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

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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-bounces@nmr.mgh.harvard.edu> on behalf of "Yendiki, Anastasia" <AYENDIKI@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-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

 

        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