Find the orientation of planar gradiometers
The Neuromag/Elekta/Megin Vectorview and Triux scanners have a combination of magnetometers and two planar gradiometers at each sensor location.
This is further detailed in the Neuromag/Elekta/MEGIN System Hardware Users’ Guide and on this page.
The following code demonstrates how you can determine the orientation of the two planar channels at each location.
grad = ft_read_sens(filename, 'senstype', 'meg');
sel = find(strcmp(grad.chantype, 'megplanar'));
for i=1:numel(sel)
j = sel(i);
clear coilindex*
coilindex1 = find(grad.tra(j,:)>0)
coilindex2 = find(grad.tra(j,:)<0)
pos(i,:) = (grad.coilpos(coilindex2,:) + grad.coilpos(coilindex1,:))/2;
ori(i,:) = grad.coilpos(coilindex2,:) - grad.coilpos(coilindex1,:);
ori(i,:) = ori(i,:)/norm(ori(i,:));
end
figure
quiver3(pos(:,1), pos(:,2), pos(:,3), ori(:,1), ori(:,2), ori(:,3))
axis equal
axis vis3d
