EEG ELECTRODES + TECHNICAL

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Why 10-20 system?

Why 10-20 system?

Developed in 1953 for EEGers because they wanted an international standard format for labelling electrode locations in all languages; allows for serial EEGs to be compared and exchanged among EEGers internationally, and shared through literature.

Theory of 10-20 system

based on relationship between a measured electrode site and underlying cortical structures; Each electrode is given an alphabetic abbreviation which identifies with the lobe or area of the brain to which it refers; Percentages (10,20,25%) are used when measuring, as opposed to absolute distances

Advantages of 10-20 system

Allows for precise location of equally spaced electrode positions on the scalp using identifiable skull landmarks; allows for normal and abnormal differences in head size and shape between different patients ; allows for placement of additional electrodes

Landmarks (4)

Nasion; inion; Rt and Lt preauricular

Nasion

Indentation between forehead and the nose

Inion

Ridge that you can feel, found by running finger up/down back of head down to the neck

Pre-auricular

indentation just above cartilage (tragus)

How many electrodes are used on standard 10-20?

19 standard electrodes (does not include non-cortical electrodes such as EKG, does not include REFERENCE and GROUND)

FP1 FP2 F3 F4 C3 C4 P3 P4 F7 F8 T3 T4 T5 T6 O1 O2 FZ CZ PZ

"z" subscript

Z electrodes indicate they are in the midline

Even vs. Odd numbers

Even = Right Odds = Left

Non cortical leads (commonly used x5)

EOG A1 A2 / M1 M2 SP1 Sp2 EKG

Rules for additional electrode coverage

New electrodes would be labelled the same alphabetical character that it was placed BEHIND and then given ' after the letter

FC'

Behind F; in front C

CP4

behind C4, in front P4

CP6

behind (nonstandard) C6, infront of (nonstandard) P6

Indications for modifications to 10-20 (5)

neonates post Nsx Head injury head/scalp deformities Electrocerebral silence EEGs

Rules for modifications to 10-20

Electrode should be moved to avoid and lesions or tender areas, or if IVs or ICP monitors are present; try to maintain more/less equal electrode distances Should move the homologous electrode on the opposite side Modifications should be annotated as such

Different electrode metals used

Au, Ag-AgCl, Steel, Platinum

Theory of electrodes

Electrodes are transducers that convert energy from a flow of ions (brain tissue) to a flow of electrons (electrode) without losing information content Ideally the electrode will sense all ionic changes that occurs below it (in the brain) but will NOT participate in the flow of current

Electrode impedance

Impedance Is the opposition to A/C flow by all components that oppose current flow • i.e. resistors, inductors, capacitors etc • Symbol = Z • Measured in kΩ

How to check impedance

• Electrode Z is checked by applying a weak AC signal and comparing resistance of each electrode to that of the reference and ground • Z should be checked at onset of every recording to ensure good contact between the electrode and the scalp

Impedance = opposition to the flow of AC current Resistance = opposition to the flow of DC current

Desirable electrode impedances

• Impedance readings should be between 0.1-5kΩ & equal among electrodes

• Very low impedances are undesirable because they act as a shunt/short circuit between electrodes • Very high impedances are undesirable because connecting a high Z and a low Z electrode will create imbalance at the amplifier which then favors electrical interference

Different electrode types

Nasopharyngeal ( INVASIVE- inserted through nostril 2cm, looks @ anterior-mesial surface of T lobe)

Sphenoidal (INVASIVE Platinum wire inserted through cannula, looks @ inferior lateral surface of T lobe / NON INVASIVE on mandibular notch, looks @ anterior T lobe)

Surface (NON INVASIVE, applied with paste on the scalp)

Elecs pros/cons

Advantages: ease of application; closer/deeper recording Disadvantages: may require local anesthesia; allergies to metal; cost; pt discomfort

Pre-amplifier

responsible for accommodating, amplifying and converting the analog electrical signals from the electrode into a digital signal that can be processed by the computer

ADC

Analogue to Digital Conversion

Process at which a digital EEG system converts analogue waveform into a series of numerical values to be digitally displayed.

Allows for (computer) data that is easily copied, shared, transported, manipulated.

Sample Rate

of times per second a waveform is sampled at regular intervals; measured in Hz

Think of horizontal resolution

GUIDELINE: Sampling rate of at least 200 samples per second per channel

Dwell Time

Time between each sample point; Aka intersample interval

Any analogue data in the "dwell time" will be LOST and not seen on digital.

Nyquist Theory

"Minimum Sample Rate required to accurately reproduce analogue waveform must be twice the highest frequency we see" But this isn't actually good enough... In reality, EEG needs 10-2x this.

GUIDELINE: sampling rate must be 3x the HFF setting (70Hz) = 210hz sampling rate at least 200 samples per second per channel

Sample Skew

Occurs when all channels are not sampled simultaneously. The time lag between sampling of each channel is known as sampling skew.

Often not that noticeable, unless we are working with >128inputs

(Computers often sample 8 inputs at time, so groups of 8 inputs will be msec apart)

Aliasing

Distortion of frequencies/amplitude/morphology due to inadequate sampling rate

Amplitude resolution

The accuracy of voltage of analogue signals when digitally reproduced; the vertical axis gets broken up in to increments called "bits". The more bits, the better amplitude resolution. Inadequate # of bits = loss of data in-between the bits

think Vertical Resolution

Bits

Total number of voltage increments along the vertical axis available

2^x where x = bits. 2^12 = 4096 bits.

GUIDELINE: minimum 12bits

Screen resolution

Aka screen display

GUIDELINE: Screen resolution of at least 4-pixels per vertical millimeter and at least 1024 x 1280 pixels requiring no less than a 17 inch monitor

Differential amplifier

Electronic device that discriminates against in-phase/common signals and amplifies out-of-phase/different signals

CMRR

Common Mode Rejection Ratio

How well differential amplifiers can cancel common signals Vo:Vi -- Out-of-phase:In-phase

Optimized by by having low equal electrode impedances and a high amplifier impedance

How to measure CMRR

• Apply an in-phase signal to both inputs • Input enough voltage to produce a 10mm deflections • Apply an out-of-phase signal and see how much input V is needed to get same 10mm deflection

-Ex: if it takes 500mV for in-phase and 50uV for out-of-phase, then500mv (1mv/1000uV):50uV = ratio of 10000:1

Analogue Calibration

A procedure done to ensure the EEG acquisition machine is fit for its purpose.

When we used paper machines, was VERY important follow ALL sequential steps for calibration, including checking the following, as we could not do any post-recording manipulations or modifications

-ink -paper -pen alignment -over/under dampening -LFF/HFF

Digital Calibration

Square Wave calibration; much quicker and less intensive than analogue calibration since our computers do all the "thinking" with their algorithms.

"square wave" is created when the same voltage/frequency is applied to all channels. A successful calibration means ALL channels are showing the same square wave; any deviation from this likely means poor electrode quality or issues with the headbox/jackbox input

Why System reference?

Allows for reformatting of montages (during and after digital recordings) as all electrode inputs are FIRST referred to SysReF, which allows data from each electrode to be collected separately.

System reference is added to 10-20 system, often @ FCz.

Poor system reference will affect ALL channels, and will results in high impedances.,

System reference

EEG uses differential amplifiers which require two inputs. We get around this issue by using a system reference electrode.

Not actually FP1 - A1

Think (Fp1-SysRef) - (A1-SysRef) where each ( ) = differential amplifier

System Reference will cancel out and give us the true value of Fp1 - A1

ohm's law

V=I/R Voltage(v) = current(mA) x resistance (ohms)

Series circuit

a circuit having its parts connected serially; one pathway; constant current

Vt = V1 + V2 it = i1 = i2 Rt = R1 + R2

parallel circuit

a closed circuit in which the current divides into two or more paths before recombining to complete the circuit; constant voltage

Vt = V1 = V2 It = i1 + i2 (1/Rt) = (1/R1) + (1/R2)

capacitor

an electrical device characterized by its capacity to store an electric charge

EEG applications??

Electrodes on a patient's head with paste creates a capacitor (double layer generates a +ve and -veside)

Filter

“an electronic device comprised of capacitors and resistors that discriminate against a particular frequency or band of frequencies”

Filters Act by:

1. Attenuating input signals by decreasing the amplitude gradually

2. Shifting input signal peak in time (phase shift)

FILTERS DO NOT alter the frequency input signals.

3 common types of filters in EEG

HFF LFF Notch filter

LFF

Lower frequency filter; aka high pass filter

“attenuates slow frequencies and allows fast frequencies to pass and be seen at 100% amplitude output”

• Electrical Construction: a resistor and capacitor in series where the voltage is read across the Resistor

• Property of the capacitor

• RULE 🡪 at the turnover frequency/LFF setting, peak is shifted 45° or 1/8th of it’s original duration to the LEFT or EARLIER in time (this is enough to be noticeable on EEG)

HFF

High frequency filter; aka low pass filters

“attenuates fast frequencies and allows slow frequencies to pass and be seen at 100% amplitude output”

• Electrical Construction: A.) a resistor and capacitor in series where the voltage is read across the Capacitor B.) a resistor and capacitor in parallel where the voltage is read across the Resistor

• Property of the capacitor • RULE 🡪 at the turnover frequency/HFF setting, peak is shifted 45° or 1/8th of it’s original duration to the RIGHT or LATER in time (this is enough to be noticeable on EEG)

Why do we need notch filter?

aka 60hz Filter

electrical interference can arise from electrical devices which transport electrical power at a level of 110-230 Volts AC. This power is alternating 50 or 60 times per second and therefore called “alternating current” or AC.

This 50 or 60 Hertz activity can show up in the EEG, especially where the electrode does not make good contact, or where there are simply too many cables and electrical devices around. (stretcher beds, IVs, TVs, ventilators, tube feed pumps, calf compressors, etc).

frequency response curve

“graphical representation of the % amplitude output of a band of frequencies based on a set of low and high frequency filter settings”

bandwidth

area under the curve of frequencies between the LFF and HFF that are relatively unfiltered =100% output amplitude seen/recorded

Turnover frequency

frequency at which 30% of the original signal is attenuated, given a roll-off of -3dB

•Occurs at the LFF and HFF settings

Roll off

The rate at which attenuation of original signal occurs; this is what makes filtering a gradual process; Is a function/process of the capacitor

• Measured in dB

🡪 -3dB gives 30% attenuation at turnover freqs 🡪 -2dB gives 20% attenuation at turnover freqs

Why do we ground patients?

1. to protect pt from injury in the event there is an electrical fault in the equipment.

2. reduces amount of artifact/noise in the recording.

Why do we ground our machine?

To protect pt and tech from electrical shock.

ground loop

Ground loops occur if a patient is connected to more than one piece of equipment using two separate grounds

• If the grounds are not at the same voltage level then one machine has more leakage current than the other and current will flow through the patient to the machine with the lower leakage current • Also one machine develops a short-circuit and a broken chassis ground, current will flow through the patient to the machine with the good ground

How to prevent ground loop?

• The hospital has a policy that ensures that all outlets in patient areas are at the same ground level • Equipotential grounding • There is safe equipment that limits the amount of current that can flow through it: • Isolated jack-box (eeg machine headboxes)

Leakage current

the value electrical current that inherently flows from the energized electrical parts of an instrument to the metal chassis

Normally electrical current flows through ground. If the pathway is broken and someone touches the machine and a grounded device, the current will flow through them to the ground

Results from stray C and R (insulators are not perfect and so some current will flow through them) • Main sources of stray C is through the power cord • Using extension cords increases stray C

Canadian Standards Association (CSA) • At the chassis 100uA • At the electrodes 50uA • At the electrodes of a susceptible patient 20uA

EEG Convention (polarity)

• When two electrodes are attached to input 1 and input 2 of an amplifier, the voltage difference is reflected by a “pen” deflection, either up, down, or not at all.

• EEG convention is what dictates which way the pen deflects.

If input 1 is more negative than input 2, pen deflection is

Upward

If input 1 is more positive than input 2, pen deflection is

downward

If there is no difference between input 1 and input 2

There is no deflection

Contamination

occurs when reference is picking up extraneous or brain activity

Cancellation

occurs when there is little or no difference in activity at two or more electrodes; flat EEG channel

Summation

occurs when activity between electrodes has opposite polarity therefore resulting pen deflection is addition of both

Montage

the way electrodes are logically configured and/or referenced to each other

Polarity

whether activity is positive or negative/up-going or down-going

Derivation

two electrodes referenced to each other in a montage, can be referential or bipolar, also called channel

Localization

process of determining where brainwaves are coming from and which electrodes are most involved and the polarity of the event

Phase Reversal

opposing pen deflections on two channels with common electrode in input 1 in channel 1 and input 2 of channel two

End of Chain

when most active electrode is at the beginning or end of a bipolar chain of electrodes

Field Diagram

head diagram of electrodes with areas of activity mapped by most to least active

Dipole

occurs when a positive and negative event occur at the same time

what is a common epilepsy in pediatrics with dipoles are expected on EEG? Where will the +ve and -ve occur on EEG?

How to localize on bipolar

Phase reversal - common electrode is most active End of chain-electrode - activity at the end of chain is most active, then determine fields,

*always remember to account for any cancellation or summation

How to localize on referential

Timing Amplitude Sharpness

Advantages of Bipolar

Can use phase reversal and end of chain to localize No contamination Good for seeing low amplitude activity and focal activity Similar inter-electrode distances

Disadvantages of Bipolar

Cancellation & Summation=makes localization difficult Difficult to localize generalized/diffuse act Cant assess symmetry due to cancellation

Advantages of Referential

Easy to see spread/field Dipoles easier to see & localize No cancellation=good for generalized activity & symmetry of analogous electrodes

Disadvantages of Bipolar

Contamination can occur Lose low amplitude activity in background Unequal inter-electrode distances

artifact

any potential that does NOT originate in the brain

Artifacts are identified by: Location, Morphology, Lack of field, frequency

Using filters or changing system settings should always be the LAST resort when dealing with artifacts

Non-physiological artifacts

1. external electrical interference (IV poles; stretcher beds; photic lamp, etc)

2. internal electrical malfunction of any part of the EEG machine (headbox/jack box issues)

Physiological artifacts

1. mvmts

2. non-cerebral bioelectrical potentials (eye blinks, talking)

3. skin resistance changes (poor/imbalanced impedances, sweat, bridging)