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

[二姬]

Dear Jan-Mathijs,

Thank you very much for your insightful suggestions regarding the alignment
of the EGI 128 electrode system.

I would like to clarify my earlier mention of unusual source localization
results obtained using the 'interactive' method. Specifically, some of my
source analysis indicated significant activations in deep brain structures,
such as the cerebellum, which seemed challenging to reconcile with my
experimental hypotheses. However, after re-aligning the electrode positions
(notably adjusting the fit of the electrode system, which initially seemed
too small and tight relative to the head model), I observed nearly
identical cerebellar activations. This consistency in results, despite the
electrode position adjustments, has reassured me, especially in light of
your advice.

Moving forward, I plan to try the eloreta method for source analysis to see
if the results align with those currently obtained. Once again, I am
grateful for your guidance and the time you took to address my concerns.

Best regards,

Chengyuan Wu
Ningbo Kangning Hospital


Schoffelen, J.M. (Jan Mathijs) via fieldtrip <fieldtrip at science.ru.nl>
于2024年2月5日周一 16:33写道：

> 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> 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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