[Freesurfer] glmfit flag --frame-mask

Dear FS Experts,

I have seen few posts about this mri_glmfit flag, and I would like to know how it works. In an older post, ( https://www.mail-archive.com/freesurfer%40nmr.mgh.harvard.edu/msg32412.html), Doug suggested to use this flag when only some subjects should be exclude for each vertex (i, e. differnet number of subjects to exclude per vertex). Could anyone give me an insight on how this works?

My intention is to perform a GLM analysis of some DTI metric volumes projected into the surface. However, some of the DWI volumes are cutted, so I would like exclude those vertexs where there is no signal (i,e value 0). Will --frame-mask work for this? Any other suggestion?

Any help will be very appreciated! Thank you in advance,

Victor Montal