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Dear Dr. Douglas N. Greve,

Thanks to you, I could have a binary disk approximately 3mm in radius by the codes you told me (MailScanner has detected a possible fraud attempt from "secure-web.cisco.com" claiming to be https://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg74761.html):

mri_volsynth --dim-surf fsaverage/surf/lh.white --pdf delta --delta-crsf ${vertexno} 0 0 0 --0 delta.mgh
mris_fwhm --i delta.mgh --o delta.sm3.mgh --s fsaverage --hemi lh
mri_binarize --i delta.sm3.mgh --min 0.5 --o delta.sm3.bin.mgh

I have sincerely appreciated your help. 

Now I'm wondering how to modulate the radius. I guess I can do it by adding --fwhm option to mris_fwhm command, for example:
mris_fwhm --i delta.mgh --o delta.sm3.mgh --s fsaverage --hemi lh --fwhm 5
At the end, I will get a binary disk with 5mm in radius. Am I right? And does this mean that --fwhm 3 is default for mris_fwhm? How could you define 3mm?

Kind Regards,
Andy (Junghyun) Kim