[FieldTrip] Seeking Guidance on Aligning EGI 128 Electrode Positions with Standard_bem Head Model

[Schoffelen, J.M. (Jan Mathijs)]

Hi Chengyuan,

I think that the ‘interactive’ method is the best thing to use here. Alternatively, if the coordinate systems match approximately (i.e. if the mesh of the scalp surface, and the electrodes are already approximately in register (i.e. the axes pointing into the same direction etc.), then an ‘automatic’ iterative closest point algoritm (icp) may be used. Yet, typically - with only 128 points in one of the point clouds (electrodes) - this does not yield a result that is more trustworthy as such than a manual alignment.
It’s hard from a distance to judge what you mean with ’the source localization results appear unusal’, it could be that this - if true - is caused by something else than a coregistration misfit.

Good luck,
Jan-Mathijs

On 3 Feb 2024, at 06:41, 二姬 via fieldtrip <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>> wrote:


Dear FieldTrip Community,

I am Chengyuan Wu from Ningbo Kangning Hospital, reaching out once again in hopes of gaining insights from this knowledgeable community. Previously, I discussed my process of source analysis using EEG data collected from the EGI 128 electrode system and the challenges I faced in aligning electrode positions with the standard_bem head model in FieldTrip. Despite not having received a response to my earlier inquiry, I have progressed with the analysis but have encountered new concerns that I hope to address with your help.

After manually aligning the EGI 128 electrodes with the standard_bem model using the 'interactive' method, I proceeded with the source analysis. However, the source localization results appear unusual, raising doubts about the reliability of the manual alignment of the EGI electrode positions. The absence of MRI fiducials such as nas, lpa, and rpa in the standard_bem, and the provision of fiducial points like FidT9, FidT10, and FidNz in the EGI electrode file, has left me uncertain about how to correctly align the electrodes using the 'fiducial' method or any other more reliable method.

Here is the MATLAB code I used for the manual alignment:

templateheadmodel = 'F:\eeg_ningbo\fieldtrip-20211209\template\headmodel\standard_bem.mat';
load(templateheadmodel);
%% electrode realign as good as possible
cfg = [];
cfg.method = 'interactive';
cfg.elec = elec; %load from the egi128 electrode file
cfg.headshape = vol.bnd(1);
elecR = ft_electroderealign(cfg);


Given these concerns, I am seeking advice on the following:

  1.  1.Are there known issues or limitations with the 'interactive' method for aligning electrodes with the standard_bem head model that could lead to inaccuracies in source localization?
  2.  2.Can anyone provide guidance or suggest an alternative approach for aligning the EGI 128 electrode positions with the standard_bem model, especially considering the mismatch in fiducial points between the EGI system and the standard_bem?
  3.  3.If anyone has successfully navigated this alignment challenge or has insights into using the 'fiducial' method under these circumstances, your advice would be invaluable.

Your expertise and suggestions would be greatly appreciated as I strive to ensure the accuracy and reliability of my source analysis. I am looking forward to your responses and thank you in advance for your time and assistance.

Best regards,

Chengyuan Wu Ningbo Kangning Hospital

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https://doi.org/10.1371/journal.pcbi.1002202

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