[Simnibs-discuss] Targeting subject specific locations on the scalp

[Guilherme B Saturnino]

Hello,


SimNIBS 3 offers two ways of getting 10/10 EEG positions out of the box.


1) Use the eeg positions calculated using MNI transformations, which are 
stored in m2m_{SubID}/eeg_positions/EEG10-10_UI_Jurak_2007.csv. Those 
can also be accessed in the GUI 
<https://simnibs.github.io/simnibs/build/html/tutorial/gui.html>, which 
loads it automatically, or in  scripts (MATLAB example 
<https://github.com/simnibs/simnibs/blob/v3.1.2/simnibs/examples/simulations/TMS.m>)


2) Using the eeg_positions 
<https://simnibs.github.io/simnibs/build/html/documentation/command_line/eeg_positions.html> 
command line tool with the Nz, LPA, RPA and Iz as an input


Those two were compared and validated here 
<https://link.springer.com/chapter/10.1007/978-3-030-21293-3_1>.


I'm not sure why it is not working in your case though, might be due to 
head size, as you are moving by constant amounts?


Best Regards,

Guilherme



On 09/07/2020 19.48, Mccalley, Daniel wrote:
> Hi all,
>
> Our lab targets the FP1 with TMS. We use a simple measurement system 
> (http://clinicalresearcher.org/F3/) to locate exactly where we want to 
> place the coil for each subject. Typically, we measure the subject's 
> head, and then, starting from nasion, we'll move up Y mm and left X mm 
> (where X and Y are determined from each subject's given head size).
>
> Ideally, we'd like to create an electric field model for each subject 
> that accounts for the TMS parameters as well as the exact location 
> used. To implement this in SIMNIBS, I've tried the following:
>
> 1) Open a subjects m2m file and find the coordinates for nasion (say, 
> for example, that these coordinates are X= -2.78, Y= 117.59, Z= -20.63).
>
> 2) Determine Y and X used to locate FP1 for the given subject (say, 
> for example, that starting from nasion, we moved 37 mm in the Y 
> direction, and over 32 mm in the X direction).
>
> 3) Subtract the displacements in the Y and X directions from the 
> original nasion coordinates, resulting in X= -34.78 and Y=80.69
>
> 4) Add a new position wherein the reference coordinates match the 
> original nasion coordinates (and the Define Reference Coordinates box 
> is checked) and the coordinates calculated in step 3 are input in the 
> position fields (Z is left blank for SIMNIBs to calculate)
>
> 5) Adjust for coil orientation and TMS parameters
>
> This seems to have worked well for 90-95% of our subjects, however 
> there are a few subjects in which the resulting coil placement appears 
> to be over the eye (Note: Ordinarily I'd attach examples of the 
> working and not-working subjects, but I'm not sure if the skin 
> reconstruction of our participants falls under PHI).
>
> Am I going about this the wrong way? Could this reflect a problem with 
> segmentation/meshing? Or maybe a problem with the experimenter 
> measurements of the subjects head? Any advice would be greatly 
> appreciated. Thank you all so much!
>
> Best,
>
> Dan McCalley
> Medical University of South Carolina
> College of Graduate Studies, Neuroscience
>
>
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