Table 1. Definition and Interpretation of Diffusion Tensor Imaging (DTI) and Neurite Orientation Dispersion and Density Imaging (NODDI) Metrics in the Context of Traumatic Brain Injury (TBI).

Metric	Description of measurement	Interpretation in TBI
DTI diffusion metrics
Fractional anisotropy	Scalar value from 0 to 1, 0 = isotropic diffusion (equal in all directions)	Reduced fractional anisotropy,10,11,12,13,14 white matter tract disorganization15
Mean diffusivity	Scalar measure of water molecule diffusion within a voxel	Higher mean diffusivity, increased cellularity, edema, and necrosis10,11,12,13,16
Axial diffusivity	Magnitude of diffusion parallel to fiber tracts	Reduced axial diffusivity, axonal injury, diminished axonal caliber, or less coherent axon orientation17
Radial diffusivity	Coefficient of diffusion perpendicular to the main fiber orientation	Increased radial diffusivity,18,19,20 demyelination21
NODDI diffusion metrics
ICVF	Fraction of tissue water restricted within neuritis, amount of neurites	Reduced ICVF, axonal loss or demyelination22,23,24,25; increased ICVF, axonal swelling in acute phase and regeneration in recovery phase14,26,27
ODI	Variability of neurite orientations, neurite spatial arrangement from 0 (parallel) to 1 (random)	Increased ODI, axonal dispersion or disorganization23,25; reduced ODI, reduced axonal complexity or glial scarring14,28
ISOVF	Fraction of tissue water confined within neurites	Increased ISOVF, tissue atrophy leading to expanded cerebrospinal fluid spaces14,22

Abbreviations: ICVF, intracellular volume fraction; ISOVF, isotropic volume fraction; ODI, orientation dispersion index.