Lead Field

[Pádraig Kitterick]

The method implemented in meg_leadfield1.m gives the same result as the
equations of Sarvas. It's just a different formulation of the forward
problem. I've produced identical leadfields calculated with several
different packages, including fieldtrip, and also a direct
implementation of the Sarvas equations.

It is possible that the CTF software is accounting for the coil loop
radii and other machine-specific inputs to the calculations, but these
rarely change the fields by large amounts. Are you using the same sphere
to approximate the head in both instances? The meg_leadfield1.m code
assumes that the centre of the single sphere is at the origin, (0,0,0).
If the centre of the sphere isn't at the origin, then you need to adjust
the dipole and sensor locations by subtracting the centre of the sphere
from their coordinates. The CTF system could well be doing this
adjustment using a pre-programmed sphere centre based on either a
general estimation or coregistration information it has available to it.
Unless you can access all the information that it is using to do the
calculations, it will be very hard to get identical answers.

The best thing is to ensure that you have a good estimate of the centre
of the single sphere for your sensor array based on an average
head/brain or a particular subject you are trying to model. Then your
fields will be as accurate, despite not being identical in absolute
numerical values to those produced by the CTF software.

Padraig

Cristiano Micheli wrote:
> Hi Pádraig
> Thank you for the answer. It helped to fix the scaling factor. Now i get a
> magnetic field of order of ~1 mT for a 10nA.m dipole ~3 cm far from the pick
> up coils (with Fieldtrip routine).
> Still i cannot get the same field as for the CTF software.
> In CTF leadfield i get [min max] values of: 1e-06 *[ -67.1 100.4] Tesla (275
> channels), while in Fieldtrip i get: [-0.0011 0.0016] Tesla.
> I think the two routines use different equations. CTF implements Sarvas'
> forward model for a spherical conductor in a homogeneus medium ("Basic
> mathematical and electromagnetic concepts of the biomagnetic inverse
> problem", Sarvas J, 1987).
> I cannot get to the source of Fieldtrip's method for the forward model,
> which is mentioned in meg_leadfield1.m routine head comment (adapted from
> Luetkenhoener, Habilschrift '92). It sounds like an habilitation work.
> I found from Lütkenhöner :
> "Möglichkeiten und Grenzen der neuromagnetischen Quellenanalyse", Münster:
> LIT-Verlag, 1992
> Unfortunately i do not have access neither to the book nor to formulas.
> How can i check it?
> If it does not depend on implementation details (which sounds also plausible
> since the error is quite high), what can be again the problem to make the
> two fields match?
> Thank you
> Cristiano
>
> ----------------------------------
> 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/fcdonders/fieldtrip.
>
>
>

--
Pádraig Kitterick
Graduate Student
Department of Psychology
University of York
Heslington
York YO10 5DD
UK

Tel: +44 (0) 1904 43 3170
Email: p.kitterick at psych.york.ac.uk

----------------------------------
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/fcdonders/fieldtrip.