Diffusion Tensor Imaging or DTI can be used in order to probe, in vivo, the intrinsic, three-dimensional diffusion properties of water within tissues. DTI has been applied in several studies to infer the microstructural characteristics of the brain (Pierpaoli et al., 1996; Virta et al., 1999), the heart (Holmes et al., 2000), and the spinal cord (Fenyes and Narayana, 1999). In addition, DTI has been introduced in the diagnosis of disease conditions such as cerebral ischemia (Lythgoe et al., 1997), acute stroke (van Gelderen et al., 1994) and multiple sclerosis (Werring et al., 1999). Unlike conventional DWI (Le Bihan, 1991), where diffusion-weighted (DW) images are used to calculate the scalar ADC, DTI characterizes diffusive transport of water by an effective diffusion tensor D. The eigenvalues of D are the three principal diffusivities and the eigenvectors define the local fiber tract direction field (Basser et al., 1994). Moreover, one can derive from D rotationally invariant scalar quantities that describe the intrinsic diffusion properties of the tissue. The most commonly used are the trace of the tensor (Basser et al., 1994, Pierpaoli et al., 1996, Basser, 1995), which measures mean diffusivity, Fractional Anisotropy (FA) and Lattice Index (LI)( Pierpaoli et al., 1996, Basser et al., 1995; Basser and Pierpaoli, 1996; Pierpaoli and Basser 1996), which characterize the anisotropy of the fiber structure, meaning how much higher the diffusivity is along some directions compared to others.
We use DTI in our research in an attempt to understand the underlying cause(s) of a variety of pathologies including epilepsy, traumatic brain injury, brain tumors and arterial-venous malformations. Shown below is an absolute value DTI colormap. In this visualization of the diffusion information, we assign colors to specific diffusion directions. X, Y and Z directions are assigned red, green, blue respectively. Any other direction within the xyz octant is assigned a combination of red, green and blue. Fibers with rotational or mirror symmetry appear to have the same color. Voxel brightness is scaled by its lattice index value. In order to determine the diffusion direction from this map, one uses the color wheel shown to the right. Assume the color wheel is actually a dome and you are standing at the center. The angle that your line of sight forms with a specific color on the dome represents the angular direction of the diffusion.
The diffusion pathways that are visible in the map are white matter fiber bundles. Diffusion in gray matter is more isotropic (no preferred diffusion direction) which is reflected in this map. DTI information can be obtained in 3D allowing fiber tracking throughout the brain.
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