BESA BrainVoyager interface

For subjects with both EEG and MRI/fmri data it is possible to project source dipoles from BESA into the BrainVoyager surface module display of the subject's MRI/fmri results. In order to coregister the coordianates systems between BESA and BrainVoyager it is necessary to fit the electrode positions used to collect the EEG data to the head surface defined by the MRI scan.  It would be ideal to have 3D digitized positions for all of the electrodes used for each subject, but this would be very time consuming.  For now a single digitization obtained from EGI is being used for specifying  electrode positions (x,y,z in cm) to BESA. The positions are transformed from digitizer coordinates to BESA coordinates using three fidicual marks and solving for a best fit sphere using all electrodes except those anterior to LPA/RPA and below the plane defined by LPA, RPA and NAS.  Choose File->Head Surface Points and Sensors->Save for BrainVoyager to save an electrode position file that BV can read. Start BV from its icon or from Fit->Start BrainVoyager in the BESA source analysis window.  Load the appropriate MRI (.vmr) file into BrainVoyager, start the surface module and create or load the head surface file (see BrainVoyager pages for details on how to do this).  Below is an example of the BrainVoyager surface view with the electrodes imported from BESA:

First it is necessary to fit the fiducial points.  Right click on the right preauricular point and choose Set fiducial and check the appropriate box in the popup dialog.

  Similarly, set the nasion and left preauricular point. Then choose EEG/MEG -> Fit fiducials and EEG/MEG -> Fit surface points to let BrainVoyager fit the electrodes to the surface of the head.

 BrainVoyager uses 3 translation, 3 rotation and 3 scaling parameters to produce this fit. Now with this coordinate transfomation set up a dipole source model can be sent to BrainVoyager by selecting Fit -> Send Solution to BrainVoyager from the source modeling window in BESA or by simply clicking on the BrainVoyager button above the source waveforms in the window.  The following source model in BESA

looks like the following in BrianVoyager.