Here is an explanation of etiv calculation in Freesurfer - https://surfer.nmr.mgh.harvard.edu/fswiki/recon-all

etiv is not obtained via direct segmentation of all structures within the skull boundaries. Rather, its estimation depends on the linear transform between the T1 image and a T1w atlas.

Best,

Yujing

From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> On Behalf Of Carlos Alcocer
Sent: Friday, January 5, 2024 4:11 PM
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] poor inferior cerebellum segmentation

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Hello,

Inferior portion of cerebellum are not segmented well. It is included in skullstrip and I tried adding control points in cerebellar WM but this did not help. I am not interested in cerebellum but I do need eTIV. Will this poor cerebellar segmentation affect eTIV calculation? 

Thank you,

Carlos