Hi Nazife - That is correct, the lengths are numbers of voxels along a tract in each subject's native DWI space, so if you were to compare them across subjects you'd probably want to correct for total intracranial volume. (This is just as you would do for any other length/area/volume measurement derived in individual space, like cortical thickness, etc.) Let me know if you have any other questions!

a.y ________________________________ From: freesurfer-bounces@nmr.mgh.harvard.edu freesurfer-bounces@nmr.mgh.harvard.edu on behalf of Nazife Ayyıldız nazifeayyildiz@gmail.com Sent: Wednesday, April 8, 2020 8:43 AM To: freesurfer@nmr.mgh.harvard.edu freesurfer@nmr.mgh.harvard.edu Subject: [Freesurfer] Tracula: tract length meaning

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Dear Tracula experts,

I want to compare two groups according to the length statistics from Tracula overall.stats outputs. I just need comprehensive explanations what "tract length" statistics mean and how they are derived and what their reliability is. I checked but I couldn't find detailed information.

Are they suitable to use in group comparisons? If so, which one is more suitable to use: average, center, min or max length? I saw in some articles that they used average_length of a tract. If I want to use length statistics do you suggest taking any control variables?

I read a message from Anastasia in the archive-list saying that we should be aware that these measures are in their individual DWI spaces. If I understood correctly, I shouldn't use them directly. Should I control the head sizes of the individuals then? How would you do if you wanted to do this kind of comparative analysis?

I would really appreciate it if you answer. Thank you very much in advance!

Best regards, Nazife

-- -Nazife Ayyıldız-