[FieldTrip] LCMV source localization

[Sushain Magotra]

Hello eveyone,

I am a research fellow working in the area of brain connectivity analysis
and its applicability in Deep Brain stimulation for Parkinsons patients.
So, for this I created a model of the human head using FieldTrip and then
simulated two dipoles inside this head model with thier predefined time
series and collected the synthetic EEG data.
Then, I created a source model to reconstruct these two dipoles again
inside the head model using LCMV inverse method.
But, the reconstrcuted dipoles are way too far from the original dipoles in
terms of their location.
I think that even if they are deviated from the original positions it
should be a little difference but my results show too much difference.
I am attaching my results obtained below and also providing the code that
I've used. Please let me know where am I going wrong.

% READ MRI
mri = ft_read_mri('Subject01.mri');

%SEGMENTATION
cfg           = [];
cfg.output    = {'gray','white','csf','skull','scalp'};
segmentedmri  = ft_volumesegment(cfg, mri);

%MESH GENERATION
cfg        = [];
cfg.shift  = 0.3;
cfg.method = 'hexahedral';
mesh = ft_prepare_mesh(cfg,segmentedmri);

% HEAD MODEL CREATION
cfg        = [];
cfg.method ='simbio';
cfg.conductivity = [0.33 0.14 1.79 0.01 0.43];
vol        = ft_prepare_headmodel(cfg, mesh);

%EEG SENSOR ALIGNMENT
elec = ft_read_sens('standard_1020.elc');
cfg          = [];
cfg.method   = 'interactive';
cfg.elec     = elec;
cfg.headshape = vol;
elec_aligned = ft_electroderealign(cfg);

% DIPOLE SIMULATION
cfg      = [];
cfg.vol  = vol;
cfg.elec = elec_aligned;
cfg.dip.pos = [
   28.5 -5.5 50.5        % dipole 1
  26.5 14.5 41.5         % dipole 2
   ];
cfg.dip.mom = ...       % the vector represents [qx1 qy1 qz1 qx2 qy2 qz2]
  [ 1 0 0 0 0 0 ]' + ...% this is how signal1 contributes to the 6 dipole
components
  [ 0 0 0 1 0 0 ]';     % this is how signal2 contributes to the 6 dipole
components

time = (0:199)/200;     % manually create a time axis
signal1 = data.trial{1, 1}(1,:);
signal2 = data.trial{1, 1}(3,:);
cfg.dip.signal = {[signal1; signal2]}; % one trial only
cfg.fsample = 200;                     % Hz
raw = ft_dipolesimulation(cfg);
avg = ft_timelockanalysis([], raw);

%SOURCE MODEL
ftpath   = 'C:\Users\sushain\fieldtrip-20180129';
load(fullfile(ftpath, 'template/sourcemodel/standard_sourcemodel3d10mm'));
template_grid = sourcemodel;
clear sourcemodel;

cfg                = [];
cfg.grid.warpmni   = 'yes';
cfg.grid.template  = template_grid;
cfg.grid.nonlinear = 'yes';
cfg.mri            = mri;
cfg.grid.unit      ='mm';
grid               = ft_prepare_sourcemodel(cfg);

%LEADFIELD CREATION
cfg                  = [];
cfg.elec             = elec_aligned;  % gradiometer distances
cfg.headmodel        = vol;   % volume conduction headmodel
cfg.grid             = grid;  % normalized grid positions
cfg.normalize        = 'yes'; % to remove depth bias
lf                   = ft_prepare_leadfield(cfg);

%LCMV SOURCE RECONSTRUCTION

cfg                  = [];
cfg.method           = 'lcmv';
cfg.grid             = lf; % leadfield, which has the grid information
cfg.vol              = vol; % volume conduction model (headmodel)
cfg.keepfilter       = 'yes';
cfg.lcmv.fixedori    = 'yes';
lcmv_reconstructed_sources   = ft_sourceanalysis(cfg, avg);

In the LCMV results I plotted the two dipoles showing the maximum
power at two grid locations.Note that the originally placed dipoles
are red and the reconstructed dipoles are blue in colour.
But the results show that these reconstructed dipoles are really
deviated from their original locations.

It would be really helpful for me if I could get some suggestions on
what could I possibly be doing wrong.

Thanking you.

Kind Regards,

Sushain Magotra
Research Fellow IRTG ELAINE
University of Rostock
Institute of General Electrical Engineering
Albert-Einstein-Str. 2
D-18059 Rostcok
Germany
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