Optical Co-registration¶

When we do not have an MRI for a participant we can register the MNI template scalp to the participant’s head shape. We will then register the head shape to the helmet and subsequently to the sensor positions.

Reading the data¶

We first start by reading the data as we normally would.

%- read data 
%--------------------------------------------------------------------------
S = [];
S.data ='OPM_meg_001.cMEG';
S.positions= 'OPM_HelmConfig.tsv';
D = spm_opm_create(S);

Reading and marking the meshes¶

We now must mark a number of fiducial points on our meshes. We will need to mark

3 points on the helmet file (which contains sensor positions)
3 corresponding points on the mesh with the head and helmet present
3 points on the head shape mesh
3 point on the corresponding mesh file with the head and helmet present

Optical co-registration¶

The optical co-registration requires that the head shape file S.headfile be provided in any format that the gifti function supports as well as the 4 sets of fiducial points. Note that the fiducial points are provided to the functions in row order.

S=[];
S.D = D;
S.headfile  ='head.obj';

 S.helmetref= ...
[0,133.9,-24;...     %point 1 
-114,30.5,-61;...    %point 2
116,29,-59];         %point 3

S.headhelmetref = ...
[77,-23,608;...    %point 1
31,90,715;...      %point 2
26,-141,692];      %point 3

S.headfid = ...
[111.626,-9,504;... % nas
97, 54, 551;...     % lpa
84, -67, 548];      % rpa 

S.headhelmetfid  = ... 
[33,-22.6,654;...   % nas
14.7, 38.9, 699;... % lpa
5,-82,693];         %rpa

cD = spm_opm_opreg(S);

If the co-registration works we should get an image of a brain within the sensor array like the one below