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 i EEG EQUIPMENT AND PRINCIPLES
APPROVED BY:
Ali Ümit Keskin
………………….
DATE OF APPROVAL: 08.05.2009
ii 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 equipmen equipmentt is and and how it works. works. The conten contentt of this this report report conce concentr ntrate atess on basic 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
iii TABLE OF CONTENTS
List of Figures..................................... Figures............................................................ .............................................. .............................................. ....................................iv .............iv List of Symbols/Abbreviations...................... Symbols/Abbreviations............................................. ................................................. ................................................. .......................vv Introduction................................... Introduction.......................................................... .............................................. .............................................. .........................................1 ..................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........................... References.................................................. .............................................. .............................................. .............................................. ...........................11 ....11
iv LIST OF FIGURES
Figure 1.Basic Components of an EEG Device................................... Device.......................................................... ................................2 .........2 Figure 2.Schematic Diagram of 10-20 System for Electrode Placement.............................3 Placement.............................3 Figure 3.Example of a Normal Human’s EEG Using Unipolar and Bipolar Montage........4 Figure 4.Principle of Differential Amplifier.......................................... Amplifier................................................................. ...............................5 ........5 Figure 5.EEG Artifacts Detected between Two Electrodes Named according to 10-20 system................................ system....................................................... .............................................. .............................................. .......................................6 ................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/ABBREVI SYMBOLS/ABBREVIATIONS ATIONS
EEG
Electroencephalogram
EMG
Electromyogram
ECG
Electrocardiography
DC
Direct Current
f n
The Highest Frequency Occurring in the Signal
f s
Sampling Rate
Hz
Hertz-Frequency Unit
1 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 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.
2 1.Definition and Basic Operational Principles
The electrical signals, in other words potentials, generated by the brain’s neural activ activity ity can be observ observed ed at the scalp scalp by using using suitab suitable le ampli amplific ficati ation on method methods. s. The measur measured ed signa signall is called called EE EEG G which which stands stands for electr electroen oencep cepha halog logram ram.. It is used 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 condit condition ion.. For clinic clinical al exami examinat nation ion purpos purposes, es, the EE EEG G is record recorded ed over over a perio periodd 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.
3 2.Electrodes 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 This result resultss in a number number of differ different ent measur measured ed signal signals. s. For routin routinee applic applicati ations ons,, 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 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 4
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 different measured measured EEG signals. Thus, a different different view of EEG topography topography is obtained 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 5 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 potentials include electrostatic electrostatic charging of patient’s 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) (EMG) activ activity ity genera generated ted by scalp scalp muscle muscless if the patien patientt is not suffic sufficien iently tly relax relaxed, ed, 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. 6
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 signal signalss pass pass a coupl coupling ing capac capacito itorr that that remove removess poten potentia tiall residu residual al high high voltag voltagee 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 obtain obtained ed as the deflec deflectio tionn of pens pens as paper paper passe passess undern underneat eath. h. Howeve However, r, most 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 other input input is each each remain remaining ing electr electrode ode.. After After these these input input signal signalss are amplif amplified ied 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. 8
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*f n , where f n is the highest frequency occurring in the signal. If this rule is not obeyed, for example f s<2*f n, where f s is the sampling rate, a component at a frequency f>f n will result in a spurious frequency component of a lower frequency after digitization, with the frequency of the spurious component being given by f n-(f-f n).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 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 exami examine ne coma coma depth depth and its develo developme pment. nt. Thus, Thus, EEG is a crucia cruciall measur measureme ement nt technique for coma staging.
11 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/ELEC38 http://www.eelab.usyd.edu.au/ELEC3801/notes/electroence 01/notes/electroencephalogram.pdf phalogram.pdf
http://www.patient.co.uk/pdffiles/pilsL605.pdf http://bio-medical.com/news_display.cfm?mo http://bio-medical.com/news_display.cfm?mode=EEG&newsid=5 de=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
