Matching fiducials selection

Discussions
1

Hello,
Thanks for continued support to BST user :slight_smile:

  1. We have EEG and structural scan acquired for each subject. Surface segmentations are performed using BRAINSUITE. Inside EEG file we have electrode positions measured on individuals head w.r.t NAS, LPA, RPA points using Brainsight. As per tutorial on your website using NAS, LPA, RPA points inside EEG channel file, BST will convert existing coordinates to BST coordinate system while importing. However when we import anatomy, we mark these NAS, LPA, RPA points manually. Point we marked on subjects head while acquiring EEG and one marked on MRI while importing in BST can be slightly different. How this can be matched ? Is it taken care automatically ? I hope my question is clear " I want to mark same points while importing MRI in BST which I have marked while creating EEG channel file or EEG measurement on same subject’s head".

  2. Is it necessary to have NAS, LPA, RPA points stored in EEG channel file to import it correctly ? How we can proceed if we do not have these point already marked in EEG channel file ? Do application check for any specific field to find these fiducials?

Thanks in advance for help.
Eshwar Ghumare

2

Hello,

You have to start by importing the MRI and surfaces (using the Import anatomy folder menu), mark the NAS/LPA/RPA in the MRI and close the MRI viewer. This is your reference for the CTF/SCS/Brainstorm coordinate system. See the this page for help:http://neuroimage.usc.edu/brainstorm/CoordinateSystems

Then when you import the position of the Brainsight electrodes, it uses the NAS/LPA/RPA points of the electrodes file to try matching the electrodes positions on the MRI, by modifying the the positions of the electrodes. This is done automatically.
It is an inaccurate process, this is why to also recommend digitizing 100-200 additional points to digitize the head shape of the subject. Then when you import the file, you should get a message “Refine using head points?” If you don’t you can run this manually: right-click on the channel > MRI registration > Refine using headpoints.
http://neuroimage.usc.edu/brainstorm/Tutorials/TutDigitize

If there is no NAS/LPA/RPA points in the channel file, it just import the electrodes positions as they are. Then the registration does not depend on Brainstorm, but on the program that saved the channel file. If it uses the same coordinate system, it might be correct, if it doesn’t it would probably wrong.

Then, if the registration looks bad of you didn’t have the NAS/LPA/RPA, you can edit manually the electrodes positions.
Right-click on the channel file > MRI registration > Edit. Read the tooltips of the buttons to understand how to use them

Cheers,
Francois