MEG

Digitization

See also Section 3.6 of the Acquisition Software Manual. Some useful tips were taken from the wiki of the Cambridge MEG Center which we acknowledge.

Take a lot of care when performing this step because it is crucial for head position measurement and for further data processing.

Determination of the position of the head position indicator coils and head shape/EEG electrodes digitization is performed with the help of Polhemus 3-D digitizer. Digitization is performed outside of the magnetically shielded room, in the consultation room next to the MEG room.

Subject preparation

  • Bring the wooden chair away from the desk and from any metallic object (including your watch and mobile phone!).
  • Ask the subject to sit on the wooden chair.
  • Put the Isotrak goggles on the subject and secure their position by tightening the ropes. It is crucial that 1) the goggles remain still during all the digitization; 2) HPI coils behind the ear are accessible for digitization.

Software preparation

  • Check that the Isotrak unit is powered on (there is a switch in the rear panel).
  • Log in the laptop (same credentials as on the MEG acquisition workstation).
  • Start the Prepare Acquisition software on the laptop by clicking the corresponding icon on the desktop
  • Select your project AND load your settings.
  • Select/enter the subject info. NOTE: For new subjects you have to enter the subject's NIP (initials in the 'name' window' and the six numbers in the 'surname' window) and date of birth. For privacy reasons, it is FORBIDDEN to enter the subject's name.
  • Hit "Change" right next to HPI.
  • On the digitization window, click on "Coordinate frame alignment" to start the single-operator digitization.

Digitization

  • Now you are ready to start the digitization. Make sure your participants head does not move while you are taking the points, it’s a good idea NOT to carry on a conversation with your participant whilst digitizing.
  • When pointing the stylus on the subject's head, ensure that the angle between the stylus and the scalp surface is almost constant (one strategy is to point the stylus always towards the center of the head; be delicate though as the tip of the stylus is very pointy and may be uncomfortable for the participant).

Figure: Digitization in action.

  • Point the stylus and touch the left pre-auricular point (LPA) and click on the stylus. Wait until you hear a beep and a value has appeared on the screen before moving the pen away from the head. Train yourself to make the pen beep only once (it's sort of difficult the first time), if it beeps twice you have to start again (see last point below).
  • Repeat for the Nasion.
  • Repeat for the right pre-auricular point (RPA). Now you will hear TWO beeps, the second one indicates that cardinal points have been digitized.

Figures: Left pre-auricular point (LPA) and right pre-auricular point (RPA).

Figure: Nasion

Check that the difference in X axis between LPA an RPA is less than 5 mm (in absolute value). If it is more, redo your digitization.

  • Now digitize all four HPI coils in any order. Be very careful when digitizing these points, as they can come back to haunt you later if not done properly the first time!

Then, if you only record MEG:

  • Digitize a few more anatomical landmarks (>20) starting with three points on the side of the nose and, for instance, following three lines along the scalp (2 lateral and 1 median). Please do not skip this step: It is crucial for source localization in the analysis phase!

If scalp EEG channels are selected for acquisition, the program moves on to digitize the EEG electrode locations:

  • Digitize all EEG electrodes in the ascendant channel number order (see EEG digitization scheme), starting from the reference electrode.
  • Once all points have been digitized, click on the stylus away from the subject's head to abort the digitization process. Don't forget to click OK and to save your preparation (file/save normal preparation) !!!
  • If at any point you make a mistake, hold the stylus away, press the stylus button, and then click on the button at the top of the section you would like to do again. A selection box will appear with the choices ‘check’, ‘redo’ or ‘help’ select either ‘check or ‘redo’ and digitize the HPI points again.