Plotiing the sensors

abdallahqusaibe · June 22, 2020, 2:15pm

Dear All,

When diplaying the senors of vectorview, we saw:

2 green rectangle separated
2 red rectangle separated
1 yellow square

1)Why it is like that even there is 4 integration point (for location) for each? how this shape is drawing? it is related to the integration points? how?

computing the lead field it consider 1 point for each sensor, how this 1 point is taken among the 4 integration points?

Thanks for your answer
Abdallah

SBeumer · June 23, 2020, 8:24am

Hi Abdallah,

What you see in vectorview are representations of two gradient meters, in both planar directions, green and red and a magnitude mater in yellow. This is actually close to how a MEG sensor is in practice. I would suggest you to have a look at http://ilabs.washington.edu/what-magnetoencephalography-meg and then squid sensor.

The integration points are inside the perimeter of the sensors , though as far as I know not related to the drawing. They are there to sample the calculated magnetic field better than just using one point.

The leadfield computes just 1 point, because eventually you will get one voltage out of the sensor. The integration points are to more or less "average" the field to get a better representation of what would happen in reality.

A more thorough explanation on this topic is given in : Computing the vector field B on the MEG sensors (forward problem)

Kind regards,
Steven

Francois · June 23, 2020, 10:02am

When importing Elekta vectorview recordings, the geometry of the sensors (described in the file coil_def.dat, search the forum for more help) is added to the channel file: 4 points for the magnetometers, 8 points for the gradiometers.
https://neuroimage.usc.edu/brainstorm/Tutorials/ChannelFile#Edit_the_channel_file

These 4 or 8 points from the .Loc field of the channel file are used for displaying the sensors (green/yellow/red rectangles):

github.com

brainstorm-tools/brainstorm3/blob/master/toolbox/gui/figure_3d.m#L4038


            oriLin = [oriLoc; oriLoc + oriDir ./ norm(oriDir) .* oriLength];

            line(oriLin(:,1), oriLin(:,2), oriLin(:,3), 'Color', [.8 .8 0], oriOpt{:});

            % Channel label

            % txtLoc = oriLoc + 1.5 * oriLength * oriDir ./ norm(oriDir);

            % text(txtLoc(1), txtLoc(2), txtLoc(3), Channels(i).Name, 'Color', [.8 .8 0], textOpt{:});

        end

    end

    

    

% ===== ELEKTA-NEUROMAG =====

case 'vectorview306'

    

    % === ELEKTA: MAGNETOMETER ===

    % Plot only the magnetometer because they are all on the same chip

    if strcmpi(Channels(i).Type, 'MEG MAG') && (nPoints == 4)

        oriLength = 0.015;

        chLoc = Channels(i).Loc(:,[1 2 4 3])' .* 1.00;

        % Square of integration points is 2x smaller than the actual sensor: simply scale it

        coilScaleF=0.5;

        chLoc = [(1+2*coilScaleF)*chLoc(1,:) - coilScaleF*chLoc(2,:) - coilScaleF*chLoc(4,:);

                 (1+2*coilScaleF)*chLoc(2,:) - coilScaleF*chLoc(1,:) - coilScaleF*chLoc(3,:);

When computing the forward model: the leadfield is estimated for each of the 4 or 8 integration points, and then the Weights (last field of the channel file) are applied to compute the final single value you obtain in the Gain matrix of the headmodel file.