Diffusion tensor imaging
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Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) technique that enables the measurement of the restricted diffusion of water in tissue. The principal application is in the imaging of white matter where the location, orientation, and anisotropy of the tracts can be measured. The architecture of the axons in parallel bundles, and their myelin sheaths, facilitate the diffusion of the water molecules preferentially along their main direction. Such preferentially oriented diffusion is called anisotropic diffusion.
The imaging of this property is an extension of diffusion MRI. If we apply diffusion gradients (i.e. magnetic field variations in the MRI magnet) in at least 6 directions, it is possible to calculate, for each voxel, a tensor (i.e. a symmetric positive definite 3 ×3 matrix) that describes the 3-dimensional shape of diffusion. The fiber direction is indicated by the tensor’s main eigenvector. This vector can be color-coded, yielding a cartography of the tracts' position and direction (red for left-right, blue for superior-inferior, and green for anterior-posterior). The brightness is weighted by the fractional anisotropy which is a scalar measure of the degree of anisotropy in a given voxel. Mean Diffusivity (MD) or Trace is a scalar measure of the total diffusion within a voxel. These measures are commonly used clinically to localize white matter lesions that do not show up on other forms of clinical MRI.
Diffusion tensor imaging data can be used to perform tractography within white matter. Fiber tracking algorithms can be used to track a fiber along its whole length (e.g. the corticospinal tract, through which the motor information transit from the motor cortex to the spinal cord and the peripheral nerves).
Some clinical applications of DTI are in the tract-specific localization of white matter lesions, the localization of tumors in relation to the white matter tracts (infiltration, deflection), the localization of the main white matter tracts for neurosurgical planning, and the assessment of white matter in development, pathology and degeneration. DTI also has applications in the characterization of skeletal and cardiac muscle.
