Why is the order of magnitude of Mean diffusivity value so small?

Dear All,

I used FSL’s dtifit for the DTI fit. The generated dti_MD.nii.gz, the value of the MD in the brain is around 0.0007, why is the order of magnitude so small? Is this normal for mean diffusivity value?

dtifit -k dwmri.nii.gz -o dti_ -m b0_mask.nii.gz -r "$BVECS" -b "$BVALS"

Any help is greatly appreciated!



Yes, this is normal. MD is the average magnitude of the eigenvalues of the diffusion tensor. The magnitude is dependent on your b-value (higher b-value → more diffusion → higher MD), and where you measure MD. In CSF for example, MD is high since water is free to move in a fluid space. MD tends to be lowest in WM since movement is restricted along the axons.


Thank you!
The units for the diffusivities are mm²/s (opposite of the b -value, which is in s/mm²), and in the brain these are typically around 700-800 ×10-6 mm²/s – which matches my data.