TABLE 1.
Diffusion parameters determined by FBI and FBWM, their biophysical meanings and potential biological interpretations
| Parameters | Acronym | Meaning | Biological interpretation |
|---|---|---|---|
| Axonal water fraction (FBWM) | AWF |
Amount of intra‐axonal water divided by the total amount of MRI‐visible water within a voxel Decreases if intra‐axonal water decreases, if extra‐axonal water increases (edema), or both |
Potential loss of axons, edema |
| Intra‐axonal diffusivity (FBWM) | Da | Diffusivity of water molecules along the length of the axons | Changes in the intra‐axonal compartment (e.g., axonal beading) |
| Mean extra‐axonal diffusivity (FBWM) | MDe | Mean diffusivity of water within the extra‐axonal compartment. This compartment includes glial cells and extra‐cellular space | Analogous to MD but only for extra‐axonal water |
| Radial extra‐axonal diffusivity (FBWM) | De,⊥ |
Radial diffusivity of the extra‐axonal compartment Perpendicular to the direction of highest extra‐axonal diffusivity |
Analogous to RD but for extra‐axonal compartment |
| Axial extra‐axonal diffusivity (FBWM) | De,II | Axial diffusivity of the extra‐axonal compartment in the direction of highest extra‐axonal diffusivity | Analogous to AD but for extra‐axonal compartment |
| Intra‐axonal fractional anisotropy (pure FBI) | FAA |
The fractional anisotropy of just the water within of the intra‐axonal compartment Reflects the geometrical arrangement—if all axons in the same direction, this value is high |
Analogous to regular FA, but typically has higher values as intra‐axonal water is more organized than extra‐axonal water |
| Extra‐axonal fractional anisotropy (FBWM) | FAE | Fractional anisotropy of just the water within extra‐axonal compartment |
Analogous to regular FA but typically has lower values as extra‐axonal water is less organized than intra‐axonal water High values could represent tight packing of axons Parameter could be affected by myelination thickness |
| Zeta (pure FBI) | ζ |
Equal to the AWF divided by the square root of the intra‐axonal diffusivity (Da) Scaled version of the AWF |
Since Da is often fairly constant across voxels, ζ is usually strongly correlated with AWF, but it is easier to measure |
Abbreviations: AWF, axonal water fraction; FAA, fractional anisotropy of the intra‐axonal; FBWM, fiber ball white matter modeling; FAE, fractional anisotropy of the extra‐axonal.