Hello Mattia,
I’m assuming you’re asking how we combine these two MEG arrays, when the units are apparently so different? The magnetometers in the Elekta instrument are stored in units of T (as are the other MEG machines), but the planar gradiometers are stored in units of T/m, which because of the short baseline, give initial raw GRAD numbers that appear roughly 100 times greater than their MAG counterparts.
A naive attempt to do simultaneous source localization with both MAGS and GRADS would simply ignore the MAGS, since they are so “tiny” that they don’t contribute to the model. However, in both the XFIT software that came with your Elekta machine, and in Brainstorm, and in many other software packages, we can explicitly take into account samples of the baseline (or noise) data. In Brainstorm, we ask you first to create a “noise covariance” matrix from the recordings. We “pre-whiten” the data by pre-scaling the channels by the observed std. deviations seen in the noise data (we can use either the diagonal values of the noise covariance matrix, or the full matrix, which includes rotations into virtual combinations before scaling). This brings GRADS and MAGS into the same basic range of units. The same whitener is applied to the head modeling, to keep units and scales consistent. Source estimation then proceeds normally with a combined array of 306 pre-whitened sensors and pre-whitened data.
Does this answer address your question?
– John