Subject Preparation

From MEG Core
Jump to navigation Jump to search
Subject Preparation: De-metaling

Before entering the MSR subjects must remove any metal from their person. There are no known risks associated with MEG recordings. Although wearing metal objects while in the MSR is not harmful to the individual, bringing metallic or magnetic objects into the MSR causes interference and high noise levels in the recording. *It is recommended that subject are screened and given instructions before arriving for testing. However, if subjects arrive with metal on their clothing that can not be easily removed they may be issued non-magnetic (paper) clothing which will be provided by staff.

  • Subjects must remove:
-Bras with metal under wires
-Makeup - mascara, eye liner, eye shadow
-Clothing containing metallic shiny threads or glitter
-Metal on clothing i.e., metal buttons, snaps or trimming
-Rings or other jewelry
-Cell phones
-Credit Cards
-Small metal objects such as hair pins, paper clips, safety pins, etc.
  • Additionally, everyone entering the MSR must also remove their shoes or use protective booties to prevent magnetic dust from getting into the MSR.


If a subject arrives with embedded metal on their person, i.e., a permanent retainer, which can not be removed it may be necessary to de-magnetize the subject. Care should be taken not to use the Degausser close to the computer monitors as it may permanently damage the screens. The Degausser must NOT be used in the MSR.

  • Plug in the Degausser.
  • Move 3-4 feet away from all computer monitors.
  • Pass the Degausser in a circular motion in front of the area of interest 3 or 4 times.
  • Unplug Degausser.
  • It is recommended that the subject not have had an MRI within 3-4 days before having a MEG scan because of the possibility of residual magnetization.

Head Size/Fit Test

Used to determine whether the subject will fit the MEG sensor helmet. The helmet will fit subjects up to 59cm (within 98 percentile for head size).

  • Determine head size by measuring the:
- Head Circumference,
- Nasion-Inion Distance and
- Preauricular Point to Preauricular Point Distance.
  • Determine fit by placing the MEG helmet along with a protective cap over the head.
  • Record measurements.
Head Size
Gantry Limits Nasion-Inion Preauricular Circumference
MEG / EEG ≤ 40cm ≤ 40cm ≤ 59cm

Fiducial Points/Head Coil Placements

Used for head localization (the process of locating the subject’s head in space relative to the Dewar). Used later to co-register the MRI Coordinate System with the MEG Head Coordinate System.

  • If the subject has had previous digital pictures of the fiducial points retrieve images to match placement.
  • Place a small black dot at 1.5cm up from the nasion (indentation between the forehead and the nose) midline. *The dot may be placed slightly higher if the nasion curvature is such that it does not allow level placement of the head coil.
  • Place a small black dot below the left & right preauricular points centered (about 1.5cm) in front of the tragus. This may be measured for accuracy.
  • Take a picture of these locations using the HP 715 Digital Camera. Download and print the pictures.
  • Attach Small (MEG) Adhesive Washers to head localization coils.
  • Attach head localization coils at the fiducial marks.
Head Coil Locations
Nasion Right Ear Left Ear
Blue Red Green
  • Make sure the marks line up with the center of the head coils.
  • Place tape over the cables to secure.

Electroculogram (EOG) Eye Lead Placement

Used to identify and monitor eye blinks and saccadic eye movements.

  • Left Infra Orbital (LIO) - Place electrode on the orbital ridge centered directly under the eye.
  • Left Outer Canthus (LOC) - Place electrode at the lateral junction of the upper and lower eye lid.
  • Clean the skin on the cheek near the eyes with alcohol and Nuprep Skin Cleaner. Wipe the skin to remove the gritty residue.
  • Attach the Large EEG Adhesive Washers to the electrodes.
  • Place the Infra Orbital electrode.
  • Place the Outer Canthus electrode.
  • Press the electrodes onto skin.
  • Apply Elifix Electrolyte through the electrode opening.
  • Check the impedances using the Grass Impedance Meter.
  • Plug in the left infra orbital electrode in jack 1 and the left outer Canthus electrode in jack 2.
  • Push the red ON button.
  • The Electrode Selector should be at 1 or 2.
  • EOG impedance should be < 50k ohms.
  • Secure with tape.
  • Refer to section entitled “Electrode Impedance Meter Check” for more detailed information

on reducing impedances.

EEG Setup

Reference Electrodes / Mastoid Placement: Used to define the electronics common point.

  • Left and Right Mastoid Placement - Bone prominence behind the ear.
  • Clean the skin behind the left and right ear with alcohol and Nuprep Skin Cleaner. Wipe the skin to remove the gritty residue.
  • Attach Large (EEG) Adhesive Washers to the electrodes.
  • Press the electrodes onto the skin.
  • Apply Elifix Electrolyte through the electrode opening.
  • Check the impedances using the Grass Impedance Meter.
  • Plug in the left mastoid electrode in jack 1 and the right mastoid electrode in electrode jack 2.
  • Push the red ON button.
  • The Electrode Selector should be at 1 or 2.
  • Mastoid electrode Impedance should be less than or equal to 5K ohms.
  • Refer to the section entitled “Electrode Impedance Meter Check” for more detailed information on reducing impedances.
  • Secure with tape.
EEG Easy Cap System

The Easy Cap is a modular EEG-recording cap system with 64 ring adaptor locations using Ag/AgCl sintered electrodes. *Other application methods may be used, however, careful attention should be given to the choice of electrodes; refer to Electrode Criteria.

Measuring / Marking Frontal Polar, Cz & Occipital Locations

  • Measure the nasion – inion distance. Place a mark at 50% of the total distant. This marks Cz.
  • From the nasion place a horizontal mark at 10% of the nasion – inion distance. This marks FPz.
  • Place a vertical mark on the forehead midline nasion. For accuracy measure the distance between the eyebrows; mark midway crossing the horizontal mark. This completes Fpz.
  • From the inion place a horizontal mark at 10% of the nasion - inion distance. This marks Oz.
  • Measure head circumference from the Fpz position. Place a vertical mark at 50% of the distance of the circumference. This completes Oz.
  • Measure and mark 5% of the circumference distance on the left of FPz (on the same horizontal plane). This completes Fp1.
  • Measure and mark 5% of the circumference distance on the right of FPz (on the same horizontal plane). This completes Fp2.
  • Measure and mark 5% of the circumference distance on the left of Oz (on the same horizontal plane). This completes O1.
  • Measure and mark 5% of the circumference distance on the right of Oz (on the same horizontal plane). This completes O2.
  • Measure from left to right preauricular points. Mark the midpoint. This completes Cz.

Fp1, Fpz, Fp2 Locations Mounting The Cap

  • Select a cap size based upon the head circumference measurement:

Small Med Large ≤ 56cm ≤ 58cm ≤ 60cm

  • Rotate the adaptors in the cap so that the narrow opening is toward the back of the head.
  • Place the Large (EEG) Anchoring Adhesive Washer on the inside of the cap on the adaptors at Fp1 & Fp2 locations.
  • Press Fp1 & Fp2 with the attached adhesive washers onto the forehead at the marked locations.
  • Have the subject hold Fp1 & Fp2 in place while turning the cap over the head.
  • Gently pull the cap over the head and ears into place.
  • Check and adjust the cap so that the Cz electrode is at the Cz mark.
  • Check and adjust Fp1 & Fp2 and O1 & O2 to assure that they are placed correctly; if not, choose another cap size.
  • Anchor the cap with the chest belt (or the chin strap).
  • Snap the electrodes into the adaptors on the cap in a systematic manner starting at the back of the head. *This step isn’t necessary if the electrodes are already on the cap.
  • Insert the electrodes so that the lead wire points toward the narrow side of the adaptor.
  • Using an old ballpoint pen (or the fingers) where the mine (the point of the pen) has been removed push the electrode into the adaptor.
  • Do not to bend or place excessive pressure on the lead wire where it attaches to the electrode.
Digitizing Electrode Positions

The Polhemus Digitizer is used for the digitization of the EEG electrode positions and for the determination of head shape. Digitize all electrode positions using the Polhemus.

  • After the Easy Cap has been placed on the subject, position the small Receiver Module on the back of the head on top of the cap near midline. Secure with tape.
  • Place the large Transmitter on a non-metallic stand midline directly behind the subject. Tape the transmitter to the stand to secure.
  • Turn on the laptop computer located on the portable cart.
  • Open the Polhemus Electrode Digitizing Program:
  • Double click on the Electrode Digitizer Program.
  • The software should detect two (2) stations: the receiver and transmitter.
  • If the software does not recognize both stations, check the connections then restart the program.
  • Click on Define Head Coordinates.
  • Define the head coordinates by placing the stylus pen tip at the center of a head coil location (nasion, left preauricular, right preauricular) then press the stylus button. If a mistake is made,
    • Press the stylus pen until all head coordinates have been entered then,
    • Press define Head Coordinates again. Do not save the previously acquired locations simply start over from the beginning.
  • Click on Digitizing Start / Stop.
  • Place the stylus pen at the center of an electrode location as designated by the digitizing program then press the stylus button.
    • A single data point is collected each time the stylus button is depressed and a beep will alert the user to each acquired point.
    • The program will assign a number to the electrode position in the sequence the electrodes are selected, i.e., 1, 2, 3, 4, etc.
    • If a mistake is made, click on Remove Last Point and try that position again.
    • Click on Digitizing Start /Stop when all electrode locations have been done.
  • At the end the program a head diagram will appear on the screen.
    • If the diagram is:
    • Correct then continue with Step 8.
  • Not correct then check to make sure that the receiver or transmitter did not move then repeat digitization (steps 6a –d).
  • Click on File / Save Data.
  • Save the data to the hard drive and
  • Save to a floppy disk.
  • The filename is the current date.
  • Exit the program. Eject disk. Turn off laptop.
  • Transfer the Polhemus head coordinate file from the disk to the computer at the acquisition workstation / console.

The operator and anyone within 10 feet of the transmitter and receiver should de-metal. The general rule of thumb is three times the separation distance between the transmitter and receiver from any metal in the room. For example, if the separation distance is one foot, metal within three feet of the transmitter or receiver may cause distortion and inaccurate measurements.

Reducing Skin Impedances

Nuprep is intended for to reduce skin impedance. It is a topical abrasive used to clean the skin to remove any natural or applied oil, lotions and/or dead skin. Remove with water.

  • Insert the wooden end of a cotton swab through the opening of the electrode to push hair aside (in one direction) until the skin is clearly visible.
  • If the hair is long, lift up the electrode a little and let it down again while moving back the swab to grab a new bunch of hair (this will prevent the hair from slipping back).
  • Dip the swab into the Nuprep (Abralyt or other skin prep) and insert through the electrode opening.
  • Twirl the swab against the skin using the thumb and index finger.

Applying Electrolyte Elifix Conductive Cream is a white opaque, water soluble electrolyte used as a conductive medium. Remove with warm water.

  • Fill a syringe with Elifix (Abralyt or other electrolyte).
  • Squirt Elifix into the middle of each electrode opening, completely filling the electrode space (the space from the skin to the electrode).

When ELECTROLYTE: If the electrodes are closely spaced, draw back the syringe concurrently to prevent electrolyte from swelling underneath the adaptor rim and bridging to the adjacent electrodes.

Electrode Impedance Meter Check The Grass S88 Impedance Meter measures AC impedance (resistance and capacitance). An impedance check should be performed on all electrodes used for EEG or EMG.

When two or more electrodes are plugged into the electrode switch positions, all electrodes except the one selected electrode being measured are connected together and provide the reference for the one being measured.

1. The Electrode Selector Switch should be at position 1 or 2.

2. Plug the ground electrode (FPz) into electrode jack 1 position.

3. Plug the other electrodes into the jack 2 position in a systematic manner one at a time.

4. Press the Red ON Button to activate the impedance check. The power will automatically shutoff after approximately 2 minutes. (There is no OFF button.)

5. If the impedance is too high - take a cotton tip swab dipped in Nuprep (Abralyt or other skin prep) and twirl against the skin until impedance is at an appropriate level. Then re-insert Elifix electrolyte.

Impedance Recommendations EEG Electrodes 5Kohms EMG Electrodes (EOG, ECG, etc.) 50Kohms

6. After checking the impedance of each electrode plug the electrode into the appropriate electrode jack box position.

If the electrode impedance exceeds 199.9K ohm, a “1” will appear in the left display window with all other digits blanked out indicating the impedance is too high. Take a cotton swab that has been dipped in Nuprep and insert it through the electrode opening and twirl against the skin. Re-apply electrolyte. Re-check the impedance.

Electrode Jack Box

Electrode Jack Box Mounted On The Amplifier

ELECTRODE JACK LOCATION FPz Ground (green); Z position.

Linked Mastoids Ref position (red).

EOG – Left Outer Canthus EOG – Left Infra Orbital A+ (black); Bipolar Channel A- (blue); Bipolar Channel

      (EEG Channel 057)

Cerebral Electrodes Place according to selected montage; Numbered jacks; Unipolar Channel.

If Using EOG: 1. Plug into a bipolar channel.

If Performing EEG: 1. Plug FPz into the ground (green / Z) electrode position.

2. Plug the Mastoids into a Linked Electrode Adaptor (Y Connector) then place the single end of the adaptor into the Ref (red) position to link the mastoids together.

3. Plug the cerebral electrodes into the appropriate MEG System Jack Box unipolar connection according to the particular montage design.