Thursday, October 13, 2005

DTI: Diffusion Tensor Imaging

Google Image Result for "Diffusion Tensor Imaging (DTI) is a powerful MRI technique that enables us to translate the self-diffusion, or microscopic motion of water molecules in tissue into a MRI measure of tissue integrity and structure. Namely, the spatial characteristic of water diffusion highly depends on the barriers imposed on the water molecule motion, those barriers being the elements of tissue such as cell membranes, myelin sheath, intracellular microorganelles and others. Specifically, in white matter, water self diffusion is restricted, or hindered mostly by the intracellular axonal space, and by the interstitial, extracellular space among the well-packed axons in the fiber tract. By taking several diffusion weighted images in several dimensions, one can reconstruct the so-called diffusion tensor for each image unit, or pixel. The diffusion tensor gives a three dimensional representation of the preferred direction of diffusion, in the shape of the 3D ellipsoid. This ellipsoid can be characterized by six parameters; diffusion constants along the longest, middle, and shortest axes ( λ1, λ2, and λ3, called principal axes) and the direction of the three principal axes. Once the diffusion ellipsoid is fully characterized at each pixel of the brain images, local fiber structure can be derived. For example, if λ1 >> λ2 ≥ λ3 (diffusion is anisotropic), it suggests the existence of dense and aligned fibers within the each pixel, whereas isotropic diffusion (λ1 ≈ λ2 ≈ λ3) suggests sparse or unaligned fibers. When diffusion is anisotropic, the direction of λ1 indicates the direction of the fibers. Recently, such DTI techniques in combination with 3-D fiber reconstruction algorithm was used to generate spectacular images of the axonal connectivity pattern in vivo both in humans, rodents, and recently also in cats."
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