Hi Colleen

After correction the Euler number should be exactly 2. If it is not, it means the surface is not topologically equivalent to a sphere. Every defect (hole or handle) will decrease it by 2, so an Euler number of -18 (like your lh below) is:

# of holes = (2-euler_number)/2 = (2--18)/2 = 10

Make sense?

Cheers Bruce

-----Original Message----- From: freesurfer-bounces@nmr.mgh.harvard.edu freesurfer-bounces@nmr.mgh.harvard.edu On Behalf Of C.P.E. Rollins Sent: Thursday, September 3, 2020 12:17 PM To: Freesurfer freesurfer@nmr.mgh.harvard.edu Subject: [Freesurfer] how to calculate Euler number

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Dear Freesurfer Developers,

It is not clear to me how to calculate the Euler number. Using mri_segstats reports something like: Computing euler number orig.nofix lheno = -18, rheno = -20 orig.nofix lhholes = 10, rhholes = 11 However, I had understood from previous comments that the Euler number should be around 2. I'm wondering which of the above numbers I could report.

Thanks, Colleen

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