Hi Cat - The FA/MD/RD/AD at a specific voxel comes from first fitting a tensor
to the intensity values of all the DWIs at that voxel and then finding the
eigenvalues of the tensor at that voxel. The tracts work like ROIs - the
FA/MD/RD/AD values are averaged over all the voxels in a tract.
Best,
Anastasia.
________________________________
From: freesurfer-boun...@nmr.mgh.harvard.edu
<freesurfer-boun...@nmr.mgh.harvard.edu> on behalf of Chong, Catherine (Cat),
Ph.D. <chong.cather...@mayo.edu>
Sent: Wednesday, October 21, 2020 1:17 PM
To: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] Tracula: Calculation of MD values from other eigenvectors
External Email - Use Caution
Dear Tracula Experts,
We are trying to address a question from a reviewer who is wondering why we
included all of the average diffusion parameters in our model ( i.e. FA,
MD,FA, RD, and AD) for distinguishing two groups of patients . The reviewer
comments that MD values are calculated from AD and RD henceforth, including of
all of the three parameters ( AD, RD, and MD) might have biased our analysis. I
understand the reviewer’s concern that MD is calculated from adding all three
eigenvalues/3, but I am uncertain about the relationship between the
eigenvalues and the calculation of ‘average’ parameters for a given fibertract.
I am looking forward to your guidance,
With kind Regards,
Catherine
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