When importing directing from the native MEG format (Yokogawa, Elekta...), Brainstorm has access to the precise geometry of each type of sensor. Each gradiometer is modeled by 8 integration points, and each magnetometer with 4 integration points, to compute a forward model with a higher accuracy. The forward model is computed for each integration point, and then averaged per sensor.
To get an illustration, go back to the introduction tutorials, right-click on the channel file > Edit channel file, and see the number of location/orientation columns you have per sensor:
When importing from a SPM .mat/.dat file, this information is not available. SPM already decided that gradiometers would be represented by one point only, and gradiometers by two points (one for each coil, at the center of the coil). Right-click on your channel file > Edit channel file, you should see a maximum of 2 location and 2 orientation columns.
How much overall precision do you lose because of this: I have no idea. Maybe not much, but we haven't quatified this formally.
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Another slightly annoying display limitation is that the device not being identified, there is not correct display of the MEG helmet in the 3D views.
This is something you can fix manually with the following procedure:
- Rename the channel from the Brainstorm database explorer (ie. change the comment of the file) to: "KIT channels"
- Rename the channel file from your file explorer (ie. the actual file name) to: "channel_kit.mat"
- Right-click on folder > Reload