Not exactly because volumes are computed from soft segmentations rather than discrete labels, but yeah, it should be very close.

--

Juan Eugenio Iglesias

http://www.jeiglesias.com

From: Marco Ganzetti <marco.ganzetti@biogen.com>
Date: Tuesday, July 1, 2025 at 3:55 AM
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] Re: eTIV in SynthSeg

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Thanks. In SynthSeg, if I set a threshold at 0.5 on the segmentation output, should I get exactly the TIV?

Label 0 is the background, therefore all the labels above 0 should constitute the TIV. Is this the case?

Best,

Marco

From: Iglesias Gonzalez, Juan E. <JIGLESIASGONZALEZ@mgh.harvard.edu>
Date: Tuesday, 1 July 2025 at 05:51
To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] Re: eTIV in SynthSeg

EXTERNAL SENDER

The former!

--

Juan Eugenio Iglesias

http://www.jeiglesias.com

From: Marco Ganzetti <marco.ganzetti@biogen.com>
Date: Monday, June 30, 2025 at 05:57
To: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] eTIV in SynthSeg

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Dear,

I would like to know how the eTIV is estimated in SynthSeg and SynthSeg+ .

Is it a combination of CSF+ventricles+WM+GM masks?

Or do you estimate is using the registration to MNI space approach?

Best,

Marco