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Table 1 Common scalar metrics for diffusion tensor imaging analysis

From: Neuroimaging in repetitive brain trauma

Metric Description
Fractional anisotropy (FA) Describes the anisotropy of water diffusion. Measured as a unitless scalar value between 0 and 1 with 0 being completely isotropic. Thought to be related to WM integrity. Changes in FA have been associated with TBI
Relative anisotropy (RA) Represents the ratio of the anisotropic part of the diffusion tensor to its isotropic part. An alternative measure for FA
Axial diffusivity (AD) Describes the magnitude of diffusion along its principal orientation. In WM this will measure diffusion along the axon. May be more specific to axonal degeneration
Radial diffusivity (RD) Describes the magnitude of the diffusion in the plane perpendicular to the principal orientation. In WM, radial diffusivity is perpendicular to the fiber orientation. May be modulated by myelin in the WM
Mean diffusivity (MD) Describes the magnitude of diffusion regardless of direction. Proportional to the trace of the diffusion tensor, with units of m2/s. Also known as the apparent diffusion coefficient (ADC). Increased MD has been associated with TBI
  1. TBI, traumatic brain injury; WM, white matter.