EEG EQUIPMENT AND PRINCIPLES

Term Paper

By

Mehmet Serdar Teke

ID Number: 260701050-072

BME-102 Introduction to Biomedical Engineering

Section 1

Submitted to: Associate Prof. Ali Ümit Keskin

Yeditepe University

Faculty of Engineering and Architecture

Department of Biomedical Engineering

8 May 2009

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EEG EQUIPMENT AND PRINCIPLES

APPROVED BY:

Ali Ümit Keskin ………………….

DATE OF APPROVAL: 08.05.2009

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LETTER OF TRANSMITTAL

May 8, 2009

Ali Ümit Keskin

Yeditepe University

Dear Mr. Keskin;

I am submitting to you the report, due May 8, 2009, that you requested. The report is

entitled EEG equipment and principles. The purpose of the report is to mention about what

EEG equipment is and how it works. The content of this report concentrates on basic

principles of EEG equipment and which methods are used to solve problems related to it. If

you should have any questions concerning my paper, please contact me.

Sincerely,

Mehmet Serdar Teke

Biomedical Engineering-Double Major Program Student

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TABLE OF CONTENTS

List of Figures.......................................................................................................................iv

List of Symbols/Abbreviations..............................................................................................v

Introduction..........................................................................................................................1

1.Definition and Basic Operational Principles……………………………………………..2

2.Electrodes Detecting the Signals, Generated by Brain’s Neural Activity, at the Scalp….3

2.1.Electrode Positioning System…………………………………………………...3

2.2.Electrode Montages Used to Measure Signals at the Scalp……………………..3

3.Amplifier Unit of EEG………………………………………………………….……….5

3.1.Differential Amplifiers Used to Eliminate the Effects Generated by the

Environment…………………………………………………………………..…5

3.2.Analog Low Pass Filters…………………………………………………….......6

4.Main Unit of EEG…………………………………………………………………....….7

4.1.Analog to Digital Conversion……………………………………………..…….8

4.2.Aliasing……..…………………………………………………………..…….....9

5.Detecting Malfunctions on the Brain Using EEG……………………………………….9

References...........................................................................................................................11

iv

LIST OF FIGURES

Figure 1.Basic Components of an EEG Device...................................................................2

Figure 2.Schematic Diagram of 10-20 System for Electrode Placement.............................3

Figure 3.Example of a Normal Human’s EEG Using Unipolar and Bipolar Montage........4

Figure 4.Principle of Differential Amplifier.........................................................................5

Figure 5.EEG Artifacts Detected between Two Electrodes Named according to 10-20

system.....................................................................................................................6

Figure 6.Amplitude versus Frequency Graphic in a Practical Low Pass Filter……………7

Figure 7.Technical Blog Diagram of an EEG Recorder…………………………………...8

Figure 8.Principle of Analog to Digital Conversion…………………………………….....8

Figure 9.Aliasing:A High Frequency Component(dotted line) mimics a low frequency

component if sampled with a too low rate………………………………………..9

v

LIST OF SYMBOLS/ABBREVIATIONS

EEG Electroencephalogram

EMG Electromyogram

ECG Electrocardiography

DC Direct Current

fn The Highest Frequency Occurring in the Signal

fs Sampling Rate

Hz Hertz-Frequency Unit

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INTRODUCTION

The presence of electrical activity in the brain was discovered by Richard Caton,

who was an English physician, in 1875. His findings were about electrical phenomena on

rabbits’ and monkeys’ brains. Hans Berger, a German neurologist, started to study on

electrical activity of human brain. In 1924, he used his ordinary radio equipment to amplify

the brain’s electrical activity so that he could record it on graph paper. He realized that

rhythmic changes in brain waves varied with the individual’s state of consciousness. In

other words, he made the first EEG device. Thus, he is known as the inventor of EEG. His

findings about the first human electroencephalogram were published in 1929.

In this research, it is aimed to define EEG and to examine its operational principles.

First of all, main operational principles are intended to consider. After that, details of

operational principles of EEG are examined in this report. Electrode positioning system

and electrode montages used to measure signals at the scalp are intended to examine in the

first part of detailed operational principles of EEG. Then, the reasons why differential

amplifiers and analog low pass filters are used are explained in details. After explaining

these reasons, analog to digital conversion and aliasing problem are targeted to discuss.

Finally, another purpose of this research is to go over how to detect malfunctions by using

EEG.

Main problems associated to EEG device are artifacts produced by the ambient

environment and aliasing problem while converting analog signals into digital signals. To

eliminate artifacts produced by the ambient environment, differential amplifiers are used.

The reason why differential amplifiers are used for this purpose is explained in details in

the upcoming part of the report. To solve the aliasing problem, analog low pass filters are

used before digitization. The reason of that is also explained in the upcoming part of this

report.

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1.Definition and Basic Operational Principles

The electrical signals, in other words potentials, generated by the brain’s neural

activity can be observed at the scalp by using suitable amplification methods. The

measured signal is called EEG which stands for electroencephalogram. It is used to

examine global brain function of the person whose brain signals are recorded. However,

brain function related to the performance of specific cognitive tasks is not evaluated using

this method. Therefore, EEG serves to provide initial information about global brain

condition. For clinical examination purposes, the EEG is recorded over a period of

approximately 15 to 20 minutes. While the electrical signals produced by someone’s brain

are recorded, he should sit relaxed in a comfortable chair and keep his eyes closed.

Figure 1.Basic Components of an EEG Device

Basically, EEG is constructed by electrodes, amplifier unit and main unit. Electrodes

at the edge of the cables detect the signals generated by the brain’s neural activity. These

signals are sent to the amplifier unit via cables. Then, they are amplified and filtered in this

unit. Finally, these filtered signals are sent to the main unit which converts these signals

from analog to digital and shows them in a display. This is the main operating principle of

EEG.

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2.Electrodes Detecting the Signals, Generated by Brain’s Neural Activity, at the Scalp

2.1.Electrode Positioning System

In order to serve as an index of the function of a spatially distributed neural network,

the EEG must be recorded from multiple measurement positions distributed over the scalp.

This results in a number of different measured signals. For routine applications, the

measurement positions are arranged according to an international standard which is called

the 10-20 system. This system contains 19 electrodes on the scalp and 2 ear electrodes.

Modern EEG devices record at least 21 different signals, referred to as channels. For

advanced applications, systems that have 32 channels or more, up to 512 channels, are

used.

Figure 2.Schematic Diagram of 10-20 System for Electrode Placement

2.2.Electrode Montages Used to Measure Signals at the Scalp

The voltages between two electrodes in 10-20 system are measured. Therefore,

electrode pairings are important while evaluating brain’s global function. The EEG has

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been examined using a number of different electrode montages. That means the EEG can

be recorded with different types of electrode pairings. Each type of pairing results in

different measured EEG signals. Thus, a different view of EEG topography is obtained

with different types of electrode pairings.

Although the set of montages used within a particular clinical EEG laboratory is

typically standardized, there are still no standard montages with international acceptance

across laboratories. Traditionally, there are two groups of montages which are bipolar and

unipolar montages. The voltages between arbitrary pairs of electrodes are recorded with

bipolar montages while all electrodes in each hemisphere of the head are paired with one

common electrode per hemisphere by using unipolar montages, this common electrode is

usually located at the mastoid or at the ear lap. EEG recordings based on a referential

montage are preferable, because the recorded voltage differences can be recalculated after

recording with respect to a virtual reference electrode.

Figure 3.Example of a Normal Human’s EEG Using Unipolar and Bipolar Montage

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3.Amplifier Unit of EEG

3.1.Differential Amplifiers Used to Eliminate the Effects Generated by the

Environment

The voltages observed at the electrodes are a combination of the EEG and potentials

induced by the ambient environment. These external potentials need to be removed to

evaluate the global brain function in a good way. This signal filtering is made by the

amplifier unit of EEG. External potentials include electrostatic charging of patient’s body

and static electrode potentials. They contribute to the total voltage picked up at the

amplifier input. The amplitude of these external potentials usually exceeds the EEG by

several magnitudes. Thus, differential amplifiers are used to minimize these external

voltages since they magnify only the differences between EEG signals picked up at pairs of

electrodes, and not the absolute amplitudes as shown in figure 3. Since the external

potentials are almost identical at all electrode sites they cancel each other by the use of

differential amplifiers.

Figure 4.Principle of Differential Amplifier

There are different types of artifacts which are power line artifact, electromyogram

(EMG) activity generated by scalp muscles if the patient is not sufficiently relaxed,

movement of electrodes on the skin, eye movements, and electrocardiographic (ECG)

activity embedded in the ongoing EEG. The moving eye, as an electric dipole, generates

slowly varying electrical potentials that are picked up by the EEG electrodes.

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Figure 5.EEG Artifacts Detected between Two Electrodes Named according to 10-20

system

Standard low noise amplifiers with high input impedance are used to amplify the

voltage differences detected between pairs of electrodes. The amplifier is split into two

modules. First stage provides a gain of about 10. Before entering the second stage, the

signals pass a coupling capacitor that removes potential residual high voltage DC

potentials that might occur if electrode potentials are not equal. The overall gain in most

EEG systems is about 10000 to 20000, yielding EEG amplitude of about 1 volt at the

amplifier’s output.

3.2.Analog Low Pass Filters

Signals amplified are sent to a low pass filter in analog EEG. Then, the output is

obtained as the deflection of pens as paper passes underneath. However, most EEG

recorders are digital today. In digital EEG, before sending the signals to a digital low pass

filter they must be converted from analog to digital. However, analog low pass filters are

used before analog to digital conversion to eliminate aliasing effect mentioned in the next

part of the report.

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Figure 6.Amplitude versus Frequency Graphic in a Practical Low Pass Filter

An ideal low pass filter completely eliminates all frequencies above the cut-off

frequency. However, there is a transition band before eliminating all frequencies above the

cut-off frequency in practical low pass filters. Also, all frequencies above the cut-off

frequency cannot be eliminated in practice.

4.Main Unit of EEG

Main unit of EEG is where analog signals generated by the brain’s neural activity

and filtered at the amplifier unit of EEG are converted into digital signals to show them on

a display.

Technical block of an EEG recorder in which unipolar montage is used is shown in

figure 7. One of the inputs of amplifiers is connected to the reference electrode and the

other input is each remaining electrode. After these input signals are amplified by

differential amplifiers and filtered by analog low pass filters, analog to digital conversion

of these analog signals is made in analog to digital converters at the main unit of EEG.

However, some manufacturers provide only one analog to digital converter scanning all

channels periodically by means of an analog multiplexer located between the analog to

digital converter and the low pass filter. During recording, the incoming actual EEG is

displayed on the screen.

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Figure 7.Technical Blog Diagram of an EEG Recorder

4.1.Analog to Digital Conversion

Analog to digital conversion is made by taking samples from the analog signal.

There is a sampling interval which is the time passed between two samples. While the

sampling interval decreases digital signal becomes more accurate because digital signal is

more similar to analog signal as the sampling interval decreases. Sampling rate is the

reverse of sampling interval, which means sampling rate=1/sampling interval.

Figure 8.Principle of Analog to Digital Conversion

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The sampling rate and the number of samples called bits needs to be adapted

appropriately. Also, amplitude range must be determined in a good way to measure the

voltages accurately. In a digital signal, there exist numbers which are equivalent to volts in

an analog signal.

4.2.Aliasing

According to Shannon Nyquist theorem, the minimum sampling rate for an adequate

digital representation of analog signals should be equal to 2*fn , where fn is the highest

frequency occurring in the signal. If this rule is not obeyed, for example fs<2*fn, where fs

is the sampling rate, a component at a frequency f>fn will result in a spurious frequency

component of a lower frequency after digitization, with the frequency of the spurious

component being given by fn-(f-fn).This effect is known as aliasing shown in figure 9. To

prevent such distortion, analog low pass filters with a suitable cut-off frequency are used to

suppress high frequencies before digitization.

Figure 9.Aliasing:A High Frequency Component(dotted line) mimics a low frequency

component if sampled with a too low rate.

5.Detecting Malfunctions on the Brain Using EEG

The ability of EEG to provide a quick overview of global brain function is used for

routine neurological examinations. Also, it is used for long-term brain function monitoring

during operations and in the intensive care units of hospitals.

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The EEG is one of the most sensitive measures for brain death diagnosis. The EEG

can prove brain death if there is clinical evidence for this pathological status. Also, it can

prove that if no brain related electrical activity can be recorded even at an increased

amplifier gain. Moreover, the EEG is able to prove brain death if the low cut-off frequency

is extended to 0.16 Hz and the recording time is extended to 30 minutes.

The EEG is also used to classify the type of epilepsy and to identify the epileptic

focus driving the pathological activity. In general, most epileptic patients exhibit specific

EEG patterns in a restricted brain area, which can be observed in a subset of skull

electrodes located near these areas.

The EEG is a sensitive measure to evaluate coma depth and its development.

Suitable therapy and prognosis depend on the results coming from the EEG since it is used

to examine coma depth and its development. Thus, EEG is a crucial measurement

technique for coma staging.

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REFERENCES

Books:

Biomedical Technology and Devices Handbook edited by James Moore and George

Zouridakis, CRC Press 2004

Encyclopedia of Biomedical Engineering edited by Metin Akay, Wiley 2006

Internet Sources:

http://www.eelab.usyd.edu.au/ELEC3801/notes/electroencephalogram.pdf

http://www.patient.co.uk/pdffiles/pilsL605.pdf

http://bio-medical.com/news_display.cfm?mode=EEG&newsid=5

http://faculty.washington.edu/chudler/hist.html

http://www.epilepsy.org.uk/info/eeg.html

http://www.nlm.nih.gov/medlineplus/ency/article/003931.htm

http://www.measurement.sk/2002/S2/Teplan.pdf