[Freesurfer] Question about brain masking

Dear FreeSurfers,

I'm having some trouble with my brain masks. Below I have inserted a subject's T1.mgz and brain.nii overlay (made with preproc-sess). As you can see, the mask goes well beyond the boundaries of the brain. Any ideas what could cause this-perhaps noisy functional scans? The majority of my subjects' masks look like this.

Should I worry about this? My brainmask.mgz files are all fine, and I can use those to mask out voxels for visualization purposes-Do I need to be concerned if my brain.nii volumes look this way? Would I fix by running mkbrainmask-sess and tweaking the "-threshold" argument?

Thanks!

Dan Dillon

Affective Neuroscience Lab

Harvard University