Diffusion Tensor Images can be viewed as an overlay of colored voxels. Voxels are colored red, blue, and green to represent the direction of the vectors in the DTI data.
DTI volume data sets consist of two volumes: a three frame eigenvector volume (EV volume) and a fractional anisotropy volume (FA volume). These must be specfied in the File->DTI, Load DTI Volumes... dialog box. There will be a considerable delay while loading DTI volumes as TkMedit does some pre-processing.
The eigenvector volume describes the directionality of diffusion in a particular direction (with the first EV being the direction of greatest mean diffusion within each voxel), and the FA volume describes how homogenious the diffusion is in a particular direction (high values mean that diffusion is primarily occuring in a single direction- and FA is calculated from something like the variance of the three eigenvalues).
Head points are displayed in full color. The red, green, and blue spectrums represent the directionality of the EV volume. The opacity of the overlay can be configured in the dialog box brought up by choosing View->Configure...->DTI Display.... The overlay can be hidden by unchecking View->DTI Overlay.