Diffusion Tensor Imaging (DTI)
Diffusion tensor imaging (DTI) is an extension of diffusion weighted imaging that allows data profiling based upon white matter tract orientation.
Diffusion weighted imaging is based on the measurement of Brownian motion of water molecules. This motion is restricted by membranous boundaries. In white matter, diffusion follows the ‘pathway of least resistance’ along the white matter tract; this direction of maximum diffusivity along the white-matter fibers is projected into the final image.
Clinical Applications
Display of the DTI data
Tensor maps and maps of metrics
Several methods are used to visualize the large amount of data obtained at DTI. Diffusion tensor maps may be generated using a workstation with 3D display capability. In addition, the metrics FA, relative anisotropy (RA), or MD may be calculated on a voxel-by-voxel basis and displayed as 2-dimensional (2D) color or gray-scale images; the major and minor eigenvalues may also be displayed in this fashion. From these images, the average metric values within user-defined regions of interest (ROIs) may be calculated. In addition, the maps may be interrogated using methods such as histogram analysis.
3D tractography
In white matter, the direction of the major eigenvector tends to be parallel to the orientation of axonal fibers. Using this observation, algorithms have been developed that may generate 3D representations of axonal fibers, or 3D fiber tractography. These algorithms in effect attempt to “string together” adjacent voxels based on similarity in the direction of their major eigenvectors.
Although useful in tract visualization, white matter fiber tractography represents a more postprocessed representation of DTI data (than do visualization of tensor maps and maps of metrics) and is, therefore, prone to the addition of error. In voxels that contain crossing fiber tracts from ≥2 directions, the association between the diffusion tensor measurement and the axonal fiber direction is less direct; algorithms have been developed to mitigate this problem, which arises commonly in CNS structures (such as the brainstem and in areas with complex crossing association fibers).