Hi François, Hi John,
Thank for your long answers, It's very helpful for me.
I read and check in different sources about the Elekta and the MNE modling principles.
If I'm right,
in the hardwear of Elekta Neuromag, magnetometre has one coils with only one Josephson boucle
and in the gradiometer two coils and only two josephson boucle.
But in the soft, we can use more than one point to calculate the magnetic field, for a more accurate modeling. ( it's not according to the construction of the sensor)
each point with a value of weight.
The values that are saved by the acquisition software.
Magnetometers are magnetic fields in Tesla, gradiometers are spatial derivatives of the magnetic fields. Check the Elekta documentation for more information.
I chek in the documentation, the result is dBn/dx and dBn/dy, it's the normal on the the coil thank you.
If you are developing your own forward model, you are supposed to know/define the way you want to model your sensors.
Actually, I'm working in the FE model, but to validate my results,
I need some real mesures, so I have to adapt my results in simulation into the result with the mesures in ordre to check and validate.
This is the reason why I need to understand the manner of modeling the sensors of the Neuromag in brainstorm.
About the reply of John, it's verry important, thank you very much.
I read all the tutoriel of Brainstorm, and try most of the examples.
About the head models, I read some articles and among them I find yours, very intersting, Thank you for your work.
For the sensor models,
actually I have only some results of thumb stimulation, from the MEG of Neuromag, and I don't have information about the "FIF" data.
I can saw the caracteristique of the sensors on brainstorm right-click > edit channel file, as Fracois explains.
Now we could just model the magnetometer by computer the forward field at the center of the coil. But we felt that's too approximate. Instead we evaluate the forward field at four points in the magnetometer coil, and weight each one by 1/4, to get an average of the flux passing through the relatively large coil. You see these as "weights" of 0.25 in the Channel editor
To confirm a confusion with my colleague, there is only one Josephson boucle in the magnetometre and not 4 ? The four points are only for the calculation and modeling.
an other question :
to get an average of the flux passing through the relatively large coil. You see these as "weights" of 0.25 in the Channel editor.
to calculate the flux passing through the coil, we need the surface of the coil, but I dont find it in the calculation ? or we don't use it ?
The planar gradiometers are a little trickier. The gradiometer comprises two smaller coils counter wound. We could simply calculate the magnetic field in the center of the first coil, then subtract the magnetic field in the center of the second coil, to yield the gradiometer effect. Again, we instead chose to average the field in the first coil by picking TWO points in that coil and averaging them, then picking TWO points in the second coil, averaging them, then subtracting them from the first coil's average. We did not pick four points in each coil, because these are smaller coils than the magnetometer. But because there are two coils, then we still have four points. Note in the weights, however, that two of the weights are negative, since they correspond to the second gradient coil.
great, very clarifying,and helpful, Thank you.
But why are these gradiometer weights not simply "0.5 0.5 -0.5 -0.5", instead of the stranger [29.76 29.76 -29.76 -29.76] values (in one type of Neuromag, there are other variations)?
The strange weights therefore also convert the magnetic field values so that the right units are obtained.
do you know how they had do to get these value? its very strange?
The Neuromag writes the magnetometer data in units of Tesla, but writes out the gradiometer data in units of Tesla/Meter
In brainstorm, in the variable F we find the field in Tesla for the Magnetometer, and the gradient of the field for the gradiometer in Tesla/meter.
but when we plot or visualize the MEG recording, "http://neuroimage.usc.edu/brainstorm/Tutorials/TutImportRecordings" and "http://neuroimage.usc.edu/brainstorm/Tutorials/TutExploreRecodings ",
the graphe is (Time (s) VS Amplitude (fT)) , which kind of data are we plotting on?
I understand that it is the value of the field in each sensor,
for the magnetometer (Tesla) it's OK, but how is the visualisation for the value of the magnetometer (Tesla/Meter).
Thus you are correct that you will make 1,224 calculations of the magnetic field outside the head, but the way you combine these calculations is reflected in the Weights associated with each sensor. The resulting 306 data values are in units of either T or T/m, corresponding to the 306 channels of data you see in the "F" field of Brainstorm's "data" structure.
Thank you for this explanation, it's very kind.
Best regards
Takfarinas