactive and Stereotactic Human Brain Atlas of White Matter Tracts

We present a human brain atlas of white matter (WM) tracts containing 40 major tracts, which is three-dimensional (3D), segmented, labeled, interactive, stereotactic and correlated to structure and vasculature. We consider: (1) WM accuracy by correlating WM tracts to underlying neuroanatomy and quantifying them; (2) balance between realism and completeness by processing a sequence of track volumes generated for various parameters with the increasing track number to enable a tract "shape convergence". MPRAGE and DTI in 64 directions of the same subject were acquired on 3 Tesla. The method has three steps: DTI-MPRAGE registration, 3D tract generation from DTI, to WM reconstruction from MPRAGE to parcellation into 17 components. 82 track volumes were generated for a wide spectrum of parameter values: Fractional Anisotropy threshold in [0.0125, 0.55] and trajectory angle lower than 45°, 60°, 65°, 70°, 75°, 80°, 85°, 90°. For each tract, a sequence of track volumes was processed to create/edit contours delineating this tract to achieve its shape convergence. The parcellated tracts were grouped into commissures, associations, projections and posterior fossa tracts, and labeled following Terminologia Anatomica. To facilitate that, a dedicated tract editor is developed which processes multiple track volumes, handles tracts in three representations (tracks, contours, envelopes); provides editing/visualization simultaneously on axial, coronal, sagittal planes; enables tract labeling and coloring; and provides numerous tools (track counting, smoothing and length thresholding; representation conversion and saving; structural atlas support). A stereotactic tract atlas along with parcellated WM was developed to explore in real-time any individual tract or their groups along with surrounding neuroanatomy.

Diffusion tensor imaging (DTI) and colored fractional anisotropy (FA) mapping of the subthalamic nucleus (STN) and the globus pallidus interna (GPi)

IntroductionThe subthalamic nucleus (STN) and the globus pallidus internus (GPi) are the most common surgical targets for the treatment of Parkinson’s disease. We studied directionally colored fractional anisotropy (FA) and diffusion tensor imaging (DTI) sequences to better target these anatomical regions.MethodsFour patients undergoing stereotactic surgery for movement disorders were studied. Stereotactic targets and fiber tractography were determined on MRIs using the Schaltenbrand–Wahren atlas for definition in the iPlan software. In addition, post-operative imaging was fused to preoperative FA sequences for end-result identification. Axial, sagittal, and coronal images of the FA sequence were studied. DTI parameters used ranged from 2 to 4 mm for voxel size in the x/y/z planes, fiber length was kept constant at 15 mm and FA threshold of 0.25.ResultsColored FA maps resulted in a key signature in and around the STN and GPi. Regions identified include, but were not limited to: the internal capsule, nigral projections, the thalamic fasciculus, Forel’s fields H1 and H2, zona incerta, suthalamic fasciculus, tegmental tracts, and cerebello-rubro-thalamic tract.ConclusionsColored FA maps allow a potential method to identify the STN and GPi accurately. DTI has proven to be a powerful tool that can be used to augment identification of the STN nucleus and GPi used for stereotactic surgery.

Caveats in diffusion tensor imaging interpretation

Diffusion tensor imaging (DTI) utilizes the diffusion of water to create detailed maps of brain white matter structure. DTI provides information not apparent on conventional magnetic resonance imaging, and has proved to be more sensitive to disruption of white matter development than any other imaging method. In children with cerebral palsy, DTI can add anatomical specificity and quantitative information about the white matter tract size and microstructure. It has been of great benefit in children born preterm with periventricular leukomalacia (PVL).1 Studies have variably shown correlations between corticospinal tract injury2 and thalamocortical projections (posterior thalamic radiation)3,4 in the pathophysiology of motor disability in these children. Yoshida et al.5 reported that when compared with controls, children with PVL have reduced fractional anisotropy and number of fibers in the region of corticospinal tract injury and posterior thalamic radiation, and that these measurements correlate with a clinical measure of function. These findings are a quantitative confirmation of the qualitative data from previous studies and from the visual impression of experienced clinicians. This categorization of variable injury in sensory and motor pathways provides further support for the development of rehabilitative interventions that selectively target affected pathways. The interpretation of results from DTI studies is predicated on an understanding of the strengths and limitations of the methodology employed. The study by Yoshida et al. illustrates important questions regarding the interpretation of quantitative DTI data and the limitations of analysing individual tracts in disorders affecting multiple brain regions such as in children with PVL.6 In regard to interpretation of DTI data, three points need to be carefully considered. First, brain volume and diffusivity have a strong time-dependence, particularly in the first 2 years of life; therefore, studies involving participants in their infancy and childhood may be influenced by this age effect. Second, measurements based on regions of interest are largely evaluator-dependent and interrater agreement should always be presented, as it is an important indicator of the reliability and reproducibility of the results. Finally, measures of imaging contrast and volume are interrelated, as the fractional anisotropy threshold setting will partially determine the volume calculated and when regions of interest are generated, the volume will influence the fractional anisotropy measurements. This type of coupling of the ‘anatomical definition’ and ‘intensity shift’ is a common issue in image analysis and requires careful interpretation. DTI tractography will only describe specific tracts. As the injury in children born preterm is varied and diffuse to both gray and white matter, a global and integrative analysis of neuroanatomy is essential for understanding the pathophysiology of PVL. Recently, state-of-the-art non-linear normalization techniques, such as large-deformation diffeomorphic metric mapping, have been developed to obtain global and automated analysis of the whole brain.7 When combined with other imaging methods, such as functional resting connectivity,8 these tools have the potential to describe the pathophysiology of brain injury in children with cerebral palsy and other childhood neurological disorders accurately and automatically. This is an essential step for detailed anatomy function correlation studies and to provide crucial information that can be useful in intervention studies and for the determination of prognosis.

A role of diffusion tensor imaging in movement disorder surgery

The safe and reversible nature of deep brain stimulation (DBS) has allowed movement disorder neurosurgery to become commonplace throughout the world. Fundamental understanding of individual patient’s anatomy is critical for optimizing the effects and side effects of DBS surgery. Three patients undergoing stereotactic surgery for movement disorders, at the institution’s intraoperative magnetic resonance imaging operating suite, were studied with fiber tractography. Stereotactic targets and fiber tractography were determined on preoperative magnetic resonance imagings using the Schaltenbrand–Wahren atlas for definition in the BrainLab iPlan software (BrainLAB Inc., Feldkirchen, Germany). Subthalamic nucleus, globus pallidus interna, and ventral intermediate nucleus targets were studied. Diffusion tensor imaging parameters used ranged from 2 to 8 mm for volume of interest in the x/y/z planes, fiber length was kept constant at 30 mm, and fractional anisotropy threshold varied from 0.20 to 0.45. Diffusion tensor imaging tractography allowed reliable and reproducible visualization and correlation between frontal eye field, premotor, primary motor, and primary sensory cortices via corticospinal tracts and corticopontocerebellar tracts. There is an apparent increase in the number of cortical regions targeted by the fiber tracts as the region of interest is enlarged. This represents a possible mechanism of the increased effects and side effects observed with higher stimulation voltages. Currently available diffusion tensor imaging techniques allow potential methods to characterize the effects and side effects of DBS. This technology has the potential of being a powerful tool to optimize DBS neurosurgery.

Differences in the architecture of low-grade and high-grade gliomas evaluated using fiber density index and fractional anisotropy

Accurate pre-operative assessment of tumor grade is important for the selection of appropriate treatment strategies. The aim of this study was to retrospectively evaluate whether the fiber density index (FDi) and fractional anisotropy (FA) via diffusion tensor MRI (DTI) could assist with pre-operative diagnosis of glioma grade. A total of 31 patients who had histologically confirmed gliomas underwent DTI performed using a 1.5-Tesla magnetic resonance scanner. To reconstruct the white matter adjacent to the tumor, DTI fiber tracking (DTI-FT) using an FA threshold of 0.15 was implemented. Regions of interest (ROIs) were defined (i–iv) as: the tumor center (ROI i); white matter adjacent to the tumor (ROI ii); contralateral centrum semiovale (ROI iii); and the homologous fiber tracts to ROI ii in the contralateral hemisphere (ROI iv). We calculated six parameters from different ROIs and compared high-grade and low-grade gliomas: FA values and ratios; FDi values and ratios with an FA threshold of 0.15; and FDi values and ratios with an FA threshold of 0.25. The results showed that FA ratios and FDi ratios with FA thresholds of 0.25 were significantly different between patients with high-grade and low-grade gliomas. This may be useful for developing surgical strategies and appraising patient prognosis.

Diffusion Tensor Imaging Study of the Cortical Origin and Course of the Corticospinal Tract in Healthy Children

Several studies have questioned the traditional belief that the corticospinal tract (CST) arises exclusively from the precentral gyrus and passes through the anterior half of the posterior limb of the internal capsule (PLIC) in humans; however, no direct evidence existed from structural imaging, and developmental aspects of CST origin have not been clarified. We used diffusion tensor imaging (DTI) tractography to test the hypotheses that CST can originate from both pre- and postcentral gyri and is located posteriorly in the PLIC, and we also determined how age, sex, or handedness affected these locations. Forty-two healthy children (2.6-17.5 years of age; 20 girls) underwent DTI. Subsequently, tractography was performed on the basis of fiber assignment by continuous tracking (FACT) algorithm and brute force approach, with a fractional anisotropy (FA) threshold of <0.2 and an angle threshold of >50 degrees . The CST was isolated by using a knowledge-based region-of-interest approach, and its cortical origin and location on the PLIC was determined. DTI revealed that the CST originated from both pre- and postcentral gyri in 71.4% of hemispheres, from the precentral gyrus only in 19%, and from the postcentral gyrus only in 7.1%. The overall distribution was similar in both hemispheres. However, children with CST originating from both pre- and postcentral gyri were older (mean, 11.1 years of age) than those with precentral origin (mean, 5.8 years of age) or postcentral origin (mean, 7.8 years of age) only (P = .00003). The center of the CST was localized at 65% of the length (from its anterior margin) of the PLIC, and the CST occupied 26.5% of its anteroposterior length. There was a significant positive correlation between age and FA of the CST (r = 0.49; P = .002). The volume of the precentral portion of the left CST was significantly higher than that of its postcentral portion (P = .01) and that of the right CST (P = .0002). The pattern of cortical origin of CST, its location at the level of PLIC, and its volume and FA were unaffected by sex or handedness. The CST most frequently originates from both pre- and postcentral gyri, especially in older children, and is typically centered approximately two thirds of the distance from the anterior margin of the PLIC and occupies about a quarter of its anteroposterior length. In young children, the CST can often be seen originating exclusively from the precentral gyrus by DTI.

Semiautomatic tractography: motor pathway segmentation in patients with intracranial vascular malformations

Object The visualization of white matter tracts using tractography has previously been achieved by displaying streamlines that pass between regions of interest (ROIs). These techniques require a significant amount of user interaction, and their results are entirely dependent on the positioning of the ROIs. Furthermore, in patients with intracerebral hemorrhage secondary to intracranial vascular malformation, there is often significant cerebral edema and susceptibility artifact from the hematoma, which degrade the reliability of tractography. In this paper, the authors' objectives were to visualize the motor pathways of patients with hemorrhagic and nonhemorrhagic vascular malformations by using a novel semiautomated technique that functions without the need for multiple ROIs. Methods The authors investigated the tractography appearance of the descending motor pathways in 6 patients with intracranial vascular malformations. Of these patients 4 presented with a spontaneous intracranial hemorrhage, 2 of whom were clinically hemiparetic. Diffusion tensor imaging was performed using a 1.5-T clinical MR imaging system, and whole-brain tractography was performed after reconstruction of the data. A fractional anisotropy threshold of 0.05 was used to terminate the tractography. The semiautomatic motor pathway segmentation technique required definition of a single voxel within the corticospinal tract of the medulla from which the descending motor pathways were automatically defined by grouping together all streamlines within the entire image with a geometry similar to that of the single streamline generated from this initial voxel. The results of this segmentation were then visually assessed and compared with the patient's motor function. Results The authors' semiautomatic algorithm consistently visualized the location of the descending motor pathways in patients with nonhemorrhagic and hemorrhagic vascular malformations. In 1 patient whose complete right hemiplegia (complete paralysis) was caused by a large left frontal hematoma that bisected the descending motor pathways, the authors were unable to reconstruct the motor pathways due to severe tract degeneration. However, in all cases in which motor function was intact or only mildly impaired, the technique clearly delineated the motor pathways, even in the presence of large anatomical displacement by the vascular abnormality or associated hemorrhage. Conclusions Semiautomatic tractography allows consistent and rapid demonstration of the descending motor pathways in patients with hemorrhagic and nonhemorrhagic intracranial vascular malformations. The technique allows the use of a comparatively low fractional anisotropy threshold and does not require the definition of multiple ROIs. These techniques may help to improve the clinical feasibility and potentially the reliability of tractography for the evaluation of patients with intracranial vascular malformations as well as other space-occupying lesions of the brain.

Fractional Anisotropy–Threshold Dependence in Tract-Based Diffusion Tensor Analysis: Evaluation of the Uncinate Fasciculus in Alzheimer Disease

Tract-based analysis can be used to investigate required tracts extracted from other fiber tracts. However, the fractional anisotropy (FA) threshold influences tractography analysis. The current study evaluated the influence of the FA threshold in measuring diffusion tensor parameters for tract-based analysis of the uncinate fasciculus in subjects with Alzheimer disease (AD). Subjects included 30 patients with AD and 10 healthy controls. We acquired tractographies of the uncinate fasciculus by using different FA thresholds. We measured mean FA and the apparent diffusion coefficient (ADC) along the uncinate fasciculus for different FA thresholds and evaluated the correlation between diffusion tensor parameters (FA, ADC) and the Mini-Mental State Examination (MMSE) scores. The uncinate fasciculus showed lower mean FA and higher mean ADC values in cases with more severe AD. A higher FA threshold led to a lower mean ADC value and a higher mean FA value along the uncinate fasciculus, whereas the relative order of measured values according to the severity of AD was not influenced by the FA threshold. An FA threshold of 0.2 showed higher correlation between mean ADC values and MMSE scores. FA thresholds of 0.15 and 0.20 showed higher correlation between mean FA values and MMSE scores. Appropriate selection of the FA threshold leads to higher correlation between diffusion tensor parameters and the severity of AD. For tract-based analysis of degenerative diseases such as AD, appropriate selection of the FA threshold for tractography is important.

Diffusion tensor imaging and tractography of Rasmussen encephalitis

A 5-year-old girl presented with intractable seizures and nonfocal hemispheric slowing on EEG. Blood, urine, and CSF laboratory values were all normal. MR imaging of the brain demonstrated diffuse volume loss of the entire left hemisphere. Clinical and imaging findings were consistent with the diagnosis of Rasmussen encephalitis. Diffusion tensor imaging demonstrated increased ADC values and decreased fractional anisotropy (FA) of the affected (left) hemisphere. Tractography data showed decreased numbers of lines drawn in the affected hemisphere, with fewer lines drawn at higher FA thresholds, relative to the contralateral hemisphere, or to normal age-matched controls. Diffusion tensor imaging offers an adjunct MR imaging modality in diagnosis and monitoring of this rare condition.

Fiber Tracking with Distinct Software Tools Results in a Clear Diversity in Anatomical Fiber Tract Portrayal

Fiber tract portrayal, based on diffusion tensor imaging (DTI), is becoming more and more important in functional neuronavigation. No standard exists to guarantee anatomically correct fiber tract depiction for neurosurgical purposes. Therefore, showing the anatomically correct extension of fiber tracts beyond the pure connection of functional areas remains an area of important research and investigation. Standards for fiber tracking software applications are elusive. The purpose of this study was to compare the performance of different fiber tracking software tools (FT-tools). We tested the software performance, comparability and anatomical accuracy of the tracking results of several programs. A single DTI dataset of a healthy control subject was submitted to four different fiber tracking software applications (two commercial, two freeware), three of them based on Fiber Assignment by Continuous Tracking, one based on the Tensorline Propagation Algorithm. The corticospinal tract (CST) was investigated. The tracking procedure was controlled by the following input variables: single regions of interest (ROIs): brain stem, or internal capsule, or subcortical white matter of the precentral gyrus; background threshold, fractional anisotropy (FA) threshold, maximum fiber angulation and fiber length. Tracking results were compared for 2-D correlated triplanar images (axial, coronal, sagittal) and in 3-D. For all FT-tools, the time used to generate the CST was measured. The inter-rater variability for tracking time and for the tracked CST volumes was recorded for two of the four FT-tools. Distinct FT-tools performed very differently with respect to the time required to achieve CST portrayal (track generation time varied between 16 and 50 min). None of the software applications was able to display the CST in its full anatomical extent. Especially the lateral precentral areas were not pictured. Surprisingly, the application of the four distinct FT-tools did not lead to comparable tracking results. As very similar or identical tracking algorithms were used, this difference cannot be easily explained. Clearly, neurosurgeons have to be cautious about applying fiber tracking results intraoperatively, especially when dealing with an abnormal or distorted fiber tract anatomy. The authors recommend the use of adjunct strategies such as intraoperative electrophysiology to enhance patient safety and improve anatomical accuracy when using tracking results for surgical procedures.

Visualization of peripheral nerve degeneration and regeneration: Monitoring with diffusion tensor tractography

We applied diffusion tensor tractography (DTT), a recently developed MRI technique that reveals the microstructures of tissues based on its ability to monitor the random movements of water molecules, to the visualization of peripheral nerves after injury. The rat sciatic nerve was subjected to contusive injury, and the data obtained from diffusion tensor imaging (DTI) were used to determine the tracks of nerve fibers (DTT). The DTT images obtained using the fractional anisotropy (FA) threshold value of 0.4 clearly revealed the recovery process of the contused nerves. Immediately after the injury, fiber tracking from the designated proximal site could not be continued beyond the lesion epicenter, but the intensity improved thereafter, returning to its pre-injury level by 3 weeks later. We compared the FA value, a parameter computed from the DTT data, with the results of histological and functional examinations of the injured nerves, during recovery. The FA values of the peripheral nerves were more strongly correlated with axon-related (axon density and diameter) than with myelin-related (myelin density and thickness) parameters, supporting the theories that axonal membranes play a major role in anisotropic water diffusion and that myelination can modulate the degree of anisotropy. Moreover, restoration of the FA value at the lesion epicenter was strongly correlated with parameters of motor and sensory functional recovery. These correlations of the FA values with both the histological and functional changes demonstrate the potential usefulness of DTT for evaluating clinical events associated with Wallerian degeneration and the regeneration of peripheral nerves.

Influence of User-Defined Parameters on Diffusion Tensor Tractography of the Corticospinal Tract

This study discusses the influence of user-defined parameters on fiber tracking results obtained from a standard deterministic streamline tractography algorithm. Diffusion tensor imaging with fiber tractography was performed in five healthy volunteers. A region of interest was highlighted in the ventral part of the pons at the level of the middle cerebellar peduncle. The parameters studied were angle threshold, fractional anisotropy threshold, step length and number of seed samples per voxel. Changes in fiber tracts were described for increasing values per parameter. Increasing the angle threshold resulted in more and longer fibers. A higher fractional anisotropy threshold resulted in decreased length and fiber tracts that were not representative. Increasing the step length decreased the fiber continuity and altered its position. A higher number of seed samples per voxel resulted in a higher fiber tract density. When interpreting diffusion tensor images, the reader should understand the influence of user-defined settings on the results, and should be aware of the inter-dependency of fiber tracking parameters.

Analysis of acute traumatic axonal injury using diffusion tensor imaging

Traumatic axonal injury (TAI) contributes significantly to mortality and morbidity following traumatic brain injury (TBI), but is poorly characterized by conventional imaging techniques. Diffusion tensor imaging (DTI) may provide better detection as well as insights into the mechanisms of white matter injury. DTI data from 33 patients with moderate-to-severe TBI, acquired at a median of 32 h postinjury, were compared with data from 28 age-matched controls. The global burden of whole brain white matter injury (GBWMI) was quantified by measuring the proportion of voxels that lay below a critical fractional anisotropy (FA) threshold, identified from control data. Mechanisms of change in FA maps were explored using an Eigenvalue analysis of the diffusion tensor. When compared with controls, patients showed significantly reduced mean FA (p < 0.001) and increased apparent diffusion coefficient (ADC; p = 0.017). GBWMI was significantly greater in patients than in controls (p < 0.01), but did not distinguish patients with obvious white matter lesions seen on structural imaging. It predicted classification of DTI images as head injury with a high degree of accuracy. Eigenvalue analysis showed that reductions in FA were predominantly the result of increases in radial diffusivity (p < 0.001). DTI may help quantify the overall burden of white matter injury in TBI and provide insights into underlying pathophysiology. Eigenvalue analysis suggests that the early imaging changes seen in white matter are consistent with axonal swelling rather than axonal truncation. This technique holds promise for examining disease progression, and may help define therapeutic windows for the treatment of diffuse brain injury.

Diffusion tensor imaging and optimized fiber tracking in glioma patients: Histopathologic evaluation of tumor-invaded white matter structures

Fiber tracking is increasingly used to plan and guide neurosurgical procedures of intracranial tumors in the vicinity of functionally important areas of the brain. However, valid data concerning the reliability of tracking with respect to the actual pathoanatomical situation are lacking. We retrospectively correlated fiber tracking based on magnetic resonance (MR) DT imaging with the histopathological data of 25 patients with WHO grade II and III gliomas. Fiber tracking using the Fiber Assignment by Continuous Tracking (FACT) method was performed to investigate the integrity of white matter tracts in the surrounding border zone of the lesions. The tracking procedure was stopped when fractional anisotropy (FA) thresholds=0.1, 0.15, 0.2, 0.25, and 0.3, or a tract turning angle >60° were encountered. In 9 patients we were able to reconstruct brain fiber tracts at biopsy loci (2–32% tumor infiltration) using an FA threshold of 0.15 and 0.2, but not for a threshold of 0.25 or 0.3. The neurological outcome demonstrated potential tumor cell infiltration of functionally intact brain fiber tracts in the range of 2–8%. These findings may be useful in planning therapeutic approaches to gliomas in the vicinity of eloquent brain regions.