Table II.
Measurement | Definition | Formula |
---|---|---|
Axial diffusivity (AD) | Magnitude of the microstructure oriented in the direction of the principal axis; first eigenvalue | λ1 |
Radial diffusivity (RD) | Magnitude of the microstructure oriented in the direction perpendicular (i.e., orthogonal) to the principal axis; Average of the second and third eigenvalue | |
Fractional anisotropy (FA) | Normalized difference between axial diffusivity and radial diffusivity | |
Mean diffusivity (D av) | Weighted average of axial and radial diffusivity |
These formulas assume that the diffusion tensors are diagonalized to yield three eigenvalues: λ1, the eigenvalue representing the magnitude of the principal axis, and two eigenvalues (axis λ2 and λ3) which represent the magnitude of the two axes perpendicular to the principal axis.