Fig. 2.

Diffusion anisotropy and diffusion tensor imaging. In the presence of anisotropic diffusion the ADC, as in white matter, depends on the measurement direction. A left to right: Measurement direction was vertical (yellow arrow). Vertical tracts (such as pyramidal tract) have high ADC, while horizontal tracts (as in corpus callosum) are dark. This results from the fact that diffusion is reduced perpendicularly to the white matter fibers due the presence of plasma membranes and myelin. With Diffusion Tensor Imaging it becomes possible to characterize diffusion in all 3 dimensions and to determine the direction of fastest diffusion. For each image voxel an ellipsoid can be produced the nature of which is related to key DTI parameters: overall ellipsoid volume and mean diffusivity, the shape (oblong) to the degree of fractional anisotropy and the orientation to the fiber main direction. B left to right: After the ellipsoids have been obtained for all voxels of the image (here for the cortico-spinal tract out of the motor cortex in red) an algorithm is used to determine whether adjacent voxels are likely to be connected (here with the FACT algorithm from Mori et al., 1999). Connected voxels within putative tracts are then displayed using pseudo-colors. It should be noticed that such color tracks are purely the results of a software and do not represent genuine anatomical structures.