Gradiometer

Discussions
1

Dear BST expert,

When clicking “Display time series” for “MEG (gradiometers)”, things are shown with the unit of “fT” rather than “fT/cm”. What did BST do to convert the gradiometer data into a “magnetometer” type of display?

Thanks,

Zhimin

2

Dear Zhimin,

This question has been answered in the following thread:
http://neuroimage.usc.edu/forums/showthread.php?686-Elekta-grad-mag-scaling

Cheers,
Francois

3

Thanks a lot, Francois. That explains it.

BTW, I notice that, for my planar gradiometer topography display, when clicking “No magnetic interpolation” followed by “2D sensor cap”, things look fine as what planar gradient should look like. But when only clicking “MEG (gradiometers)” and “2D sensor cap”, things look like a magnetometer topography with the typical butterfly pattern. The tutorial explained a little by saying “A simple inverse problem + forward problem are solved to reconstruct the magnetic field on a high-resolution surface”. But what “simple inverse problem” was used — single ECD? Also, is it implemented differently for different sensor type — i.e. the “simple inverse problem” was solved for gradiometers by using only all good gradiometer channels, and it was solved independently for magnetometers by using only all good magnetometer channels? Finally, to display field topography in the sensor space based on this simple inverse solution, is it true that the relevant forward model was constructed based only on magnetometer configuration, even though the original data before solving the “simple inverse problem” was from gradiometers? That can explain why I saw a butterfly pattern even for planar gradiometers.

Thanks,

Zhimin