[FieldTrip] combining gradiometer/magnetometer sensors - neuromag data

Sarang S. Dalal sarang at cfin.au.dk
Wed May 10 10:59:39 CEST 2017

Dear Andria and Nicole,

Combining sensor types (gradiometers and magnetometers, or MEG and EEG) is indeed something that is not documented, and there is not yet a unanimous agreement in the community about how to do this best.

But, I can suggest one simple solution that was discussed at a recent beamformer workshop hosted by the Aston MEG Centre in February. (I believe John Mosher made the initial suggestion.)

As you probably know, the covariance matrix is an essential ingredient to constructing beamformer solutions. Different sensor types will have different scales and different noise properties, which then complicates the covariance between channels of different types.

So, one way around this is to zero out the ‘cross’ terms of the covariance matrix, i.e., set to 0 the elements of the covariance matrix that result from the product of a magnetometer channel and a gradiometer channel.  This can be accomplished with something like the following code:

timelock = ft_timelockanalysis(cfg, data);



In my experience, this allows source reconstruction using all channels, with results at least as good as analyzing the channels separately. This is not to say that is the best possible solution, but it is a simple one that should work well!

Best wishes,

On 10 May 2017, at 09:33, Klink-3, N.E.C. van <N.vanKlink-2 at umcutrecht.nl<mailto:N.vanKlink-2 at umcutrecht.nl>> wrote:

Hi Andria,

We have been struggling with the same problem, also using a 306-channel Neuromag system. We ended up correcting for the scale difference between magnetometer and gradiometer signals (i.e. noise 5e-15T vs 5e-13T/m). So we multiplied the signals of the magnetometers and the leadfield values of the magnetometers by a factor 100. This gave us the best result.

Kind regards,

Van: fieldtrip-bounces at science.ru.nl<mailto:fieldtrip-bounces at science.ru.nl> [mailto:fieldtrip-bounces at science.ru.nl] Namens Andria Pelentritou
Verzonden: 10 mei 17 1:52
Aan: fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>
Onderwerp: [FieldTrip] combining gradiometer/magnetometer sensors - neuromag data

Greetings from Melbourne!

We've been using fieldtrip (v-20160801) for a while now and we are having issues when it comes to combining magnetometer and gradiometer MEG sensors types during (LCMV) beamforming for the Elekta Neuromag Triux 306-channel system. Beamforming the magnetometers or gradiometers separately on auditory evoked data works well, but we cannot successfully combine the two sensor types. We've played with the cfg.coilaccuracy variable as advised in one of the tutorials with no luck as there seem to be no obvious effects on the output. The documentation provides very little information on the topic.

What strategies would you recommend when trying to combine the Neuromag 306-channel system magnetometer/gradiometer data?

Many thanks in advance for your help and many more thanks for releasing such a valuable toolbox!
Warm regards


De informatie opgenomen in dit bericht kan vertrouwelijk zijn en is uitsluitend bestemd voor de geadresseerde. Indien u dit bericht onterecht ontvangt, wordt u verzocht de inhoud niet te gebruiken en de afzender direct te informeren door het bericht te retourneren. Het Universitair Medisch Centrum Utrecht is een publiekrechtelijke rechtspersoon in de zin van de W.H.W. (Wet Hoger Onderwijs en Wetenschappelijk Onderzoek) en staat geregistreerd bij de Kamer van Koophandel voor Midden-Nederland onder nr. 30244197.

Denk s.v.p aan het milieu voor u deze e-mail afdrukt.


This message may contain confidential information and is intended exclusively for the addressee. If you receive this message unintentionally, please do not use the contents but notify the sender immediately by return e-mail. University Medical Center Utrecht is a legal person by public law and is registered at the Chamber of Commerce for Midden-Nederland under no. 30244197.

Please consider the environment before printing this e-mail.

fieldtrip mailing list
fieldtrip at donders.ru.nl<mailto:fieldtrip at donders.ru.nl>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20170510/91606564/attachment-0002.html>

More information about the fieldtrip mailing list