Difference between revisions of "Subject Preparation"

From MEG Core
Jump to navigation Jump to search
Line 276: Line 276:




::::{| class="wikitable"
::::::::::{| class="wikitable"
|-
|-
! ELECTRODE !! Description !! Jack Position
! ELECTRODE !! Description !! Jack Position

Revision as of 12:06, 30 March 2018

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
-Belts
-Keys
-Watches
-Rings or other jewelry
-Coins
-Eyeglasses
-Cell phones
-Pagers
-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.


Degaussing

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.
*Please note: If used improperly you can magnetize your subject so always ask for assistance from MEG staff before using the Degausser!


Head Size/Fit Test

Use the MEG helmet to determine whether: a) the subject will fit the MEG sensor helmet/dewar; b) how much padding is needed to fill the excess space to prevent head movement and c) how high up in the dewar helmet the subject can be raised for optimal placement. The helmet/dewar will fit subjects up to about 59cm head circumference (within 98 percentile for head size).

  • Determine the subject's head size by measuring the:
- Head Circumference,
- Nasion-Inion Distance and
- Preauricular Point to Preauricular Point Distance.
Head Size
Gantry Limits Nasion-Inion Preauricular Circumference
MEG / EEG ≤ 40cm ≤ 40cm ≤ 59cm
  • Determine the subject's fit by placing the MEG helmet along with a protective cap over the head.
- Note the fit; if the helmet does not fit (with the coils on and inside the helmet) then the subject is not a good candidate for MEG; if the subject fits then note the amount of space the subject has to move his/her head.
- Place enough folded towels/towels over the subject's to restrict head movement then re-try the helmet. Note: you want the helmet to be tight enough to restrict head movement but not so tight as to cause a headache.
  • Determine how to place the subject's head properly - at the top the dewar helmet.
- Note how far down the helmet comes during this process as this will give you a general idea of how high up in the dewar you can go in order to position the subject's head at the top of the dewar (closest to the sensors at the top of the helmet).
  • Note or record measurements.


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. Measure 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. This is a one channel montage which is designed to pick up horizontal and vertical eye movement from both eyes. There are many ways to monitor eye movements- some montages use one channel and some use two channels. Selection is usually based upon the reason(s) for monitoring the eye movement.


  • 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 and behind the left ear 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.
  • Place a ground electrode on the left mastoid bone behind the ear.
  • 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 and the ground into jack# 3.
  • 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 impedance.
If you require further assistance, please request technical assistance from MEG staff.


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 methods and electrode selection; 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 across the top of the head. Mark the midpoint. This completes Cz.


Mounting The Cap

  • Select a cap size based upon the head circumference measurement:
EasyCap Size Small Medium Large
Head Circumference ≤ 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.

1) 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.
2) Place the large Transmitter on a non-metallic stand midline directly behind the subject. Tape the transmitter to the stand to secure.
3) Turn on the laptop computer located on the portable cart.
4) Open the Polhemus Electrode Digitizing Program.
5) Double click on the Electrode Digitizer Program.
6) The software should detect two (2) stations: the receiver and transmitter.
7) If the software does not recognize both stations, check the connections then restart the program.
8) Click on Define Head Coordinates.
9) 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.

10) Click on Digitizing Start / Stop.
11) 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.

12) At the end the program a head diagram will appear on the screen.

If the diagram is:
- Correct then continue with Step 13.
- Not correct then check to make sure that the receiver or transmitter did not move then repeat digitization (steps 5-11).

13) Click on File / Save Data.
14) Save the data to the hard drive and
15) Save to a floppy disk.
16) The filename is the current date.
17) Exit the program. Eject disk. Turn off laptop.
18) Transfer the Polhemus head coordinate file from the disk to the computer at the acquisition workstation / console.

Note: 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 reducing 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 and Ten/20 Conductive Creams are opaque white, water soluble electrolytes substances used as a conductive and fixing/adhering mediums to attach electrodes. Removed with warm water.

  • If using Elifix or Abralyt - fill a syringe with the cream.
- Squirt into the middle of each electrode opening.
  • If using Ten/20 scoop paste into a cup electrode (this paste is too thick to squirt from a syringe).
- Press firmly on the skin.
  • Completely fill the electrode space (the space from the skin to the electrode) with cream / paste so that there are no air pockets and the electrolyte makes good contact with the skin.

If the electrodes are closely spaced, in order to prevent "electrode bridging" draw back the syringe concurrently to prevent electrolyte from swelling underneath the electrode 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 Jack position 1 or 2.
2. Plug the ground electrode (FPz) into electrode Jack 1 position.
3. Plug the other electrodes into the Jack (2, 3, 4, etc) positions 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/EOG/ECG ≤ 50Kohms
6. After checking the impedance of each electrode plug the electrode into the appropriate Electrode Jackbox / Amplifier 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.


MEG Electrode DSQ 3500 Amplifier

Located inside the MSR on the table near the MEG machine. The DSQ 3500 Amplifier has inputs for:

  • Fudicial Coils - Nasion, Left Ear, Right Ear, Inion and Vertex;
  • 21 Unipolar Jacks - there are 21 unipolar channels which measure the difference between an electrode and a common reference channel. The reference channel is common to all of the EEG unipolar channels;
  • 4 Bipolar Channels - that measure the potential difference between two individual electrodes. These channels are not numbered on the jackbox, however, labelled channels 56 to 59 in the parameters;
  • ZIF Connector/Jig Connector - for the EEG EasyCap System which can accommodate up to 56 electrode positions;
  • Ground Jack - Z;
  • Reference Jack - Ref.
Please note: There is a Static Dissipation Button which you must touch before touching anything else on the amplifier and before plugging any electrodes into the amplifier.

If Performing EEG:
Place cerebral electrodes in the numbered jacks / unipolar channels or plug the EasyCap JIF Connector into the input module on the amplifier.
  • Plug FPz into the ground (Z / green) electrode position.
  • Plug the both 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.


ELECTRODE Description Jack Position
Ground Fpz Z (green)
Reference Linked Mastoids Ref (red)


If Using a One Channel Bipolar EOG:
Plug the electrodes into a bipolar channel.
EOG ELECTRODE Location Bipolar Channel 56

(Channel 56 is the first Bipolar Channel on the Amplifier)

Lt OC Left Outer Canthus Channel-056 A+ (black)
Lt IO Left Infra Orbital Channel-056 A- (blue)
Note: if recording only EOG (no EEG) you will still need to have a ground electrode.