[FieldTrip] Magnetic dipole fit vs Equiv. Current dipole fit

Robert Oostenveld r.oostenveld at donders.ru.nl
Tue Mar 3 21:54:55 CET 2015

Hi Jim

You can do an inverse solution with any method (including dipole fitting) by specifying cfg.vol=[] in the corresponding high-level function (e.g. ft_prepare_leadfield, ft_dipolefitting or ft_sourceanalysis). This causes the forward model to be computed with a magnetic dipole.

If you want to track this down to the lower lever code, have a look at these lines in the code

“forward/ft_voltype.m" line 115 of 138 --83%-- col 11
“forward/ft_compute_leadfield.m" line 280 of 611 --45%-- col 12

My appologies for this not being documented better. I have been using this myself to check on the localization of the headcoils in the CTF “hz.ds” datasets, which worked fine.

I hope you can get it to work for the Sandia Labs system. For the CTF system it is not needed to do these computations in MATLAB, since the CTF electronics does this in (alomst) real-time and the positions are streamed along with the MEG channel data. That makes this this http://fieldtrip.fcdonders.nl/getting_started/realtime_headlocalizer easy for the system we have in Nijmegen. Arjen and I have also been working on making it work for the Elekta system where the fitting has to be done in MATLAB, but have not been able sofar to get it to work robustly. You can find the experiences and (still open) report at http://bugzilla.fcdonders.nl/show_bug.cgi?id=1792. 

best regards

On 25 Feb 2015, at 22:40, Jim McKay <jim.mckay at candoosys.com> wrote:

> Hello Fieldtrippers,
> I am consulting with the Sandia Labs on development of an atomic magnetometer based MEG system prototype. One of the areas I am working on is head localization, so I was looking at the code for the realtime head localization in Fieldtrip. I was surprised to see that although the comments talk about using a magnetic dipole forward solution, it actually used the FT dipolefit code which is based on an equivalent current dipole, as far as I can tell.
> There should be a significant difference in the forward solutions between MD and ECD, so how does this work? Or am I just missing something?
> Cheers,
> Jim
> -- 
> Jim McKay
> Candoo Systems Inc. - Magnetic field sensors, systems, and site surveys
> Tel. 778-840-0361
> jim.mckay at candoosys.com
> www.candoosys.com
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> fieldtrip mailing list
> fieldtrip at donders.ru.nl
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