MEG data acquisition QA metric: distance from helmet to headpoints?

Hi all,

Does the median distance between the MEG headpoints and the helmet surface (from get_meg_helmet_surf) seem like a reasonable way to catch significant problems that may have occurred during data acquisition? I’m thinking of cases when a subject may have been in the MSR for awhile and their posture degraded, or an inexperienced RA didn’t position the subject correctly under the dewar, etc.

I made a gist using the sample data: Quick little script to get median distance between sensor helmet and head surface points (excluding the face) · GitHub

The resulting values are larger than what I’d expect – for the sample data, the median distance is 38.39 mm – but in neuromag datasets this approach does seem to identify cases where something went awry with the subject’s head position. I think the helmet surface might not be the room temperature helmet, and I did use a hidden function to get the distances, but aside from that, does this seem OK to do?