TABLE 1.

Main characteristics and interpretations of diffusion magnetic resonance imaging (MRI) metrics.

Name	Measure	Interpretations
Fractional anisotropy	Rotationally invariant index of the degree of microstructural organisation and diffusion directionality	Reduced fractional anisotropy might reflect  ● damage or degeneration to axons or myelin  ● reduced axonal packing density  ● reduced axonal directional coherence
Mean diffusivity	Rotationally invariant index of the average diffusivity or water mobility within each image voxel	Increased mean diffusivity might reflect  ● reduced white matter density due axonal degeneration  ● increased extracellular space due to myelin degeneration  ● decreased axonal packing
Axial diffusivity	Rotationally invariant index of diffusivity computed along the direction of maximum diffusivity of the diffusion tensor	Reduced axial diffusivity might reflect  ● axonal injury/degeneration  ● increased tortuosity  ● less coherent axons orientation
Radial diffusivity	Rotationally invariant index of diffusivity computed perpendicularly to the direction of maximum diffusivity of the diffusion tensor	Increased radial diffusivity might reflect  ● myelin loss  ● axons loss or decrease axonal density  ● reduced axonal packing  ● increased extra-axonal space