Diffusion tensor imaging

An imaging approach describing direction-dependent water diffusion using a tensor model. In white matter it provides indirect information about tissue organisation; crossing fibres, motion and model limitations affect interpretation.

A dimensionless measure of dispersion among the diffusion tensor eigenvalues. With eigenvalues ${\lambda }_i$ and mean $\bar{\lambda }$, $\mathrm{FA}=\sqrt{\frac{3}{2}\frac{{\sum }_i({\lambda }_i-\bar{\lambda }{)}^2}{{\sum }_i{\lambda }_i^2}}$ when the denominator is nonzero. It is not a direct measure of axon number or functional necessity.