ElectroCardiogram

Electroencephalogram (EEG): Measuring Brain Waves

Electroencephalogram

• Instrument for recording electrical activity that generates in the individual neuron of the brain.

• Effective method for diagnosing many neurological illness and diseases such as epilepsy, tumor, sleeping patterns, mental disorders, etc.

• Electrodes around the head (scalp or cerebral cortex) measures and records activity of the brain.

Where and why is EEG used?

EEG is performed predominantly in Neurology/Neurophysiology, but as it provides a measure of cerebral function, it is also used in other specialities;

Neurology

• Epilepsy

•Head Injury

• Brain Tumor/Lesions

• Cerebrovascular disorders

• Neurosurgery

Psychiatry

• Epilepsy & Related Disorders

• Organic brain diseases

• Developmental Disability

• Head Injury

Pediatrics

• Pediatric Epilepsy

• Down’s Syndrome

• Other diseases/syndromes

Internal Medicine

• Endocrine/Metabolic Diseases

• Hepatic Diseases

• Toxemia

EEG waveforms

Alpha waves - 8-13 Hz

Beta waves - Greater than 13 Hz

Theta waves - 3.5-7.5 Hz

Delta waves - 3 Hz or less

Gamma – 22-30 Hz

Alpha Wave

Characteristics: - frequency: 8-13 Hz-amplitude: 20-60 µV

Easily produced when quietly sitting in relaxed position with eyes closed (few people have trouble producing alpha waves)

Alpha blockade occurs with mental activity

Rhythm > Alpha

Frequency Component

8.1 to 13 Hz

Amplitude Infant: 20µV Child: 50µV Adult: 10µV

Main Scalp Area Occipital - Parietal

Patient Condition

Resting, Eyes Closed

Beta Waves

Characteristics:-frequency: 14-30 Hz-amplitude: 2-20 µV

The most common form of brain waves. Are present during mental thought and activity

Rhythm > Beta

Frequency Component

Over 13 Hz

Amplitude 10 to 20 µV

Main Scalp Area Frontal

Patient Condition

Resting, Eyes Open

Theta Waves

Characteristics:-frequency: 4-7Hz-amplitude: 20-100µV

Believed to be more common in children than adults

Walter Study (1952) found these waves to be related to displeasure, pleasure, and drowsiness

Maulsby (1971) found theta waves with amplitudes of 100µV in babies feeding

Rhythm > Theta

Frequency Component

4.1 to 8 Hz

Amplitude Child: 50µVAdult: 10µV

Main Scalp Area Temporal

Patient Condition

Drowsiness

Delta Waves

Characteristics:-frequency: upto 4Hz-amplitude: 20-200µV

Found during periods of deep sleep in most people

Characterized by very irregular and slow wave patterns

Also useful in detecting tumors and abnormal brain behaviors

Rhythm > Delta

Frequency Component

Up to 4 Hz

Amplitude 100µV

Main Scalp Area Frontal

Patient Condition

Deep Sleep(Adult)

Gamma Waves

Characteristics:-frequency: 36-44Hz-amplitude: 3-5µV

Occur with sudden sensory stimuli

Less Common Waves

Kappa Waves:-frequency: 10Hz-occurred in 30% of subjects while thinking

Lambda Waves:-amplitude: 20-50µV-last 250 msec, related to response of shifting visual image-triangular in shape

Mu Waves:-frequency: 8-13Hz-sharp peeks with rounded negative portions (7% of population)

EEG Electrodes

• Transform ionic currents from cerebral tissue into electrical currents used in EEG preamplifiers.

• Types of electrodes : Peel and stick electrodes Silver plated cup Scalp electrodes Intracerebral needle electrodes

EEG Electrodes ChatracteristicsEEG Electrodes Chatracteristics

One of the keys to recording good EEG signals is the type of electrodes used. Electrodes that make the best contact with a subject's scalp and contain materials that most readily conduct

EEG signals provide the best EEG recordings. SmallSmall Easily affixed to scalpEasily affixed to scalp Remain in place (glue, hat, strap)Remain in place (glue, hat, strap)

The EEG be recorded with Scalp electrodes through the unopened skull or with electrodes on or in the brain.

A normal EEG

Types of EEG electrodes

Reusable disks

These electrodes can be placed close to the scalp, even in a region with hair because they are small. A small amount of conducting gel needs to be used under each disk. The electrodes are held in place by a washable elastic head band. Disks made of tin, silver, and gold are available. They can be cleaned with soap and water or Cidex. The cost of each disk and lead is dependent on the type of metal used as a conductor, the gauge of wire used as a lead, and the type of insulation on the wire lead. Since these electrodes and leads can be used for years, their expense is low.


EEG Caps with disks

Different styles of caps are available with different numbers and types of electrodes. Some caps are available for use with replaceable disks and leads. Gel is injected under each disk through a hole in the back of the disk. Since the disks on a region of the scalp covered with hair cannot be placed as close to the scalp as individual disc electrodes, a greater amount of conducting gel needs to be injected under each. After its use, more time is required to clean the cap and its electrodes, as well as the hair of the subject. Depending on the style and longevity of the cap and the electrodes, their expense can be moderate to high.


Adhesive Gel Electrodes

These are the same disposable silver/silver chloride electrodes used to record ECGs and EMGs, and they can be used with the same snap leads used for recording those signals. These electrodes are an inexpensive solution for recording from regions of the scalp without hair. They cannot be placed close to the scalp in regions with hair, since the adhesive pad around the electrode would attach to hair and not the scalp. When purchased in bulk, their expense is very low.


Subdermal Needles

These are sterilized, single-use needles that are placed under the skin. Needles are available with permanently attached wire leads, where the whole assembly is discarded, or sockets that are attached to lead wires with matching plugs. Since they are a sterile single-use item, the expense of needle electrodes is moderate to high. Also, human subjects and, in some situations, regulatory committees need to approve the use of these electrodes before they are used.


Cup Electrode

Silver plated Reusable Center hole in cup allows for easy fill

of electrode gel after adhesion of cup to scalp

Size : Adult- 10 mm diameter

Pediatric- 6 mm diameter• Most widely used electrodes

Periodic replacement required incase of reusable electrodes.

Electrodes should be cleaned properly Sometimes the signal may be impeded due

to increased impedance of the electrode cable or due to improper contact.

Constraints

Basic Terminology

Montage: patterns of connection between electrodes; usually 16 or more electrodes

Referential: background rhythm interpretation

Electrode position of O1and O2

Commonly used montages

Recording the EEG

In clinical EEG recording, the waveforms are recorded in a series of Montages.

What is a montage?

Each EEG trace is generated from an active and a reference electrode. Different patterns of electrodes are selected and the traces grouped to provide data from different areas of the scalp.

Types of Montage

Common Reference

All active electrodes are referred to a single point, e.g. Linked Ears (LE), or Cz.

Types of MontageAverage Reference

Here, all active electrodes are referred to a calculated average of other electrodes.

Electrodes which are likely to show artefacts are generally not included in this average calculation

Types of Montage Bipolar

Each trace is made up of an electrode referred to a neighboring electrode. Traces are typically grouped in patterns such as anterior-posterior (front to back of head) or transverse (side to side)

Bipolar leads

More popular.

Voltage between the two scalp electrode is recorded.

Types of Montage Laplacian

Also known as Source Derivation, the Laplacian montage shows each active electrode referred to a weighted average of all of its neighbors.

Laplacian montages are very good for localising the focus of abnormal activity

Electrode montage Selector

Analog to Digital Converter

Electrode Test/ Calibrate

Subject

Amplifiers

Hi-Pass Low-Pass

Oscillator Computer

Notch SensitivityFiltetrs

Writer Unit

Chart Drive

Ink writing Oscilloscope

EEG

Jack Box

Preamplifier

Multistage, ac coupled, with differential input and adjustable gain in a wide range.

Calibration signal typical value : 50uV/ cm. High gain , low noise preamplifiers. High CMRR. Free from drift. Noise performance is crucial.

Sensitivity Control

Gain of the amplifier * sensitivity of the writer Gain of the amplifier is set, so that signals of

about 200 uV deflects the pen over their full linear range.

Artifacts cause excessive deflection by charging the coupling capacitors by large voltages.

This can be avoided by using Automatic gain control.

FILTERS

Common EEG artifacts

Electrical mains Eye movement ECG Head movement Muscle Sweating Electrode

Artifacts in EEG Recordings

EEG activity is very small in voltage terms – it is therefore very

easy to record artifacts in the EEG. It is important to be able to determine real EEG

from many kinds of interference.

Artifacts in EEG Recordings

Artifacts

Three sources– 60-cycle noise– Muscle artifact– Eye Movements

Movement in reference lead

Chewing

Vertical eye roll

Excessive muscle – notice saturation of T5

Talking and moving head

Eye Closure and reopening

Blink and Triple blink

Dealing with artifacts

60-cycle noise– Ground subject– 60 Hz Notch filter

Muscle artifact– No gum!– Use headrest– Measure EMG and reject/correct for influence – Statistically control for EMG

Eye movements– Reject ocular deflections including blinks– Computer algorithms for EOG correction

High and low pass filtering

Do not eliminate frequencies of interest Presence of a notch filter tuned at 50Hz,

eliminates mains frequency interference. Undesirable property of Notch filter is

‘Ringing’. Typical range of frequency of standard EEG

machines : 0.1-70Hz

Noise

EEG Amplifiers are selected for minimum noise levels.

2uV is stated as the acceptable figure for EEG reading.

Recorded noise increases with the BW of the system.

Display Device

Strip Chart Recorder Cheaper Limited hours of recording possible

Monitor Accuracy & Precision Post Processing Changes can be done through software

EEG Telemetry system (Joseph.J.Carr)

ElectroCardiogram

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