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<p>Joel,</p>
<p><br>
</p>
<p>when you have small electrodes as is usually the case for
multi-electrode montages, a simple solution is to lower the
conductivity of the modelled electrode materials. This will not
affect the field distribution in the brain, but reduce the amount
of shunting when you place many electrodes on the head.</p>
<p>This solution will be fine for small electrodes, where it is not
important that the modelled electrode properties correspond well
to the ones used in the real experiment.<br>
</p>
<p><br>
</p>
<p>Best regards,</p>
<p>Axel</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 15-11-2022 20:33, Joel Upston wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CY1PR07MB2714C39B3348383DA106608EBF049@CY1PR07MB2714.namprd07.prod.outlook.com">
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255, 255);" class="elementToProof">
Axel,</div>
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<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
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. </div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
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.
<span style="background-color:rgb(255, 255, 255);display:inline
!important" class="ContentPasted0">
Where now the fields look very similar between the two
different pipelines. </span>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.</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
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<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
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? </div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
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<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
Thanks,</div>
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<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0); background-color: rgb(255,
255, 255);" class="elementToProof">
Joel<br>
</div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>From:</b> Axel
Thielscher <a class="moz-txt-link-rfc2396E" href="mailto:axelt@drcmr.dk"><axelt@drcmr.dk></a><br>
<b>Sent:</b> Wednesday, November 2, 2022 10:21 AM<br>
<b>To:</b> Joel Upston <a class="moz-txt-link-rfc2396E" href="mailto:jupston@unm.edu"><jupston@unm.edu></a>;
<a class="moz-txt-link-abbreviated" href="mailto:discuss@simnibs.org">discuss@simnibs.org</a> <a class="moz-txt-link-rfc2396E" href="mailto:discuss@simnibs.org"><discuss@simnibs.org></a><br>
<b>Subject:</b> Re: [Simnibs-discuss] Differences in Efield
depending on Processing</font>
<div> </div>
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<p class="x_MsoNormal" style="line-height:12.0pt;
background:#FF6666"><b><span style="font-size:10.0pt;
color:white"> [EXTERNAL]</span></b></p>
<b>
</b>
<div>
<p>Hi Joel,</p>
<p><br>
</p>
<p>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?</p>
<p><br>
</p>
<p>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.<br>
</p>
<p><br>
</p>
<p>Best regards,</p>
<p>Axel</p>
<p><br>
</p>
<div class="x_moz-cite-prefix">On 19-10-2022 21:07, Joel
Upston wrote:<br>
</div>
<blockquote type="cite">
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Hi there,</div>
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<br>
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<div class="x_elementToProof"
style="font-family:Calibri,Arial,Helvetica,sans-serif;
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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 2<span><sup>nd</sup> 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.</span></div>
<div class="x_elementToProof"
style="font-family:Calibri,Arial,Helvetica,sans-serif;
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<span><br>
</span></div>
<div class="x_elementToProof"
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt; color:rgb(0,0,0)">
<span>Thanks,</span></div>
<div class="x_elementToProof"
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt; color:rgb(0,0,0)">
<span><br>
</span></div>
<div class="x_elementToProof"
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt; color:rgb(0,0,0)">
Joel<span><br>
</span></div>
<br>
<fieldset class="x_moz-mime-attachment-header"></fieldset>
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</pre>
</blockquote>
<pre class="x_moz-signature" cols="72">--
Dr. Axel Thielscher
Professor of Neurophysics and Neuroimaging
Danish Research Center for Magnetic Resonance
Copenhagen University Hospital Hvidovre
DK-2650 Hvidovre, Denmark
<a class="x_moz-txt-link-abbreviated" href="http://www.drcmr.dk" moz-do-not-send="true">www.drcmr.dk</a>
&
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
<a class="x_moz-txt-link-freetext moz-txt-link-freetext" href="http://www.healthtech.dtu.dk/" moz-do-not-send="true">http://www.healthtech.dtu.dk/</a></pre>
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<pre class="moz-signature" cols="72">--
Dr. Axel Thielscher
Professor of Neurophysics and Neuroimaging
Danish Research Center for Magnetic Resonance
Copenhagen University Hospital Hvidovre
DK-2650 Hvidovre, Denmark
<a class="moz-txt-link-abbreviated" href="http://www.drcmr.dk">www.drcmr.dk</a>
&
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
<a class="moz-txt-link-freetext" href="http://www.healthtech.dtu.dk/">http://www.healthtech.dtu.dk/</a></pre>
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