Hi Cat - I am not sure what precise means. If the effects on these measures are
the same over the entire tract, averaging the measures over the tract would
actually increase SNR and make the effects (and their relationship) cleaner. If
there are opposing effects in two different parts of the tract, averaging over
the tract would wipe out the effects (and their relationship).
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: Tuesday, October 27, 2020 12:09 PM
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: Re: [Freesurfer] [EXTERNAL] Re: Tracula: Calculation of MD values from
other eigenvectors
External Email - Use Caution
Dear Dr. Yendiki,
Thank you very much for your reply. Is it correct to assume then that the
FA/MD/RD/AD values that are averaged over all the voxels for a given tract do
not necessarily have a ‘precise’ relationship to one another?
With warm Regards,
Cat
From: <freesurfer-boun...@nmr.mgh.harvard.edu> on behalf of "Yendiki,
Anastasia" <ayend...@mgh.harvard.edu>
Reply-To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Date: Monday, October 26, 2020 at 8:26 AM
To: "freesurfer@nmr.mgh.harvard.edu" <freesurfer@nmr.mgh.harvard.edu>
Subject: [EXTERNAL] Re: [Freesurfer] Tracula: Calculation of MD values from
other eigenvectors
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
_______________________________________________
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
