[Simnibs-discuss] Differences in Efield depending on Processing

[Joel Upston]

Axel,

Thanks for the clarification. In the inital post I modeled the second pathway with only the electrodes that were used in the pipeline, but your reasoning makes sense about the conductive paste being used essentially covers a majority of the head.

I reran using all modeled electrodes, and the electrodes not used are modeled with 0 mA current. I have attached the images where the right is the non-leadfield model and the left is the leadfield model. Where now the fields look very similar between the two different pipelines. It looks like it does lower the field by about ~10% (max given with all electrodes 0.47 V/m and only used electrodes 0.521 V/m). This would be consistent with what I have found with other stimulations. The benefit is that although it does seem to change the magnitude of the electric field, the spatial distribution does not seem to change.

So, if we were to target a specific value efield magnitude for a ROI, any optimization problem using the leadfield might be incorrect due to the lower magnitude of the leadfield processing. I wonder how to correct for this during any optimization, or maybe not necessary since it could be that the scale is similar no matter what stimulation is modeled. So maybe you could just correct in post optimal solution found. Do you think it is worth looking at the spherical model with a possible additional tissue representing the conductive paste compared to the one without the additional conductive paste tissue, to get a sense of the scale estimate?

Thanks,

Joel
________________________________
From: Axel Thielscher <axelt at drcmr.dk>
Sent: Wednesday, November 2, 2022 10:21 AM
To: Joel Upston <jupston at unm.edu>; discuss at simnibs.org <discuss at simnibs.org>
Subject: Re: [Simnibs-discuss] Differences in Efield depending on Processing

You don't often get email from axelt at drcmr.dk. Learn why this is important<https://aka.ms/LearnAboutSenderIdentification>

  [EXTERNAL]

Hi Joel,


a question for better understanding: When you set up the 2nd pathway, do you then model ALL electrodes of the EEG10-20 system, or only those which you actually need for the montage?


I am asking because adding many well-conducting electrodes to the model will slightly increase the amount of current shunted through the scalp(+electrodes). Think of it as covering many parts of the scalp with well-conductive paste. We never quantified the size of this effect, but it will cause the leadfield e-fields in the brain to be consistently lower.


Best regards,

Axel


On 19-10-2022 21:07, Joel Upston wrote:
Hi there,

I have noticed a systematic difference between the two different lines of simulating the same currents, electrodes and underlying dataset. One is through processing through the leadfield and using only the electrodes and currents as given and the other is through only modeling those electrodes and currents without the leadfield. Now the 2nd pathway (no leadfield) is giving higher E_mag than the leadfield version (~7-12%), the spatial maps look similar but just the intensity differences. I think it has to do with the scaling that is done after the solve by scaling the estimated current calibration error, which if I understand correctly isn't done in the leadfield processing.  If this is the case I am trying to establish which would be more accurate to use in your view if I need a specific target value of (0.3 V/m for example). I have attached an image of a case where this is shown with the leadfield processed is on the left and the non-leadfield is on the right. I have tried this on about 5 different datasets and the pattern is consistent.

Thanks,

Joel



_______________________________________________
Simnibs-discuss mailing list
Simnibs-discuss at drcmr.dk<mailto:Simnibs-discuss at drcmr.dk>
https://mailman.drcmr.dk/mailman/listinfo/simnibs-discuss


--
Dr. Axel Thielscher
Professor of Neurophysics and Neuroimaging
Danish Research Center for Magnetic Resonance
Copenhagen University Hospital Hvidovre
DK-2650 Hvidovre, Denmark
www.drcmr.dk<http://www.drcmr.dk>
&
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
http://www.healthtech.dtu.dk/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.drcmr.dk/pipermail/simnibs-discuss/attachments/20221115/6690cfd0/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Screenshot from 2022-11-15 11-55-21.png
Type: image/png
Size: 846900 bytes
Desc: Screenshot from 2022-11-15 11-55-21.png
URL: <https://mailman.drcmr.dk/pipermail/simnibs-discuss/attachments/20221115/6690cfd0/attachment-0001.png>