Electroencephalography (EEG)

Electroencephalography (EEG)

• Electroencephalography (EEG) is a record of electric signals

produced by synchronous action of brain cells.

Electroencephalograph system diagram [4]

EEG with Special Electrodes

• Sphenoidal Electrodes • Foramen Ovale Electrodes

EEG with Special Electrodes

• Epidural Electrodes • Subdural Electrodes

EEG with Special Electrodes

• Depth Electrodes • Nasopharyngeal

Electrodes

EEG with Special Electrodes

Scalp EEG Recording Methods

• Electrodes properties and general technical requirements

– Silver/silver chloride (Ag/AgCl), gold, tin, platinum or other metals, which do not

interact chemically with the scalp. Only high- purity metals are used.

– Low contact impedances (less than 5 k). Very low impedances of less than 100

Ω usually relate to accidental short-circuits between electrodes caused by

electrolytic gel or other conductive materials bridging the gap between sites.

– Use a fairly large contact area and using amplifiers with high input impedance

(problematic when trying to record low frequency EEG), which keeps the current

density low to reduce polarization and bias potentials (a result of the exchange of

metal ions and electrolytes in the absence of current flow) (Fisch, 1991 and

Webster 1999).

– A ground electrode should be added and connected properly

– All electrodes should be made of the same material.

– Care in storing and cleaning is necessary to prevent surface contamination.

– Electrode pastes and gels should be protected from contaminated from foreign

metal ion.

– The sensitivity of amplifiers should initially be set at 5-10µV/mm and then

adjusted as necessary.

Electrodes Positions • Electrode positions in 10-20

system. (Malmivuo and Plonsey,

1995)

• In the standard 10-20 system,

there are 19 EEG sites plus 2 ear

references.

– F, T, C, P, and O = Frontal, Temporal,

Central, Parietal and Occipital.

– Even numbers (2, 4, 6, and 8) = right

hemisphere.

– Odd numbers (1, 3, 5, and 7) = left

hemisphere.

– z = midline.

(smaller the numbers are

closer to the midline)

Electrodes Placements

Multichannel EEG Recordings

• Referential montages or monopolar montages: the reference

electrode may be placed on the earlobe or the mandibular angle.

Usually, this electrode is connected to the negative input of the

amplifier.

• Bipolar montages: a pair of different electrodes are connected to

the positive and negative poles of each amplifier,

• Laplacian montage. It references each electrode to the four closest

neighbours.

• Common Average Reference (CAR) uses the average of signals

from every electrode as a reference signal.

• Others

• Monitoring of extra cerebral activity

Brain Anatomy [1]

Hemispheric Lateralization [1]

Sources of EEG

• The sources of EEG are the summed extracellular

synaptic potential fields generated by inhibitory and

excitatory postsynaptic activity of nerve cells in the

cerebral cortex and its underlying nuclei in response to

various kinds of input.

• The amplitude of an EEG measured with the scalp

electrodes is approximately 50 µV to 200 µV (Webster,

1999).

Neuron

Generation of Action Potential

Sources of EEG

• The action potential in the

afferent neuron causes the

release of a neurotransmitter

from its nerve terminal that

diffuses across the synaptic

cleft. This causes a local

change in the postsynaptic

potential. The potential

difference between the

postsynaptic membrane and

other parts of the neuronal

membrane causes an

electrical current to flow along

the neuronal membrane and

to change the membrane

potential of the perikaryon.

Generation of EEG (Events at a

Cholinergic Synapse)

1. An Action Potential

Arrives and Depolarizes

the Synaptic Terminal.

2. Extracellular Calcium

Ions Enter the Synaptic

Terminal, Triggering the

Exocytosis of ACh.

Generation of EEG (Events at a

Cholinergic Synapse)

3. ACh Binds to

Receptors and

Depolarizes the

Postsynaptic

Membrane.

4. ACh Is Removed

by AChE

EEGs

The Rhythmic Activity

• Alpha waves - the most prevalent normal brainwave in the EEG of a person who is

awake but relaxed.

– Frequency 8 - 12 Hz .

– Maximum amplitude over the posterior head region.

– Decrease in adult life. Disappear in drowsiness and sleep and can be blocked by eye opening.

(Berger, 1929).

• Beta waves-the normal brainwave in the EEG, which is characteristic for alertness,

focused attention, concentration or even stress and psychological tension.

– Frequency 12 - 30 Hz.

– Very low amplitude.

– Disappear in drowsiness and sleep (Berger, 1929).

The Rhythmic Activity

• Theta waves-the normal brainwave in the encephalogram of a person who is awake

but relaxed and drowsy.

– Frequency 4 - 8 Hz.

– Low amplitude.

– Children, stressed adults, during rapid-eye-movement sleep.

– The presence of theta waves under other circumstances may indicate an underlying brain

disorder (Walter and Dovey, 1944).

• Delta waves are normally seen in the EEG of a person in deep dreamless sleep or

an awake infant. It also presents during states of high conscious focused attention.

– Large amplitude (75µV to 200 µVp-p)

– Low frequency (0.5 - 4 Hz).

The Rhythmic Activity

• Gamma waves is associated with perception and consciousness (Keiser and

Lutzenberger, 2003).

– Occur as high frequency bursts with a frequency more than 35 Hz (30-100 Hz)

– Sometimes, it is included in beta waves classification (Jasper and Andrews, 1938).

• µ (mu) waves are EEG waves that show a shape suggestive of a wicket fence with

sharp tips and rounded bases.

– Frequency is generally half of the fast activity present.

– Younger adults (the central part of the head over the motor cortex)

– Can be blocked by movement, by intention to move, or by tactile stimuli.

– The EEG is not abnormal if it shows only a few trains of µ waves in one side (Sterman et al,

1974).

The Rhythmic Activity

The Rhythmic Activity

a) Normal, awake EEG; similar features

between hemispheres; and no epileptiform

activity.

b) Abnormal discharge called a generalized

spike and wave. This EEG pattern is typical

for absence seizures.

c) Abnormal discharge called focal spike. This

examples occurs over the right temporal

region of the brain.

a b

c

ERP and ERSP

• ERP (Event-related Potential) and ERSP (Event-related

Perturbation)

ERP and ERSP of the EEG

related to the real movement

transition from 0, Subject No.4,

electrode C3, CAR montage.

The time at 0 s was the

movement onset time.

ERP and ERSP of the

Independent Components

representing a typical auditory

response

Artefacts • Blinking and other eye movements. Eye movement artefacts usually

are identified by their frontal distribution, symmetry, amplitude, and their

characteristic shape.

• Muscle artefact. Muscle activity causes very brisk potentials which

usually reappear.

• Movement artefact. Movement artefacts are often erratic and not

repetitive unless the movement is rhythmical or is triggered.

• ECG. Potential changes generated by the heart are picked up in the

EEG mainly in recordings with wide inter-electrode distances, especially

in linkages across the head and to the left ear, and in subjects with

short necks. It is rarely a problem in bipolar montages.

• Pulse wave artefacts. An electrode may pick up periodic waves of

smooth or triangular shape, on or near a scalp artery due to pulse wave

producing slight changes of the electrical contact between electrode

and scalp.

Artefacts

• Perspiration artefact consists of slow waveforms that are normally

longer than 2 seconds.

• Galvanic skin response consists of slow waves, each with the period

of 0.5-1 second

• Movement of the tongue and other oropharyngeal structures may

produce irregular or repetitive slow waves in a wide distribution, often

with a maximum in the middle of the head.

• Dental restorations with dissimilar metals may produce spike-like

artefacts whenever the metal pieces are moved against each other.

• External electrical interference from the other power sources such as

power lines or electrical equipment. Normally, this form of noise is

observed as a 50 Hz or 60 Hz signal.

• Internal electrical malfunctioning

• Skin temperature difference at electrode sites can cause residual

potentials (depends on electrode’s temperature coefficient).

Deep Brain stimulation for

movement disorders

Deep Brain Stimulation • Parkinson’s Disease

Midbrain neurons project to the putamen and caudate, where

they release dopamine. When more than half of the dopaminergic

nerve terminals are affected, the motor impairments of

Parkinson’s disease arise. (resting tremor, rigidity, slowed

movement, decreased dexterity, small handwriting, flexed

posture, gait disorder, and imbalance .dementia can develop over

several years.

• Essential Tremor

begins in the arms and then spreads

to these other regions. not present

at rest.

• Dystonia

Deep Brain Stimulator System

Diagram

• Deep brain stimulation (DBS)

It is now hypothesized that DBS

increases output from the stimulated

structure, in addition to suppressing local

neuronal activity. The exact mechanism of

action remains a matter of debate (Hiner

et al., 2009). It is nondestructive,

reversible, and adjustable.

Deep Brain Stimulator

• Patient Programmer: on-off, battery

status, stimulator status, change

therapy settings (basic).

• Clinician Programmer: advance

therapy settings.

• Implantable pulse generator (IPG)

and electrodes.

• Battery: Primary cell (based on

lithium thionyl chloride (Li/SOCl2)

chemistry), lasted 3-5 yrs (bilateral

stimulation). Or, Rechargeable

secondary Li ion (9 yrs).

Electrodes

• Conductor wire Platinum-iridium,

Conductor wire insulation,

Electrodes Platinum-iridium.

• Diameter 1.27 mm; lead length 10-50 cm.

• Four electrodes at the end of a lead are

spaced either 0.5 or 1.5 mm apart

(Medtronic, 2008) and span a distance of

7.5 to 10.5 mm of brain tissue.

• Some DBS centers outside the United

States commonly use 5-contact

electrodes. (Hardesty, 2006)

Immunity from Electromagnetic

Interference

• must not be susceptible to electrical

influences due to external electromagnetic

fields in the range of 10 Hz-30 MHz.

• ANSI/AAMI/ISO 14708-3:2008 assess

protection from static magnetic fields,

electromagnetic fields in the range of 30-

450 MHz, and electromagnetic fields in the

range of 450 MHz3 GHz (AAMI, 2009).

Waveforms

• A rectangular waveform of adjustable duration (pulse width:

60–450 μs) and amplitude (voltage difference between anode

and cathode: 0–10.5 V). Typical settings are 2.5-3.5 V, 60-

120 μs pulse width, and 130-185 pulses/s (Medtronic, 2008).

• Reductions in tremor are typically observed only when the

frequency of stimulation is > 90Hz; conversely, low-frequency

DBS (<50Hz) often worsens symptoms [b]

• Average rate of 130 Hz was more effective at reducing tremor

when pulses were evenly spaced (Birdno et. al, 2007).

• Waveforms must be biphasic to prevent tissue damage.

• The cathodic (negative) phase of the waveform has the

greatest effect on neural activation.

Waveforms

Indication Stimulation Target Site

Dystonia Unilateral or bilateral STN or GPi

Essential Tremor Unilateral VIM

Parkinson’s Disease Bilateral STN or GPi

In vivo stimulus waveform recordings. A) Voltage-

controlled (IPG) and current-controlled stimulus

waveforms at 20 Hz and 185 Hz recorded across a 1

kΩ resistor (top) and corresponding voltage responses

recorded in vivo (bottom).

B) The peak voltage recorded in vivo was proportional

to the applied stimulus amplitude

Surgery Complications

• Death is exceedingly rare.

• Symptomatic hemorrhage, infection, and seizure each

occur in about 1% to 4% of cases.

• Complication rates increase with longer operations and

multiple electrode insertions.

• Keeping leads externalized for more during assessment

of stimulation effects increases the risk of infection.

• Repeat surgery may be necessary for subsequent

complications, such as electrode migration, skin erosion,

lead fracture, or battery failure. Batteries usually must be

replaced every 3 to 5 years.

Results

• In patients with advanced PD, bilateral stimulation of

STN or GPi over 3 months was associated with

significant improvement in motor function [a].

• Trial of 255 patients over 6 months of bilateral

stimulation showed the improvement in motor function.

However, this study demonstrated that DBS is

associated with increased risks of falls and dystonia

events (Weaver et al., 2009).

• In 21 explanted leads, recovered after 331 months from

patients, foreign body giant cells were observed (Moss et

al., 2004). Increased tissue impedance over time may

contribute to the required increase in stimulation current.

Contraindications

• Patients who will be exposed to MRI using a full body radio-

frequency (RF) coil or a head transmit coil that extends over

the chest area.

• Patients who are unable to properly operate the

neurostimulator.

• Patients for whom test stimulation is unsuccessful.

• Diathermy (e.g., shortwave diathermy, microwave diathermy or

therapeutic ultrasound diathermy), and MRI (sometimes) can

cause tissue damage resulted from heating of DBS electrodes

due to excessive energy deposition and can result in severe

injury or death (Dommerholt and Issa 2001).

• Transcranial Magnetic Stimulation (TMS).

Warnings, Precautions

• There is a potential risk of tissue damage using stimulation

parameter settings of high amplitudes and wide pulse widths.

• Extreme care should be used with lead implantation in patients with

a heightened risk of intracranial hemorrhage.

• The lead-extension connector should not be placed in the soft

tissues of the neck due to an increased incidence of lead fracture.

• Theft detectors and security screening devices may cause

stimulation to switch ON or OFF, and may cause some patients to

experience a momentary increase in perceived stimulation.

• For patients with Dystonia, age of implant is suggested to be that at

which brain growth is approximately 90% complete or above.

• Abrupt cessation of stimulation should be avoided as it may cause a

return of disease symptoms, in some cases with an greater intensity

(“rebound” effect).

Adverse Events

• Depression, suicidal ideations and suicide have been

reported, although no direct cause and effect relationship

has been established.

• Stimulation not effective, cognitive disorders, pain,

dyskinesia, dystonia, speech disorders including

dysarthria, infection, paresthesia, intracranial

hemorrhage, electromagnetic interference,

cardiovascular events, visual disturbances, sensory

disturbances, device migration, paresis/asthenia,

abnormal gait, incoordination, headaches, lead

repositioning, thinking abnormal, device explant,

hemiplegia, lead fracture, seizures, respiratory events,

and shocking or jolting stimulation.

Target Localization 1. The patient’s head is fit with a rigid frame that provides a three-

dimensional coordinate system for target localization. The patient’s

brain is imaged using magnetic resonance imaging (MRI).

2. After the brain is exposed, a platinum-iridium alloy, insulated with

glass, microelectrode recording (MER) is advanced from the

cortical surface to the target. As the electrode tip passes, voltage

characteristics of the signal enable structures to be identified.

Target Localization

3. Associated movement, is used to fine-tune the microelectrode

location. The clinician listens for the modulation of action

potential discharge in relation to passive movements of

opposite side limbs. (Sillay & Starr, 2009).

Programming

4. The leads are then secured in a

lead anchoring device on the

skull, before the pulse generator

is implanted several weeks later.

Many programmers wait 2 to 4

weeks to allow for resorption of

edema near the lead. Settings

are programmed on a handheld

computer that communicates to

the IPG via a magnetic interface.

References

1. Fundamental of Anatomy and Physiology, Frederic H.

Martini

2. Biomedical Instrumentation: Application and Design, John G.

Webster

3. Introduction to Medical Electronics Applications, D. Jennings

4. Medical Device Technologies: A Systems Based Overview

Using Engineering Standards, Gail D. Baura

5. Bioimpedance and Bioelectricity Basics, Orjan G. Martinsen

6. The Biomedical Engineering Handbook, Joseph D. Bronzino

7. www.medtronic.com