Dear all, I am sorry to bother you. I have some questions about the coregistration between MEG and MRI.
1、Three Fiducials marked before the MEG experiment decide the coordinate. If the NAS point is wrong, it can not consist with the Nasal root point on MRI head model，however the other digitized head points are consistent with the head model. In this condition, if i register MEG helmet and MRI with the other digitized head points instead of fiducials. What effect will this have on the source imaging results or no effect? And how should i adjust this?
2、What is the difference between SCS system and CTF system? Maybe they are the same ?
3、The head model deviates from the helmet. eg. The distance between the two is too far or The head model is biased to one side of the helmet
What effect will this have on the source imaging results or no effect?
Very looking forward to your replies.
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Three Fiducials marked before the MEG experiment decide the coordinate. If the NAS point is wrong, it can not consist with the Nasal root point on MRI head model，however the other digitized head points are consistent with the head model. In this condition, if i register MEG helmet and MRI with the other digitized head points instead of fiducials. What effect will this have on the source imaging results or no effect? And how should i adjust this?
The head model deviates from the helmet. eg. The distance between the two is too far or The head model is biased to one side of the helmet
What effect will this have on the source imaging results or no effect?
Move the sensor array, it will move the sources estimated from your MEG recordings.
Only you know why you have these head points not matchin the fiducials, and can decide how to compensate for this...
If I understand correctly, you're saying that the digitized NAS point is wrong. The usual recommended way to register MEG and MRI, as François said, is to use the digitized head points and fiducials as an intermediate step. But the NAS point is not used at all in the registration: the registration between MEG and digitization only relies on the fiducials (HPI points), and the registration between the digitization and the MRI uses the head shape points.
The NAS, RPA and LPA points picked on the MRI are used to create the SPS head coordinate system. So these digitized points are not used at all if you align with the head points. They should not affect normalization to a template or MNI coordinates either, right @Francois?
The further you are from the helmet, the worse the signal to noise ratio gets. But that's talking about the real head position. Trying to adjust the position artificially at analysis time would be bad: your model would not match reality and sources would be mis-localized. Same thing about being closer to one side: it changes SNR between the sides. But as long as your model is registered properly with the real head position, source analysis would take that into account.
The NAS, RPA and LPA points are only used to create the SPS head coordinate system. They should not affect normalization to a template or MNI coordinates either, right @Francois?
Indeed, if you are using the automate registration based on head points, the fiducials NAS/LPA/RPA are not used for the coregistration between MRI and MEG helmet.
The fiducials you placed in the MRI are used to define the CTF/SCS coordinates system (internal referential on which everything is aligned in Brainstorm) but the ones detected by the CTF MEG are not used at all.
The problem will probably be that this automatic registration works well only when you have points that are homogeneously distributed on the head (and only on the hard parts of the head). It doesn't seem to be the case here. The last option to try is to align manually the green digitized head points on the grey head surface...
Thanks for the correction. I'll edit my answer to clarify that it's the NAS, RPA and LPA from the MRI pannel that are used for generating the SPS system, and not the digitized ones.
...for defining the SCS coordinate system. But as I said they are used in registering the MEG to the digitized head points.
Thanks for your reply. I get addtional head points by following the tutorial https://neuroimage.usc.edu/brainstorm/Tutorials/ChannelFile to get the head points. Actually I want to know, when the three fiducials point can not consist with the head model, but the addtional head points can, if I ignore the fiducial points , and use the addtional points to achieve registration for MEG helmet and MRI head model, can I get the right source estimation?
