# [FieldTrip] Forward model for dipoles with fixed position and orientation

Leonardo Barbosa lsbarbosa at gmail.com
Tue Feb 1 03:05:59 CET 2011

Dear Fieldtrip users,

I'm trying to create a forward model for a fixed number of dipoles with
fixed positions and orientations (extracted from a MRI image using
freesurfer and a downsampling algorithm) so I can use it with my inverse
algorithm.
few examples<http://fieldtrip.fcdonders.nl/example/compute_forward_simulated_data_and_apply_a_dipole_fit>and
function
still didn't find exactly what I'm trying to do.

Is it possible to have as a result of *ft_compute_leadfield*,
*?,* ft_prepare_sourcemodel*?) a leadfield that depends only on the
*intensity* of the dipole? Because it has a fixed orientation, and the idea
is exactly reduce the number of unknowns. I noticed that SPM creates what
I'm trying to do (as copied below), but I'm trying to stay within fieldtrip
definitions so I can use it's inverse solutions to compare with mine,
although I only found examples for beamformers inverse solutions, and not
sparse methods. So this is the second question: is there any example of
source-space based sparse methods inverse solutions?

Thanks in advance for any help,

Best Regards
Leonardo Barbosa

Here is the SPM code (inside *spm_eeg_lgainmat*)

% Forward computation

%--------------------------------------------------------------------------
[vol, sens] = forwinv_prepare_vol_sens(vol, sens, 'channel',
forward(ind).channels);
nvert = size(vert, 1);

spm('Pointer', 'Watch');drawnow;
spm_progress_bar('Init', nvert, ['Computing ' modality '
if nvert > 100, Ibar = floor(linspace(1, nvert,100));
else Ibar = [1:nvert]; end

Gxyz = zeros(length(forward(ind).channels), 3*nvert);
for i = 1:nvert

Gxyz(:, (3*i- 2):(3*i))  = forwinv_compute_leadfield(vert(i, :),
sens, vol);

if ismember(i, Ibar)
spm_progress_bar('Set', i); drawnow;
end

end

spm_progress_bar('Clear');
spm_progress_bar('Init', nvert, ['Orienting ' modality '

G{ind} = zeros(size(Gxyz, 1), size(Gxyz, 2)/3);
for i = 1:nvert

G{ind}(:, i) = Gxyz(:, (3*i- 2):(3*i))*norm(i, :)';

if ismember(i, Ibar)
spm_progress_bar('Set', i); drawnow;
end

end

spm_progress_bar('Clear');
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