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-bounces@nmr.mgh.harvard.edu freesurfer-bounces@nmr.mgh.harvard.edu on behalf of Chong, Catherine (Cat), Ph.D. Chong.Catherine@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
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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