Warning option for sensors in neck in file: _make_forward.py

[wadelab]

Describe the new feature or enhancement

We are using an OPM-MEG system where the sensors are very close to the skull. Our MRI data are acquired when the subject is in a slightly different posture and so we sometimes end up with the neck sensors mapping >within< the surface of the head even though they were (obviously) outside during the MEG data acquisition. Currently _make_forward.py stops when it discovers this (in _prepare_for_forward). We would like the option to throw a warning (not an error) at this point and proceed. We think this is okay because the sensors are not inside the skull, just the neck fat. Currently we are commenting out the lines around line 573 in this file in our local copy...

Describe your proposed implementation

Perhaps a flag like USE_PERMISSIVE_POSITION_CHECK when calling _make_forward? If it's set then just warn, don't error?

Describe possible alternatives

I guess we could move the sensors out normal to the surface until they are outside the scalp? This feels like a worse solution.

Additional context

Thanks!

[welcome]

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[larsoner]

Describe possible alternatives

I guess we could move the sensors out normal to the surface until they are outside the scalp? This feels like a worse solution

Agreed that moving the sensors isn't the best idea. The best fix would probably be to fix/deform the BEM surface so that it reflects a more realistic / accurate subject geometry at the time of acquisition. Maybe using Blender it wouldn't be too bad?

Another alternative would be to use a 1-layer BEM for MEG. I don't know if this would be better or worse than using an incorrect / deformed (from how it was acquisition) outer skin surface, but I'd assume it would be better.

Perhaps a flag like USE_PERMISSIVE_POSITION_CHECK when calling _make_forward? If it's set then just warn, don't error?

I suppose we could consider a on_inside="raise" default that you could set to "warning" or "ignore" etc., but the bigger problem is that I am not sure that the BEM calculations are valid for sensors inside the surface. @agramfort ?