[Simnibs-discuss] Questions regarding electric fields and simulation

[Kristoffer Madsen]

I assume that you are referring to TMS stimulation - right?
It is a bit unclear to me what you mean by primary and secondary
electrical field. Sometimes this distinction is made as a theoretical
partitioning of the field due to the current in the coil, and charge
accumulation in the head due to change in conductivity. SimNIBS
calculates the effective electrical field based on the Poisson
equation where the dA/dt field (equivalent to the E-field in an
infinite homogeneous conductor with appropriate scaling) is used to
determine the RHS. In TMS eddy currents are effectively negligible so
these are ignored in the estimation which we typically base on a
dipole model (ccd file) or the A field determined in a volume (.nii.gz
file). I am unsure if simNIBS currently enables outputting the dA/dt
field directly (I think there was a request at some point but cant
remember if it was filled). Anyways you can easily determine it as it
is independent of the head model.

So in short I believe that simNIBS currently does not directly allow
this separation to be made, but it should be relatively easy to do it
using simNIBS with a bit of scripting (also for CSF). However, I am
still not very sure what information would really be gained from
having this separation of the field.
Best,
Kristoffer

On Mon, 9 Jan 2023 at 17:00, MOREAU Augustin (CEMNIS)
<augustin.moreau2 at chru-strasbourg.fr> wrote:
>
> Hello,
>
>
>
> I'm an engineer working currently at the CEMNIS at the Strasbourg University Hospitals (Hopitaux Universitaire de Strasbourg) and the University of Strasbourg. I've been using SimNIBS for several weeks now, and I have several questions concerning the simulations :
>
>
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> 1) Is the calculation during the simulation (and the results that we can see) takes into account secondary electric fields (in addition of the primary electric fields) ?
>
>
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>                 1.1) If yes, is it possible to see the results in the cerebrospinal fluid (to see how the fields propagate in this zone) ? Is it possible to make the separation between primary and secondary eletric fields and see the results in the CSF ?
>
>                 1.2) If no, do you know someone who did this or a reference in the litterature ? Does it have an interest to see the results of secondary electrid fields ?
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>
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> Thank you for your time.
>
>
>
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> Sincerely yours,
>
>
>
> Augustin Moreau.
>
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