[Simnibs-discuss] Difference in Electric field by optimization and simulation

[Axel Thielscher]

Hi Steven,

when calculating the leadfields that form the input of the optimization 
algorithm, electrodes are added to the mesh at all positions defined by 
the EEG cap layout. For large electrodes, neighboring electrodes will 
start to overlap and create a continuous well-conductive layer covering 
major parts of the scalp. When simulating TDCS current flow in a head 
model modified like that, most of the currents will then just be shunted 
within the well-conductive "electrode layer" and not enter the head nor 
brain. In other words, leadfield-based optimization algorithms are 
designed to work with small electrodes.

Best regards,

Axel


On 21-03-2023 15:54, Beumer, Steven wrote:
>
> Dear SimNibs community,
>
> I was trying to verify the electric field obtained by an optimization 
> with regular simulations.
>
> For a cortical target and 2 4*4cm patches, the resulting locations and 
> currents were fed into a regular simulation and interpolated to the 
> cortical surface.
>
> The obtained magnitude of the electric field differs by approximately 
> a factor of two and I can’t seem to figure out the reason for that.
>
> In the figure below the optimization and simulation are shown with the 
> Electric field plotted on the same scale.
>
> It might be a reasoning error somewhere, but if someone could help me 
> with that, I would be grateful.
>
> Sincerely,
>
> Steven Beumer MSc
>
> TUe-logo-descriptor-line-scarlet-rgb
>
> PhD candidate at the Electrical Engineering faculty
>
> PS: For some reason, this issue doesn’t show up with small circular 
> electrodes
>
>
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> Simnibs-discuss at drcmr.dk
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-- 
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
&
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
http://www.healthtech.dtu.dk/
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