Diffusion tensor imaging: a review for pediatric researchers and clinicians

Abstract

Diffusion tensor imaging (DTI) is a magnetic resonance imaging technique that allows for the visualization and characterization of the white matter tracts of the brain in vivo. DTI does not assess white matter directly. Rather, it capitalizes on the fact that diffusion is isotropic (equal in all directions) in cerebral spinal fluid and cell bodies but anisotropic (greater in one direction than the other directions) in axons that comprise white matter. It provides quantitative information about the degree and direction of water diffusion within individual units of volume within the magnetic resonance image, and by inference, about the integrity of white matter. Measures from DTI can be applied throughout the brain or to regions of interest. Fiber tract reconstruction, or tractography, creates continuous 3-dimensional tracts by sequentially piecing together estimates of fiber orientation from the direction of diffusion within individual volume units. DTI has increased our understanding of white matter structure and function. DTI shows nonlinear growth of white matter tracts from childhood to adulthood. Delayed maturation of the white matter in the frontal lobes may explain the continued growth of cognitive control into adulthood. Relative to good readers, adults and children who are poor readers have evidence of white matter differences in a specific region of the temporo-parietal lobe, implicating differences in connections among brain regions as a factor in reading disorder. Measures from DTI changed in poor readers who improved their reading skills after intense remediation. DTI documents injury to white matter tracts after prematurity. Measures indicative of white matter injury are associated with motor and cognitive impairment in children born prematurely. Further research on DTI is necessary before it can become a routine clinical procedure.

Figure 1

Three diffusion ellipsoids represent the diffusion profile of 3 different structures, marked 1, 2, and 3 on Figures 2C and 2D. The axes represent the x- (left-right, red), y- (posterior-anterior, green), and z-(inferior-superior, blue) directions. (A) Isotropic diffusion ellipsoid, representing a region of cerebral spinal fluid. (B) Anisotropic diffusion ellipsoid, representing a white matter tract parallel to the y-axis (superior longitudinal fasciculus). (C) Anisotropic diffusion ellipsoid, representing a white matter tract parallel to the x-axis (corpus callosum).

Figure 2

Sagittal (top) and axial (bottom) slices from a healthy 12-year-old girl. (A) Conventional T1 weighted anatomical image. (B) Mean diffusivity map calculated from diffusion tensor imaging data. High signal (white areas) represents high diffusion (cerebral spinal fluid); low signal (gradations of dark areas) represents reduced diffusion (gray and white matter). (C) Fractional anisotropy map calculated from diffusion tensor imaging data. High signal (white areas) represents high fractional anisotropy (white matter); low signal (dark areas) represents reduced anisotropy (gray matter and cerebral spinal fluid). (D) Red-Green-Blue map calculated from diffusion tensor imaging data. Voxels displayed as red represent tracts with primarily left-right orientation (x-axis); voxels displayed as green represent tracts with primarily anterior-posterior orientation (y-axis); voxels displayed as blue represent tracts with primarily superior-inferior orientation (z-axis). The superior longitudinal fasciculus, a tract containing fibers projecting along the y-axis (outlined in white, see Fig. 1B) is represented in green. The corpus callosum, a tract containing fibers projecting along the x-axis, is represented in red (see Fig. 1C).

Figure 3

Mean fractional anisotropy skeleton, in green, generated from Tract Based Spatial Statistics for a sample of 13 children born prematurely and 12 age-matched full-term controls, overlaid on a Montreal Neurologic Institute template. Regions of significant difference in fractional anisotropy (p < .05, uncorrected for multiple comparisons) between children born prematurely and controls are shown in red within the skeleton.

Figure 4

Fiber tracking. (A) Three-dimensional visualization of superior longitudinal fasciculus fibers, tracked from the region of interest outlined in white in Figure 2D. The location of the "seed" region of interest is depicted in this sagittal view as a white line. The blue fiber tract contains fibers connecting the temporal, parietal, and frontal lobes. (B) The arcuate fasciculus, the portion of the superior longitudinal fasciculus that connects the temporal lobe to the frontal lobe. Tract was defined by limiting the blue fiber group in Figure 4A to only those fibers passing through a second region of interest, depicted by the red line, thereby using a 2 region of interest approach.