megplanar for magnetometers

[jan-mathijs schoffelen]

Dear Stephan,

I would advice you to use cfg.method = 'sincos'. As a matter of fact,  
the 'orig' method has been conceived indeed for axial gradient data,  
but I believe the info about the top coil is only used to temporarily  
generate a local z-axis throught the gradiometer, to define the local  
X and Y coordinate frame for the planar gradient. Using the sincos- 
method the XY coordinate frame is different, but apart from that, the  
planar gradient field interpolation works with the same logic,  
estimating a gradient in an X and Y direction based on an  
interpolation of the gradients between the magnetic field  
(magnetometers) or axial gradient (gradiometers) at a given sensor  
and its neighbours.

Yours,

Jan-Mathijs



On May 19, 2009, at 1:28 PM, Stephan Moratti wrote:

> Hi all,
>
> I would like to transform my magnetometer maps to a  
> planargradiometer representation (with BESA I can do that, but it  
> would be nice to have it in fieldtrip). I guess, simply using  
> megplanar and combineplanar as it is, would be incorrect, as this  
> functions assume axial gradiameters. So, I tried to change some  
> code of megplanar (lines 281-287, method 'orig') simply assuming  
> that X2 Y2 and Z2 are equal to X,Y,Z in the case of magnetometers:
>
> % get the positions of bottom and top coil
>  X  = grad.pnt(:, 1);
>  Y  = grad.pnt(:, 2);
>  Z  = grad.pnt(:, 3);
>  X2 = grad.pnt(:, 1);% + grad.ori(:,1);
>  Y2 = grad.pnt(:, 2);% + grad.ori(:,2);
>  Z2 = grad.pnt(:, 3);% + grad.ori(:,3);
>
> Well, probably this is quite naive. Attached please find my results  
> (testplanar.tiff). testmagneto.tiff contains the original magnetic  
> flux map (magentometers). Can anybody figure out if this makes sense?
>
> Thanks for any comments and help,
>
> Stephan
>
>
> -- 
> *Stephan Moratti, PhD/
> /**/see also: http://web.me.com/smoratti/
> /*Centro de Tecnología Biomédica CBT,
> Universidad Politécnica de Madrid,
>
> en la actualidad (currently at) en el
> Centro de Magnetoencefalografía Dr. Perez Modrego,
> Universidad Complutense de Madrid,
> Faculdad de Medicina,
> Pabellón 8,
> Avda. Complutense, s/n,
> 28040 Madrid,
> Spain,
>
> email: moratti at gbt.tfo.upm.es <mailto:moratti at gbt.tfo.upm.es>
> moratti at med.ucm.es <mailto:moratti at med.ucm.es>
> Tel.:    +34 91 394 2292
> Fax.:   +34 91 394 2294
> */
> /*
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users  
> of the FieldTrip  toolbox, to share experiences and to discuss new  
> ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/ 
> archives/fieldtrip.html and http://www.ru.nl/neuroimaging/ 
> fieldtrip.<testplanar.tif><testmagneto.tif>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.