Hello everyone,
I am fitting the diffusion tensor with MRtrix3 and have found a small number of voxels with FA values greater than 1. In every voxel where FA > 1, one of the tensor eigenvalues is negative.
The affected voxels account for approximately 0.2% of the voxels within my analysis mask, for each participant. Most appear isolated and include voxels within white matter.
My acquisition is single-shell, with one b=0 image and the remaining volumes at b=1000 s/mm². The preprocessing includes:
-
denoising with dwidenoise
-
Gibbs-ringing correction with mrdegibbs
-
susceptibility, eddy-current and motion correction using Synb0-DISCO, TOPUP and eddy_cuda --repol
-
ANTs bias-field correction
-
brain masking based on the final corrected b=0 image
The tensor and scalar maps are calculated as follows:
dwi2tensor dwi_den_preproc_unbiased.mif dt.mif \
-mask mask.mif
tensor2metric dt.mif \
-mask mask.mif \\
-fa fa.mif \\
-adc md.mif \\
-ad ad.mif \\
-rd rd.mif \\
-value eigenvalues.mif \\
-num 1,2,3
I understand that FA > 1 is possible and that the negative eigenvalue indicates a non-physical tensor estimate. However, I am unsure what the recommended practice is before conducting a group voxel-wise analysis with permutation testing and TFCE.
Thank you in advance!