SCIRun User Mailing List

## Re: [SCIRUN-USERS] Total current going through the TDCS Mickey Mouse model

• From: Jess Tate <jess@sci.utah.edu>
• To: Tomasz Moszkowski <tmoszkowski@gmail.com>
• Cc: "scirun-users@sci.utah.edu" <scirun-users@sci.utah.edu>
• Subject: Re: [SCIRUN-USERS] Total current going through the TDCS Mickey Mouse model
• Date: Fri, 16 Sep 2016 14:58:52 -0600

Hi Tomasz,

The method that you described will give you an estimate of the total current assuming each step is done correctly.  However, it is subject to numerical errors and the resolution of the mesh of the model as well as the plane will make some difference in this estimation.  You will need to make sure that the plane separates all sources from all sinks and it is best if it is perpendicular to the current direction (or that the vector normal is parallel).  I think that you were talking about the plane normal vector that is parallel, which is correct, but if the plane itself is coincident or parallel with the electrodes, that is a problem.  One thing that you should also consider when comparing the estimated current the applied current is the electrode impedance, which is modeled in the TDCS example and is set as a parameter in one of the CreateMatrix modules (in SCIRun 4.7).

How different are the values that you are getting from what you are expecting?

cheers,
Jess

On Sep 16, 2016, at 2:33 AM, Tomasz Moszkowski <tmoszkowski@gmail.com> wrote:

Hello everyone,

I would like to ask you for help. I would like to validate a model that I am working on by calculating the current that is flowing through a simple model. For this purpose I use the Mickey Mouse TDCS model supplied in SCIRun 4.7. I would like to confirm that the flowing current that I measure equals the values given in the settings of the model (the RHS of the linear set of equations). My procedure:
1. Calculate the current density within the model (nothing out of the ordinary here: -gradPhi*sigma)
2. Map the normal component of the current density onto a plane that is a cross-section of the entire model. To simplify the problem I just picked a vertical plane parallel to both electrodes.
3. Multiply the normal component of the current density from the plane by the areas of the elements of the plane to get the flowing current per plane element
4. Sum all such "unit" currents to get the total current flowing between the two electrodes.

As you can imagine, the values that I get by doing this are off from the values set to the input of the model and it is not just a matter of scaling. When I multiply the solution matrix by the TDCS matrix, the RHS that I get equals the one that I set into the model, so here everything is fine. I tried to study the maths behind the TDCS model from:

[1] P. Hua, Eung Je Woo, J. G. Webster, and W. J. Tompkins, “Finite element modeling of electrode-skin contact impedance in electrical impedance tomography,” IEEE Trans. Biomed. Eng., vol. 40, no. 4, pp. 335–343, 1993.

Current shunting along the electrodes was mentioned in the paper. Could it be that some current set into the electrodes simply does not get through into the model? Or am I making some sort of a mistake that has been oblivious to me so far?

I would really appreciate any help!

Kind regards,
Tomasz Moszkowski

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