Fig. 8.

Considering multiple diffusion parameters, as well as findings based on tractography, can shed light on complex pathology. In this example, diffusion images were acquired in patients with Alzheimer’s Disease, Mild Cognitive Impairment, and Healthy Controls. In addition to calculating fractional anisotropy (FA), the authors also calculated the mode of anisotropy, which quantifies the degree to which the tensor is planar (disc-shaped) or linear (cigar-shaped), as shown in the schematic in the top right (taken from Ennis and Kindlmann, MRM, 2006). Mode was more sensitive than FA to differences between MCI and controls. The top left figure illustrates that patients showed a counter-intuitive increase in mode (shown in pink), along with an increase in fractional anisotropy (shown in yellow), specifically in a region where association fibers cross with projection fibers. Closer interrogation of this crossing fiber region using tractography (bottom panel), revealed that patients had a decrease in tractography particles in the association fibers (AF), with no change in particles in the projection fibers (PF).

Images taken from Douaud et al. Neuroimage (2011), with permission.