[FieldTrip] MEG lead field units

[Marc Lalancette]

Hello Fieldtrippies,

Regarding a question I asked some months ago about the physical units of the lead field in Fieldtrip, I wanted to share the fact that I was wrong, and that it might be good to either document this in the code and possibly to add a factor.  When using 'cm' units, the lead field is in Wb/cm^2/(A*m) = 1e4 T/Am = 1e2 T/(A*cm).  I started to realize my original assumption was wrong while working on my Biomag poster since I was getting very low SNR values (around 1) for typical source and system parameters.  So I looked closer at the single sphere code and my own old spherical forward solution code, and here is where the extra factor comes in:

Skipping to the last step, my formula is:
B = UnitConversion * (1e-7) *  ( (QxrQ/F - (QxrQ * rv') * GradF/F^2) * SensorOrient' );
The second factor is the constant mu0/(4*pi) = 10^-7 Wb/Am.
Assuming a source (Q) in units of Am and distances in cm, the complicated bit has units of Am*cm/cm^3.
Thus without the first factor, B would have units of Wb/cm^2 = 1e4 T, and so I was using UnitConversion = 1e4 in my code, to get B in Tesla.  If we don't put in a specific source amplitude (as in Fieldtrip), and still using UnitConversion, then B would be in T/(Am).

I guess units can be different than 'cm' though, so the factor would have to take that into account.  Note that the single shell forward solution gives the same units as single sphere and would need the same factor.  I haven't looked at other forward methods.  I also haven't verified if this is correctly dealt with when obtaining reconstructed source amplitudes, say with a beamformer.

Cheers,
Marc Lalancette
Lab Research Project Manager
The Hospital for Sick Children, Toronto, Canada




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