Hi Brainstorm team,
We have a data collection where electrode positions were captured with CapTrak, but the NAS landmark was in the incorrect place (4-5cm higher than it should be, on the forehead), meaning that the electrode positions are inaccurate as it stands.
Is it possible to fix this registration in Brainstorm? Or should we either (1) use CapTrak data from another session of this person (question posed here: Digitized electrode positions ) or (2) use standard channel positions?
I don't fully understand the 'refine using head point' function after reading this: https://neuroimage.usc.edu/brainstorm/Tutorials/ChannelFile#Automatic_registration, and whether what is described there relates to this question, so I wanted to ask whether it is something that would be useful here.
Thank you!
Adam
For the same subject some sessions where digitized with different NAS positions, is this true?
If it is, then, it cannot be fixed in Brainstorm, since all the channel files should share the same fiducials.
Does CapTrak has extra head points besides the EEG electrodes? If it does, you can give a try to this to find the best alignment between the head points and the scalp surface. If it only has the EEG electrode positions, the fit may not be optimal.
Another approach you could try is manually editing the registration.
Once the recordings is linked and the channel file is created. Right-click on the channel file > MRI registration > Edit and move the electrode cap to better fit the head and/or to alight the red dot for Nasion with the place that was really acquired (4-5 cm higher that it should be).
This tutorial section indicates how this can be done:
https://neuroimage.usc.edu/brainstorm/Tutorials/Yokogawa#Refine_the_MRI_registration
Correct. First session had the bad NAS position. The other two look fine and followed our standard protocol.
How do I find this out? Below in the first screenshot is what I see regarding head points before modifying any electrode positions. The second image is the same, just with electrodes added. The electrodes and head points are very similar, so i'm unsure the answer to your question. Perhaps these screenshots will allow you to find the answer. Otherwise, please let me know how I can find this out.
Okay sounds good. If the answer to my other thread (linked in the original post of this thread) is that I can use the Post CapTrak channel locations for another session (the one with the bad NAS position), then I will try that option as well.
Another thing I've realized, is whether this prompt when adding CapTrak electrode locations to a preexisting channel file would now be helpful:
I normally say "No" here, since we usually want to use the CapTrak fiducials (aka input file) when they are defined correctly; however, I'm wondering if saying "Yes" to this prompt would disregard the bad CapTrak fiducials in this instance and use the fiducials I marked on the MRI when importing the anatomy into Brainstorm. What do you think?
Would I need to do option for the other sessions within this subject so that the same fiducials are shared between all their sessions?
Thanks again!
Adam
Yes, white dot are the electrodes and green dots other head points.
I think you can answer No, even when NAS is out of place, the first approach to registration will not completely wrong, it will have descent alignment (NAS still in the front and LPA and RPA are in the good place. After this you can try to improve the registration with the head points.
