Lead Field

Cristiano Micheli michelic72 at GMAIL.COM
Mon Jun 23 15:59:20 CEST 2008

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

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