Victor Haughton

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. Hum. Brain Mapping 18:306–321, 2003.

Independent component analysis applied to diffusion tensor MRI.

The accuracy of the outcome in a diffusion tensor imaging (DTI) experiment depends on the acquisition scheme as well as the postprocessing methods used. In the present study, the DTI results acquired after applying different combinations of diffusion‐weighted (DW) gradient orientations were initially compared. Then, spatially independent component analysis (ICA) was applied to the T2 and DW images. In all cases a single component was detected that was similar to the map of the trace of the diffusion tensor, but contained a reduced amount of noise. Furthermore, when no correction for eddy current artifacts was used in the image acquisition scheme, the effects of eddy currents were separated by ICA into independent components. After these components were removed, conventional estimation of the diffusion tensor was performed on the modified data. No artifact was contained in the final rotationally invariant scalar quantities that describe the intrinsic diffusion properties. Additionally, independent components that mapped major white matter fiber tracts in the human brain were identified. Finally, the noise included in the original T2 and DW images was also separated by ICA into independent components. These components were subsequently removed and a reduction of noise in the final DTI results was achieved. Magn Reson Med 47:354–363, 2002.

Application of image registration to measurement of intervertebral rotation in the lumbar spine

In this study, the precision and accuracy of a new method for measuring axial rotations of the lumbar vertebrae, which are thought to be increased in patients with degenerative spinal instability, were estimated. A standard image registration algorithm involving a user‐selected center of rotation was adapted for this purpose. Accuracy was tested on a phantom containing a rotating lumbar vertebra. Calculated rotations were compared to the known values. Images were also obtained in subjects, using a tilting table that provided rotation between thorax and hips. For these measurements precision was estimated by varying the center of rotation. In the phantom the method had an accuracy of 0.2°. In the human subject measurements the precision was 0.60–1.5°. The method measured rotations of lumbar vertebrae with sufficient accuracy and precision to detect clinically significant differences in patients with back pain. Magn Reson Med 48:1072–1075, 2002.

Characterization of CSF Hydrodynamics in the Presence and Absence of Tonsillar Ectopia by Means of Computational Flow Analysis

BACKGROUND AND PURPOSE: Phase-contrast MR imaging (PCMR) has only partially characterized cyclic CSF flow and pressure, which, hypothetically, have a role in the pathogenesis of syrinx and symptoms in the Chiari I malformation. Our goal was to use computational flow analysis (CFA) to better understand CSF hydrodynamics. MATERIALS AND METHODS: High-resolution MR images were obtained in a healthy volunteer and a patient with Chiari I malformation. With standard segmentation and discretization techniques, 3D models of the subarachnoid space, cerebellum, and spinal canal were created. CSF flow during systole and diastole were simulated with the boundary element method in the models. CSF velocities and pressures computed in the patient with Chiari I malformation were compared with those in the healthy volunteer. Flow patterns were also compared with PCMR results for validation of the technique. RESULTS: The CFA and PCMR results agreed well. Inhomogeneous flow patterns characterized by fluid jets anterior and lateral to the spinal cord were demonstrated in both the Chiari I and volunteer models by CFA. Significant circumferential velocities were evident, suggesting swirling flow in the spinal canal. Higher magnitude jets were found in the patient with Chiari I than in the healthy volunteer. Relatively even pressure gradients were found along the spinal canal in both cases, with a 50% steeper gradient in the patient with Chiari I malformation. CONCLUSIONS: Circumferential velocities and pressure gradients in the spinal canal, which may be clinically relevant to Chiari I and other malformations, can be obtained by CFA in patient-specific geometries.

Functional MR imaging: paradigms for clinical preoperative mapping

Clinical applications of functional MR imaging include mapping of brain functions in relationship to intracranial tumors, seizure foci, or vascular malformations to determine the risk for performing surgical excision, the need for intraoperative mapping during excision, and selecting the optimal surgical approach to a lesion. A variety of paradigms are used to produce a blood-oxygen-level-dependent response in various brain regions, which can be identified with functional MR imaging. The paradigms used include active motor, language, or cognitive tasks, and passive tactile, auditory, or visual stimuli. Activation usually indicates the location of eloquent cortex. Lack of function in a region cannot be assumed when functional MR imaging shows absence of activation within the region.

Functional MR imaging assessment of a non-responsive brain injured patient

Functional magnetic resonance imaging (fMRI) was requested to assist in the evaluation of a comatose 38-year-old woman who had sustained multiple cerebral contusions from a motor vehicle accident. Previous electrophysiologic studies suggested absence of thalamocortical processing in response to median nerve stimulation. Whole-brain fMRI was performed utilizing visual, somatosensory, and auditory stimulation paradigms. Results demonstrated intact task-correlated sensory and cognitive blood oxygen level dependent (BOLD) hemodynamic response to stimuli. Electrodiagnostic studies were repeated and evoked potentials indicated supratentorial recovery in the cerebrum. At 3-months post trauma the patient had recovered many cognitive & sensorimotor functions, accurately reflecting the prognostic fMRI evaluation. These results indicate that fMRI examinations may provide a useful evaluation for brain function in non-responsive brain trauma patients.

CSF Flow through the Upper Cervical Spinal Canal in Chiari I Malformation

BACKGROUND AND PURPOSE: Previous studies have quantified CSF flow in patients with Chiari I at the foramen magnum with single-axial or single-sagittal PCMR. The goal of this study was to measure CSF velocities at multiple cervical spinal levels in patients with Chiari I malformation. MATERIALS AND METHODS: In a patient registry, consecutive patients without surgery who had PCMR flow images in 5–8 axial planes between the foramen magnum and C4 were identified. Four contiguous regions were defined from the foramen magnum to C4. In each region, the fastest positive flow (PSV) and fastest negative flow (PDV) were tabulated. Changes in peak velocity by cervical spinal level and age and sex were tested for significance with linear mixed-effects models. RESULTS: In 17 patients studied, PSV increased progressively and significantly from the foramen magnum to C4. PDVs increased slightly from the foramen magnum to C3. The changes in velocity over the 4 regions tended to be smaller in the 13 patients with tonsilar ectopia than in the 4 patients without it. Age and sex had an effect on peak velocities. CONCLUSIONS: Peak diastolic and systolic CSF velocities are significantly greater below than at the foramen magnum.

Axial rotation of the lumbar spinal motion segments correlated with concordant pain on discography: A preliminary study

OBJECTIVE One possible cause of back pain in patients with intervertebral disk degeneration is decreased stability of the motion segment. Axial rotations between lumbar spinal vertebrae can be measured noninvasively with CT. We tested the hypothesis that larger axial rotations are found in motion segments with disks that test positive for concordant pain, which is considered by some investigators to be a reasonable, accurate predictor of spinal instability. SUBJECTS AND METHODS Between October 2002 and March 2004, all patients undergoing discography were evaluated for inclusion in the study, with the approval of the institutional review board. All patients in whom concordant pain was detected at discography were enrolled in the study. The patients were placed supine in the CT scanner on a table that rotated the pelvis 8 degrees clockwise and then counterclockwise with respect to the thorax. CT images were obtained with the patient in the two positions of rotation. An automated program calculated the amount of rotation between each lumbar vertebra as a result of the table rotations. Rotations were stratified by disk level and by disk classification (concordant pain, nonconcordant pain, no significant pain). RESULTS We recorded the axial rotations of 94 disks in 16 consecutive patients (10 women, six men; age range, 26-53 years) after two disks were excluded because of a previous fusion. There were 68 normal disks by MRI and discography, six disks with nonconcordant pain, and 20 disks with concordant pain. Rotation averaged 0.6 degrees for the normal disks, 1.4 degrees for disks with nonconcordant pain, and 1.8 degrees for disks with concordant pain. The differences were significant (analysis of variance, p < 0.001). Disks at L3-L4 with concordant pain rotated on average 1.2 degrees , whereas disks classified as normal or nonconcordant pain rotated on average 0.7 degrees (significant at p = 0.005). Disks at L4-L5 with concordant pain rotated on average 1.9 degrees , and those without concordant pain rotated on average 1.4 degrees (significant at p = 0.05). Disks with concordant pain at L5-S1 had an average rotation of 2.2 degrees , whereas disks without concordant pain had an average rotation of 1.5 degrees (marginally significant difference at p = 0.07). CONCLUSION Concordant pain at discography predicts increased axial rotation at a lumbar disk level.

Patient-Specific 3D Simulation of Cyclic CSF Flow at the Craniocervical Region

BACKGROUND AND PURPOSE: Flow simulations in patient-specific models of the subarachnoid space characterize CSF flow in more detail than MR flow imaging. We extended previous simulation studies by including cyclic CSF flow and patient-specific models in multiple patients with Chiari I. We compared simulation results with MR flow measurements. MATERIALS AND METHODS: Volumetric high resolution image sets acquired in 7 patients with Chiari I, 3 patients who had previous craniovertebral decompression, and 3 controls were segmented and converted to mathematical models of the subarachnoid space. CSF flow velocities and pressures were calculated with high spatial and temporal resolution during simulated oscillatory flow in each model with the Navier-Stokes equations. Pressures, velocities, and bidirectional flow were compared in the groups (with Student t test). Peak velocities in the simulations were compared with peak velocities measured in vivo with PCMR. RESULTS: Flow visualization for patients and volunteers demonstrated nonuniform reversing patterns resembling those observed with PCMR. Velocities in the 13 subjects were greater between C2 and C5 than in the foramen magnum. Chiari patients had significantly greater peak systolic and diastolic velocities, synchronous bidirectional flow, and pressure gradients than controls. Peak velocities measured in PCMR correlated significantly (P = .003; regression analysis) despite differences between them. CONCLUSIONS: In simulations of CSF, patients with Chiari I had significantly greater peak systolic and diastolic velocities, synchronous bidirectional flow, and pressure gradients than controls.

Functional MRI neuroanatomic correlates of the Hooper Visual Organization Test

The Hooper Visual Organization Test (VOT), a commonly applied neuropsychological test of visual spatial ability, is used for assessing patients with suspected right hemisphere, or parietal lobe involvement. A controversy has developed over whether the inferences of this test metric can be assumed to involve global, lateralized, or regional functionality. In this study, the characteristic visual organization and object naming aspects of the VOT task presentation were adapted to a functional MR imaging (fMRI) paradigm to probe the neuroanatomic correlates of this neuropsychological test. Whole brain fMRI mapping results are reported on a cohort of normal subjects. Bilateral fMRI responses were found predominantly in the posterior brain, in regions of superior parietal lobules, ventral temporal-occipital cortex, and posterior visual association areas, and to a lesser extent, the frontal eye fields bilaterally, and left dorsolateral prefrontal cortex. The results indicate a general brain region or network in which VOT impairment, due to its visuospatial and object identification demands, is possible to be detected. Discussion is made of interpretive limitations when adapting neuropsychological tests to fMRI analysis.