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8/10/2019 Eeg(Biomed) Boc Dionne

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EEGBy: Dionne Boc


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ELELTROENCEPHALOGRAM

(EEG) a recording of the electrical activity of the brain from the

scalp.

The first recordings were made by Hans Berger in 1929.

Gives a coarse of neural activity and has been used tonon-invasively study the physiology of the brain.

EEG activity is quite small signal, measured in microvolt

with the main frequencies of interest up to

approximately 30Hz.


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Origin of EEG waves

Electrical impulses

generated by nerve

firings in the brain

can be measured byelectrodes placed

on the scalp.


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EEG Waves

Signal range: 2 V (brain death)several hundred V.

The frequency bands in EEG:

Alpha wave -- 813 Hz.

Beta wave -- >13 Hz. (1430 Hz.)

Theta wave -- 47.5 Hz.

Delta waves13.5 Hz.

D T A B


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Different types of brain waves

in normal EEG


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Alpha wave

rhythmic, 8-13 Hz

mostly on occipital lobe

20-200 V

normal,

relaxed awake rhythm with eyes closed


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Beta wave

irregular, 14-30 Hz

mostly on temporal and frontal lobe

mental activity

excitement


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Theta wave

rhythmic, 4-7 Hz

Drowsy, sleep


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Delta wave

slow, < 3.5 Hz

in adults

normal sleep rhythm


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Rhythm Amplitude

(uV)

Recording

& Location

Alpha() 50 100 Adults, rest, eyes closed.Occipital region

Beta() 20 Adult, mental activityFrontal region

Theta() Above 50 Children, drowsy adult,emotional distress

Occipital

Delta() Above 50 Children in sleep


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Components

Electrode

Amplifiers

Filters

Recording unit


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EEG Electrode

Each electrode site is labeled with a letter and a number.

The letter refers to the area of brain underlying the

electrode

e.g. F - Frontal lobe and T - Temporal lobe.

Even numbers denote the right side of the head and

Odd numbers the left side of the head.

Electrolyte: may be a conducting solution (gel or paste)

or may be fluid of living tissue as when electrodeinserted below skin. It is a salt solution, sodium chloride.

This means that current flow within brain becomes

electron flow in the electrodes and electrode wires.


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EEG Electrode Placement

Two types of EEG signals:

Scalpelectrodes are placed on the scalp with good mechanical

and electrical contact.

IntracranialEEG signal is obtained by special electrodes

implanted in the brain surface during surgery. EEG system cover all parts of brain by placing electrodes on all

parts of the head.

Two systems of electrode placement:

10-20 international system: includes 21 electrodes.

10-10 international system: includes 64 electrodes.


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Circuit Design


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Block Diagram


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Block Diagram


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Differential Amplifier

Measure the voltage difference between the two signals

at each of its inputs.


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Electrode Montages

Bipolar: The potential difference is measured between

two active electrodes.


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Filters

Notch filtera band pass filter

around the power

lines frequency (50 or 60 Hz).


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Use of EEG

Epilepsy Generalized (grandmal) seizures.

Absence (petitmal) seizures.

Localize brain tumors.

Sleep disorders (Polysomnography) Narcolepsy

Sleep apnea syndrome

Insomnia and parasomnia

Helpful in knowing the cortical activity, toxicity,

hypoxia and encephalopathy &

Determination of brain death.

Flat EEG(absence of electrical activity) on two records run24 hrs apart.


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Advantages and Disadvantages

EEG has two clear advantages:

The first is characteristic of any electrical recording

systemhigh precision time measurements. Changes

in the brains electrical activity occur very quickly, and

extremely high time resolution is required to

determine the precise moments at which these

electrical events take place.

Unlike other electrical recording devices that require

inserting electrodes into the brain, EEG electrodes aresimply stuck onto the scalp. It is therefore a non-

invasive procedure.


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Advantages and Disadvantages

The main disadvantage of EEG recording is poor spatial

resolution. The EEG signal is not useful for pinpointing the

exact source of the activity.


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EEG Artifacts

Biological artifacts

Eye artifacts (including eyeball, ocular muscles and

eyelid)

ECG artifacts

EMG artifacts

Glossokinetic artifacts (minor tongue movements)

External artifacts

Movement by the patient

settling of the electrodes

Poor grounding of the EEG electrodes

the presence of an IV drip


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