Timothy L. Davis

Language dominance determined by whole brain functional MRI in patients with brain lesions

Background: Functional MRI (fMRI) is of potential value in determining hemisphere dominance for language in epileptic patients. Objective: To develop and validate an fMRI-based method of determining language dominance for patients with a wide range of potentially operable brain lesions in addition to epilepsy. Methods: Initially, a within-subjects design was used with 19 healthy volunteers (11 strongly right-handed, 8 left-handed) to determine the relative lateralizing usefulness of three different language tasks in fMRI. An automated, hemispheric analysis of laterality was used to analyze whole brain fMRI data sets. To evaluate the clinical usefulness of this method, we compared fMRI-determined laterality with laterality determined by Wada testing or electrocortical stimulation mapping, or both, in 23 consecutive patients undergoing presurgical evaluation of language dominance. Results: Only the verb generation task was reliably lateralizing. fMRI, using the verb generation task and an automated hemispheric analysis method, was concordant with invasive measures in 22 of 23 patients (12 Wada, 11 cortical stimulation). For the single patient who was discordant, in whom a tumor involved one-third of the left hemisphere, fMRI became concordant when the tumor and its reflection in the right hemisphere were excluded from laterality analysis. No significant negative correlation was obtained between lesion size and strength of laterality for the patients with lesions involving the dominant hemisphere. Conclusion: This fMRI method shows potential for evaluating language dominance in patients with a variety of brain lesions.

Characterization of cerebral blood oxygenation and flow changes during prolonged brain activation.

The behavior of cerebral blood flow and oxygenation during prolonged brain activation was studied using magnetic resonance imaging (MRI) sensitized to flow and oxygenation changes, as well as positron emission tomography sensitized to flow. Neuronal habituation effects and hemodynamic changes were evaluated across tasks and cortical regions. Nine types of activation stimuli or tasks, including motor activation, vibrotactile stimulation, and several types of visual stimulation, were used. Both flow and oxygenation were evaluated in separate time course series as well as simultaneously using two different MRI methods. In most cases, the activation‐induced increase in flow and oxygenation remained elevated for the entire stimulation duration. These results suggest that both flow rate and oxygenation consumption rate remain constant during the entire time that primary cortical neurons are activated by a task or a stimulus. Hum. Brain Mapping 5:93–109, 1997.

Functional magnetic resonance imaging shows localized brain activation during serial transcranial stimulation in man

Area and depth penetration of transcranial stimulation methods such as transcranial electrical stimulation (TES) are poorly defined. We investigated the feasibility of a simultaneous TES and fMRI measurement. The aim was to compare the signal intensity changes measured using BOLD fMRI during sequential finger movement with the signal response during artificial transcranial stimulation. TES induced contralateral finger contractions and in T2* weighted images a transient signal increase was observed in the area underlying the electrodes. Compared with the signal obtained during sequential finger movements, the area activated by TES was more localized, signal amplitude was smaller and there was no post- stimulus undershoot. These data indicate that TES induces a local blood flow increase associated with a drop in the concentration of deoxyhaemoglobin.

Diffusion anisotropy and white matter tracts

Diffusion Anisotropy and White Matter Tracts V.J. W e d e e n 1, T.L. Davis 1, B.E. Lautrup 2, T.G. Reese ~ and B.R. Rosen ~ 1Harvard Medical School, Boston, USA 2CONNECT, The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Objectives. Water diffusion anisotropy can provide information about the tissue microarchitecture (1). Images of diffusion anisotropy in the brain reveal the orientation of axons in white matter, and diffusion anisotropy data provide information about the large-scale structure of white matter tracts. Methods. Diffusion-weighted magnetic resonance images were obtained in a normal volunteer as described by Davis et al (2). Employing single-shot echo-planar MRI at 1.5 Tesla, diffusion data were sampled for 15 contiguous coronal slices at a 3 millimeter isotropic resolution. Total acquisition time was 15 minutes. From these data the water selfdiffusion tensor field was computed at each voxel location (3). We assume that the inverse of the diffusion tensor may be interpreted as a non-Euclidean metric tensor, and we test whether the shortest curves--the geodesics--in this geometry follow known white-matter tracts in the brain. These geodics were computed by numerically integrating the second-order geodesic equation (4). Initial data were constructed by selecting a point in a region of high anisotropy and setting the initial tangent vector equal to the diffusion tensor eigenvector corresponding to the largest eigenvalue, which is the Figure 1 direction of greatest water mobility. Results. Figure 1 depicts three-dimensional water diffusion in an axial slice through the brain at the level of the lateral ventricles. MRI magnitude data are shown in grayscale, and diffusion data are represented by small 3D rhomboids at each pixel location; rhomboid vertices represent the top two eigenvectors of the local diffusion tensor. In Fig. 1, the posterior horn of the left lateral ventricle appears as a large white structure. The long axes of the rhomboids indicate the principal orientation of local white matter. We identify the splenium of the corpus callosum (long vertical arrow), the optic radiations (three horizontal arrows) and examples of gyral U-fibers (two short arrows). Note that the fibers of the optic radiations are seen to arch upwards into the plane of section. Base points selected in the region of the corticospinal tract yielded geodesic trajectories which remained within this tract over distances of several centimeters (Fig. 2). A coronal slice through the data volume provides an anatomic reference. Local diffusion tensor data in this plane are represented by rhomboids. Fibers of the corona radiata are visualized as vertically oriented diffusivities (black arrows). The 3D geodesic solutions for seed points within the corona radiata track through planes that lie in front of this image plane and conform to the coronal diffusion. These solutions exhibit the expected medial-to-lateral fan of the corona. Pairs of fibers whose . . . . trajectories are parallel for several centimeters are joined by a ribbon. Figure 2 Conclusions. Diffusion tensor anisotropy provides a means of tracking axonal bundles over macroscopic distances. The identification of white-matter tracts is less obvious when these cross and the diffusion tensor becomes less anisotropic. A systematic analysis of these data may be undertaken by massive computation of the full set of diffusion geodesics between all voxels. The use of geodesics promises to provide an anatomic substrate for functional connectivity in vivo. Acknowledgement. Funded in part by Human Brain Project R01 DA092461. References 1. Moseley, ME et al. Mag Res Med. 1991, 19:321-326. 2. Davis, TL. et al. In Soc. Mag. Res. in Med., 12th meeting, New York, N.Y. 1993, 1:239. 3. Garrido, L. et al. Circ. Res. 1994, 74(5):789-793. 4. Carmo, DO. Differential Geometry of Curves and Surfaces. Prentiss Hall. 1976.

The effect of paramagnetic contrast media on T1 relaxation times in brain tumors

Purpose: to study T1 relaxation times in brain tumors before and after paramagnetic contrast medium injection Material and Methods: Seventeen patients with a known or suspected brain tumor were studied with an echo planar inversion recovery imaging sequence using 10 different inversion times. Double injections of Gd chelate (0.1 mmol/kg+0.2 mmol/kg) were administered in 5 patients, and a single 0.2-mmol/kg dose in 12 patients Results: After the 0.2-mmol/kg dose, T1 decreased from 1121 ± 130 ms to 987 ± 103 ms in gray matter (p<0.001), and from 666 ± 29 ms to 646 ± 27 ms in white matter (p<0.001). Tumor T1 shortened from 1515 ± 319 ms to 717 ± 383 ms. After the 0.1-mmol/kg dose (n=5), tumor T1 decreased from 1116 ± 261 ms to 793 ± 202 ms and after the additional 0.2-mmol/kg dose it decreased further to 526 ± 141 ms Conclusion: Postcontrast T1 relaxation times in tumors showed considerable variation and remained, on average, relatively long compared to white matter. This should be taken into account when deciding which pulse sequences, imaging parameters, and contrast agent doses are optimal for brain tumor imaging