Dipoles outside BEM layers

Discussions
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1

I am trying to compute the headmodel using openMEEG BEM from the BEM surfaces. I am using the individual's MRI. I get the following error:
WARNING: Some dipoles are outside the BEM layers (2 dipoles).
The leadfield for these dipoles is probably incorrect.

To fix the cortex surface:
Right-click on the surface file > Force inside skull.

Can I assume that Right-clicking on the surface file > Force inside skull is the right thing to do here to correct this?

Thanks!

2

Can I assume that Right-clicking on the surface file > Force inside skull is the right thing to do here to correct this?

Given the error message you reported, this sounds like a trivial question, but it is not.

I edited a bit the error message and added a new section to the BEM tutorial:
https://neuroimage.usc.edu/brainstorm/Tutorials/TutBem#Warning:_Dipoles_outside
Please let me know if there is anything that is unclear in this tutorial.

@Alexandre @tmedani If you find 5min to have a look at this section, please share your feedback... :slight_smile:

3

Hi all,

Thanks, @Francois, for the new section, this explains the issue and give an acceptable solution.

Having dipoles outside the brain is not realistic, and from the computation side, the results will be different comparing with the other dipoles. This issue may also happen when the dipoes are too close to the interface.
@Alexandre may give a better explanation from the BEM perspective.

The proposed solution in Brainstorm is fine, since "Force inside the inner skull" will move slightly the dipoles inside (only a few millimeters) and avoid these unrealistic sources.

I think another possible solution is to keep the cortex as it is and replace the wrong lead field by interpolating the LF values of the neighboring dipoles.