Co-registration MRI and CTF MEG

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Hi everyone! I have data acquire using a CTF MEG system. I don’t have digitized headshape points, so I need to set NAS/LPA/RPA manually. The subject’s MEG folder contains a .hc file with coil positions, e.g.:

standard nasion coil position relative to dewar (cm):
	x = 5.65685
	y = 5.65685
	z = -27.00000
standard left ear coil position relative to dewar (cm):
	x = -5.65685
	y = 5.65685
	z = -27.00000
standard right ear coil position relative to dewar (cm):
	x = 5.65685
	y = -5.65685
	z = -27.00000
measured nasion coil position relative to dewar (cm):
	x = 6.45525
	y = 6.50760
	z = -21.16790
measured left ear coil position relative to dewar (cm):
	x = -4.82564
	y = 6.49774
	z = -24.60200
measured right ear coil position relative to dewar (cm):
	x = 5.64420
	y = -4.81848
	z = -24.71060
measured nasion coil position relative to head (cm):
	x = 8.99161
	y = 0.00000
	z = 0.00000
measured left ear coil position relative to head (cm):
	x = 0.06775
	y = 7.70824
	z = 0.00000
measured right ear coil position relative to head (cm):
	x = -0.06775
	y = -7.70824
	z = 0.00000

None of these coordinates seem to match Brainstorm’s MRI, SPS, MNI, or World coordinates: inserting them directly misplaces the fiducials.

How should I use these .hc coordinates to set fiducials manually in Brainstorm?

Thanks!

2

Hello @Francesca27,

There is no need to manually set coordinates for the head coils. When the CTF MEG data is reviewed, the .hc file is read and the positions of the nasion, left ear and right ear coils (relative to the dewar) are used to co-register the MEG helmet (and its sensors) to the Subject anatomy. This is to say, the initial registration is obtained aligning the anatomical fiducials with the head coils.

For sake of completeness, these coordinates are saved in the header.hc field of the Link to raw file file.

If you MRI-MEG registration seems off, it is not possible to refine it using head points, as they are not present in your data. If the co-registration is really off, double-check that the fiducial definition in the MRI corresponds to the places where the head coils were placed.

Check the information in here:
https://neuroimage.usc.edu/brainstorm/CoordinateSystems#Notes_for_CTF_users

In brief, before reviewing the CTF MEG data, set the MRI fiducials in the places where the head coils are on the subject.

  • Right-click on the MRI and then Edit... set the NAS, LPA and RPA to match the positions of the head coils during the MEG acquisition.