[FieldTrip] Lead Field Units in LCMV Beamformer

[Jasen Devasagayam]

Hello Everyone,

I ran into some confusion when looking at LCMV beamformed data, using the function, "ft_sourceanalysis". We are using a single shell forward model in these analyses. When looking at the output struct from this function, the lead fields were on the scale of 1e-10 T,  which seems much too large for MEG fields. Looking into FieldTrip archives, I found a similar question by Marc Lalencette from 2014 (https://mailman.science.ru.nl/pipermail/fieldtrip/2014-September/008438.html) where it looks like the issue stems from the fact that fields are computed with respect to 'cm'.

I applied a conversion factor, by multiplying all data by 1e-4, to get from 'cm^2' to 'm^2'. I applied this conversion factor in the function "ft_compute_leadfield" in Line 260:
      lf = lf*headmodel.forwpar.scale^2*1e-4;

I had originally tried to just adjust the scaling factor, but making that one small change severely altered the data in multiple places in our pipeline, which in turn yielded lead fields on the scale of 1e-10 once again. In this case, the scaling factor is 1.

I was wondering if this would be the correct place and method to implement this modification. If not, would you have any suggestions on how to get sensible lead field values?

Thank you.



[cid:202c3e90-7ada-4ef9-9785-989c31436ce8]

Jasen Devasagayam, MASc

Research MEG Technologist

Neurosciences and Mental Health



The Hospital for Sick Children

686 Bay St., Toronto, ON M5G 0A4

P 416-813-7654 ext. 201535

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