As the axial diffusivity represents the diffusivity along the primary diffusion direction whereas the radial diffusivity indicates the diffusivity orthogonal to the primary diffusion direction (calculated by averaging the second and third eigenvalues of the diffusion tensor), it was hypothesized that axial diffusivity is an indirect indicator of the integrity of axons.
After these processes, using DTIFit within the FMRIB's diffusion toolbox, the images were calculated to get the FA, λ1 (longitudinal diffusivity) (the magnitude of diffusion along the principal diffusion direction) and λ23 (radial diffusivity) (the magnitude of diffusion in the two orthogonal directions perpendicular to the principal diffusion direction) maps.
In contrast, AD represents the diffusivity along the primary diffusion direction and is assumed to characterize the integrity of axons (Gao et al., 2009; Glenn et al., 2003; Sun et al., 2006).
By sorting the eigenvalues in the order of decreasing magnitude for each voxel (λ1 > λ2 > λ3), λ1 represents the diffusivity along the primary diffusion direction, that is, along the fibre axis, and is referred to as the axial diffusivity λ ∥.
DW-MRI is thus sensitive not only to the extent to which protons are free to diffuse but also to their preferential diffusion direction.
RD indicates the diffusivity along directions which are orthogonal to the primary diffusion direction and is an indirect indicator of myelination (Song et al., 2005; Wu et al., 2011).
The influence of sampling frequency and diffusion direction was investigated.
Here, we considered only isotropic strains to avoid the existence of a preferential diffusion direction.
In the case of a single-tensor model, one continuously follows the principal diffusion direction [26].
Use the principal diffusion direction m j most aligned to the incoming vector to proceed 9.
m is the principal diffusion direction since it corresponds to largest eigenvalue λ1.
