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. 2010 Jul 27;32(8):1220–1235. doi: 10.1002/hbm.21103

Table II.

Definitions of white matter microstructure measurements

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 graphic file with name HBM-32-1220-e001.jpg
Fractional anisotropy (FA) Normalized difference between axial diffusivity and radial diffusivity graphic file with name HBM-32-1220-e004.jpg
Mean diffusivity (D av) Weighted average of axial and radial diffusivity graphic file with name HBM-32-1220-e002.jpg

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.