FIGURE 2.

Cartoon depictions of a voxel containing three white matter crossing fibers (i.e., vertical, horizontal, and through-plane fibers) and the corresponding diffusion tensor and fractional anisotropy (FA). (A) On the left-hand side, FA = 0 because there is no dominant fiber direction (i.e., same diameter, density and integrity of fibers arranged orthogonally), where water diffusion will be constrained equally in all directions. On the right-hand side, the vertical fibers remain unchanged while the other two fiber bundles (i.e., horizontal and through-plane) are decreased, such that λ₁ becomes larger than λ₂ and λ₃, resulting in an FA increase despite a net white matter fiber reduction. (B) On the left-hand side, there is a principle fiber orientation (i.e., in the vertical direction) and a corresponding FA > 0. On the right-hand side, all three fibers experience the same amount of tissue damage, such that all three eigenvalues (λ₁, λ₂, and λ₃) are increased proportionally, resulting in a constant (unchanged) FA despite a net decrease in white matter fiber density. Because it is a relative measure, FA cannot provide quantitative information about net tissue differences/changes in the presence of crossing fibers. Bold arrow represents a relatively large increase (λ₁ in part A), thin arrows represent a relatively small increase (λ₁ and λ₂ in part A), and medium arrows indicate a moderate increase (λ₁, λ₂ and λ₃ in part B).