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Bug 2019 - sensor fusion in neurmag system
|2013-02-28 13:39:00 +0100
|2019-08-10 12:03:36 +0200
Hamid - 2013-02-28 13:39:43 +0100Created attachment 428 mne2grad correction Gradiometer and magnetometer have different units and cannot be simply combined for source localisation. If you don't correct that the source localisation fails. You could do three things to correct this error: 1) Only use one sensor type for source localisation: preferably gradiometer 2) scale the gradiometer by factor of 0.0168 3) use a finer approximation for sensors using the attached file. It should be replace fileio/private/mne2grad For further details, I copy my email to Vladimir Litvak: There is a problem in fusion of gradiometer and magnetometer. A rough correction would be scaling the gradiometer by a factor of 0.0168, which is the distance between the gradiometer coils. If you don't do this correction, the source localisation fails. But a more accurate way is to model the magnetometer by 9 points and gradiometer by 8 points as explained in the neuramg maxfilter manual. I think the simplest approach is correcting the mne2grad.m file which has been written by Laurence and Joachim. They basically use two points for each grad and one point for each mag (510 points in total). I now use 9 points for each mag and 8 points for each grad according to the ELEKTA manual (2550 points in total). Therefore the "pnt" and "ori" are 2550x3 matrices (not 510x3). I also changed the "tra" matrix to a 306x2550 according to the weight of each point. The program then runs fine and it doesn't affect the speed. The results are also quite similar when you just scale the gradiometer, but I think it is better to use it anyway. I attached the new version of mne2grad_v2.m which should replace mne2grad.m located in fieldtrip/fileio/private/mne2grad.m. One thing you may consider is the parameters of the geometry of the sensors. These parameters are for Oxford MEG (see attached file) and it should be changed according the manual of the device (I haven't found them in the fiff file). However, these parameters are the same for different systems and I guess old ELEKTA MEG systems should change them.
Hamid - 2013-02-28 15:18:26 +0100it is duplicate of 2020. *** This bug has been marked as a duplicate of bug 2020 ***