Lead Field

[Pádraig Kitterick]

Hi Cristiano,

I'll attempt to help you with this but I'd also appreciate some input on
this topic from others on the list to check my thinking is correct.

I think your strange results are due to the mixing of several different
units of measurement. If the positions of sensor and source are
expressed in metres and the dipole moment in A-m (i.e. all in standard
units), then the field strengths due to the source as calculated by
compute_leadfield.m will be in Tesla. Of course, you can use cm or mm
for your scale but that will just linearly scale the field values, i.e.
converting the distances from metres to cm (increase of 10^2) will
_decrease_ the field values (reduction of 10^2) if the same moment value
is specified in both calculations. Similarly, expressing the moment in
units of nA-m will also scale the field values linearly, but in that
case it will increase the fields by 10^9 relative to the same moment
expressed in A-m.

For your example the use of cm will decrease the field values by 10^2
and specifiying the dipole moment in nA-m will increase it by 10^9 - all
this is relative to standard units. Therefore, if interpreted as Teslas,
your fields are probably too large by a factor of 10^7 for your desired
source stregth of 10nA-m. You could standardise your units by
multiplying the resultant fields by 1e-7 or by using the following when
you compute them:

dip_mom = [1e-8 0 0]; % 10 nA-m as A-m

but presumably as your sensor positions and orientations are in cm so
you could either convert them and the dipole position to metres
beforehand, or otherwise if you use the above moment value you will
still have to multiply the resulting fields by 10^2 to compensate for
the cm scaling, which should give you reasonable field strengths in Tesla.

Hope that helps and does not confuse,

Padraig

Cristiano Micheli wrote:
> Hi everybody
> I tested the leadfield routine (compute_leadfield.m) for current dipole in
> MEG forward solution.
> How are the units expressed?
> I expect the current dipole to be expressed in nA.m according to CTF
> convention (i am using a 275 channels MEG CTF system), the gradiometers'
> positions in cm and the lead field in Tesla.
> Nevertheless i compared it with CTF software and there is quite a high
> mismatch in the scaling factor, and the field is not perfectly distributed
> as in CTF software forward solution.
> In the Fieldtrip documentation it is mentioned a leadfield computation
> method from Lütkenhöner, Habilschrift '92 which i could not find. How do i
> get to the article?
> I attach the code:
>
> dip_pos = [2 2 10]; % cm
> dip_mom = [10 0 0]; % 10 nA*m
> % grad: gradiometers structure
> % vol:  conductive sphere model
> lf = compute_leadfield(dip_pos, grad, vol, 'singlesphere','yes')*dip_mom';
>
> ----------------------------------
> 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.