Digitized EEG position and projection on scalp

EmmanuelleKris · December 15, 2021, 2:07pm

We are building our own digitilization system based on move capture system with a marker on each electrod. Our recorded positions correspond to those of the marker on the electrode.
When aligning with MRI, how the projection of EEG position on scalp is made by bst? Does the brainstorm projection need to know the thickness of the electrode for example?

Francois · December 15, 2021, 4:42pm

When aligning with MRI, how the projection of EEG position on scalp is made by bst?

Radial projection of the electrodes on the scalp surface with respect to the center of mass of the head surface (mean of all the vertices [x,y,z]).

github.com

brainstorm-tools/brainstorm3/blob/master/toolbox/sensors/channel_project_scalp.m

function ChanLoc = channel_project_scalp(Vertices, ChanLoc)
% CHANNEL_PROJECT_SCALP: Project EEG electrodes on a scalp surface, 
% radially from the center of mass of the scalp surface points.
% 
% INPUT:
%     - Vertices : [Mx3] positions of the scalp vertices
%     - ChanLoc  : [Nx3] positions of the EEG electrodes

% @=============================================================================
% This function is part of the Brainstorm software:
% https://neuroimage.usc.edu/brainstorm
% 
% Copyright (c) University of Southern California & McGill University
% This software is distributed under the terms of the GNU General Public License
% as published by the Free Software Foundation. Further details on the GPLv3
% license can be found at http://www.gnu.org/copyleft/gpl.html.
% 
% FOR RESEARCH PURPOSES ONLY. THE SOFTWARE IS PROVIDED "AS IS," AND THE
% UNIVERSITY OF SOUTHERN CALIFORNIA AND ITS COLLABORATORS DO NOT MAKE ANY
% WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO WARRANTIES OF

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Does the brainstorm projection need to know the thickness of the electrode for example?

The positions that must be indicated in the channel file is the contact point between the electrode and the scalp. Therefore, if you set in the channel file the position of the top of an electrode that has a pedestal (eg. the EGI hydocell nets), you must project them on the head surface.

EmmanuelleKris · December 16, 2021, 7:50am

Hi

I don't perfectly understand that and how I sould indicate this in the channel file, please?

Francois · December 16, 2021, 11:13am

I don't perfectly understand that and how I sould indicate this in the channel file, please?

When you digitize your electrodes 3D coordinates, you may get positions that are 0.5-1cm away from the scalp.
Import this as your electrodes coordinates in the Brainstorm database (channel file), and then project the electrodes on the scalp with Brainstorm.