Preparing the subject

Participant preparation

Before participant arrives.

An important part of an MEG measurement is to prepare for subject arrival. Before subject’s arrival, you should have made sure that:
the lab space and shielded room are tidy and things there are in order
there are clean measurement clothes just in case
your experiment is ready to go, i.e. channels, stimuli, triggers etc. set up and tested
all the equipment are there (e.g. for attaching electrodes)
you have a kind and professional mindset

Once the subject arrives

interview the subject and tell what you need and want about the experiment
take signature for consent and do possible other paper work
set aside personal metals and phone of the subject, as well as yours
test in MEG if the subject is magnetic (movements, breathing)
prepare HPI, EEG, EOG, EKG, etc. (see below)
give task instructions at least twice during preparation and finally in the shielded room
give MEG instructions (don’t move or blink, relax)

HPI coil preparations

1) Preparative steps

Turn Polhemus on
Check that the chair is not near any metallic material or electromagnetic sources
Check that there are no metallic objects in the subject or yourself
Attach the Polhemus transmitter on the back of the chair, wire going downwards
Make sure that you have prepared the data acquisition software

2) Position HPI coils to the scalp or cap

Attach the 5 coils firmly to the scalp (not hair!) with your favourite method, lab provides transparent tapes for this purpose
Recommended coil positions for accurate head position estimation are as follows:
as high as possible so that MEG detects the coil signals
2 behind ears, 2 on the temples, one on the forehead
take care to not cross the eyes, ears, or back of the head with the cables
if you use EEG, fix the coils evenly around the top of the cap instead.

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3) Document nasion and preauricular points

- It will prove helpful in the later stages of the project, if you mark the landmark points with a marker pen and take a few photos from different angles
- Photos illustrate the recommended anatomical landmark points
- it is most important that the points are known and consistent and that they can be found on the MRI images

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 4) Put the glasses or the receiver element on the subject
Check that the points you need to digitise are not covered by the glasses and that they are easily accessible with the digitisation pen
Tighten the glasses to make sure they don’t move on the subject during digitization
Instead of glasses, you can use a separate receiver and fix it e.g. to the EEG cap
Glasses (or receiver) must stay stable throughout the digitization
From this step on, minimise subject head movements (avoid interaction for a while)

5) Digitize anatomical coordinates

Select HPI digitization in the acquisition software
Click Coordinate frame alignment
Ask the subject to stay still during digitisation
Make sure the pen cable does not cross the transmitter in the back of the chair
Digitize the nasion and the left and right preauricular points with the Polhemus pen by clicking the button on the pen.

In the data acquisition software, check the difference found between the left and right pre-auricular points. If this is over 4 mm, redo measurement by clicking Coordinate frame alignment again and repeating the procedure.
Optionally, you can have an assistant on the MEG control computer to help in the digitization. Instead of selecting Coordinate frame alignment, the assistant can mark the points by clicking the corresponding buttons on preparation software.

6) Digitize HPI coils, EEG electrodes, and extra points

Click HPI coils
Digitize the center of all five attached HPI-coils
If the suggestion in HPI coils is REDO -> check if coils are loose and try again

Click EEG electrodes
Digitize the centre of each electrode in specified order, starting with the Ref.
If you make a mistake, you can go back and forth using the arrow buttons

Click Pen in the bottom right of screen
Gently stroke the pen over the head while keeping the button down, or click several dozens of extra points around the head.
Good candidates for extra points are bony skull regions and the nose
When enough extra points have been marked, click the pen away twice
On the data acquisition computer, select → File → Save preparation

7) Check the HPI coil digitization

- select Check and proceed with HPI coil digitisation again. Errors should be only a couple of millimetres. Otherwise, Redo. You can also perform this check an earlier stage.

8) Save the preparation
Saving the HPI digitisation helps if the data acquisition program or even the whole computer has to be restarted.
9) Do one-time or continuous HPI measurement when subject is ready
Remove glasses, but be careful to not move the HPI coils, electrodes etc.!
Once the subject is seated or lying in the MEG device, attach the HPI coil socket in the red plug on the left hand side of the gantry
Once you start data acquisition, the software asks to do an HPI measurement. Check that the digitisation and MEG coil fit correspond well to each other (reported coil-to-coil distances around 2 mm). Take a note of the head coordinate origin.
If you are doing continuous head position monitoring, click the cHPI button.
If you want to monitor head movement, but do not want to use cHPI all the time, you can turn cHPI on for a couple of seconds every minute or two. MaxFilter then has the option -movecomp inter for intermittent movement correction.

Head Position Indicator (HPI) measurement in nutshell

Preparative steps
Position HPI coils to the scalp or cap
Document nasion and preauricular points
Put the glasses or the receiver element on the subject
Digitize anatomical coordinates (nasion, LPA, RPA)
Digitize HPI coils, EEG electrodes, and extra points
Check the HPI coil position digitization
Save the preparation and note the saving time
Do one-time or continuous HPI measurement when subject is ready

NOTE: The HPI measurement is one of the most important steps in data acquisition. For the sake of quality, please do it carefully and in a consistent manner for each and every subject. In case of problems at this stage, contact CIBR support for help.

MEG study checklist (pdf)