Electroencephalograph Machine

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ElectroencephalographyMachine

KoushaTalebian

February25,2013

BMEG550ProjectReport


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ExecutiveSummary

An electroencephalography machine is an instrument that measures the neural

activityofthebrain.Themachineconsistsofelectrodes,anamplifier,filters,analog

todigitalconvertor, acomputermodule, and amonitoringdevice.The amplifiers,

filters, analog to digital convertor are all part of the same circuitry. The biggest

challengetotheEEGmachineisnotthedatacollection,butratherthephysiological

interpretationofthedata.Sincethecollectedsignalcontainsartifactsfromdifferent

biopotentials, isolating the EEG signal is to some measures a difficult task.

Measuring these biopotentials simultaneously using extra electrodes, and then

removingthemfromthesignaliscommonpractice.

The specification of an EEG machine is not universal, as different applications

requireslightlymodifiedEEGmachine.ThefrequencyofanEEGfallsbetween0.01

to70Hz,andanEEGmachineshouldatleastrespondtothisband.Thehighercutoff

frequencymaybelargeriffastsamplingrateisneeded.Theelectrodesthemselves

requireaveryinformlowresistancefornosignaldistortion.Theamplifiersinput

resistanceontheotherhand,shouldbeashighaspossiblewithacommon-mode

rejection-ratioofatleast100dB.

AnEEGsignalconsistsof4wavelets,labeledasalpha,beta,deltaandtheta,covering

the frequency range of 0.01 to 70Hz. Each has its own characteristics and is

correlatedwitha certainphysiological feature. The digitalizeddata isoften then

decomposed into these wavelets and the doctor can interpret the results and

determinethecauseofproblemwiththepatientsbrain.


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TableofContents

ListofFigures............................................................................................................................4

ListofAbbreviation.................................................................................................................51. Introduction.......................................................................................................................61.1. History......................................................................................................................................6

2. SourceofEEGActivity.....................................................................................................82.1. AlphaWave.............................................................................................................................92.2. BetaWave.............................................................................................................................102.3. DeltaWave...........................................................................................................................102.4. ThetaWave...........................................................................................................................10

3. UsagesofEEG..................................................................................................................11 3.1. ClinicalUsage.......................................................................................................................11

3.2. ResearchUsage...................................................................................................................124. EEGMachine....................................................................................................................134.1. Electrode...............................................................................................................................134.2. AmplifiersandFilters.......................................................................................................174.3. A/DConvertor.....................................................................................................................214.4. ComputerProgram/Monitoring...................................................................................214.5. Calibration............................................................................................................................224.6. TechnicalSpecification.....................................................................................................23

5. EEGSignal........................................................................................................................25 5.1. SensitivityDistributionofEEGSignal.........................................................................255.2. BehavioroftheEEGSignal..............................................................................................26

5.3. BasicPrinciplesofEEGDiagnostics.............................................................................286. Risks&Hazards.............................................................................................................29

7. Artifacts............................................................................................................................307.1. PatientArtifacts..................................................................................................................307.2. TechnicalArtifacts.............................................................................................................307.3. ArtifactCorrection.............................................................................................................30

Conclusion...............................................................................................................................31


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ListofFiguresFigure1:Frequencyandvoltagerangeofsomeofthemostcommonbiopotential

signals(Webster,2010)................................................................................................................6

Figure2:FirstrecordingofEEGperformedbyDr.Bergerin1924(Wallis,2007)......7Figure3:Theconnectionoftwoneurons.TheEEGmachinemeasurestheSP

produced(Adeli&Ghosh-Dastidar,2010)...........................................................................8Figure4:ThefourmajortypesofEEGwaves(Webster,2010)............................................9

Figure5:Theeffectofopeningandclosingtheeyereplacesthealphawavesby

asynchronoushigherfrequencywaves(Webster,2010)...........................................10Figure6:AtypicalEEGmachine.Theinstrumentontherightistheamplifier/filter

(takenfromhttp://emgneedleelectrode.com)................................................................13

Figure7:Asubduralelectrode..........................................................................................................14Figure8:Asurfaceelectrode.............................................................................................................14

Figure9:Anelectrodecap(Teplan,2002)..................................................................................15

Figure10:TheStandardInternational10/20configuration(Kropotov,2005).........16Figure11:TheStandardInternational10/10configuration(Kropotov,2005).........16

Figure12:AnEEGchannelisthedifferencebetweentwoelectrodesvoltage(EBME.co.ukLtd,2008)..............................................................................................................17

Figure13:DifferentialamplifierproducingtheEEGchannel(EBME.co.ukLtd,2008)

...............................................................................................................................................................18Figure14:Calculationshowingwhythedifferencebetweentwochannelsisalways

thedifferencebetweenthetwoelectrodes,irrelevanttothemannerthechannelswereproduced........................................ ............................................ ........................18

Figure15:(A)Bipolarand(B)commonreference.NotethatthemontageoftheEEG

channeldependsonthemeasurementlocation(Malmivuo&Plonsey,1995).19

Figure16:Theamplifier/filtercomponentoftheEEGmachine.Allelectrodesplusthereferenceandthegroundsignalsareinsertedintothismachine.Theoperatorcanusethecomputertoconfigurethedifferentialmannerandview

themontageonthemonitor(PicturecourtesyoftheMitSarMedical)................19

Figure17:AsamplescreenshotoftheMitSarprogramusedtomonitorEEG............22Figure18:D179PerformanceCheckerusedtocalibrateanEEGmachine...................23

Figure19:TheRush-Discrollmodel.Thesensitivitytothebrainregiondecreasesas

theelectrodesmoveclosertoeachother(Malmivuo&Plonsey,1995)..............26Figure20:Summaryofthesignaldecompositionshowinghowdifferentwavesare

extractedfromtheEEGsignal(Adeli&Ghosh-Dastidar,2010)...............................27Figure21:AnexampleofadecomposedEEGsignalintowavelets(Malmivuo&

Plonsey,1995)................................................................................................................................27

Figure22:EEGsignalofanawake,lightsleep,REMsleepanddeepsleeppersonalongwithapersonincoma(Malmivuo&Plonsey,1995)........................................28


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ListofAbbreviation

A/D AnalogtoDigital

AP ActionPotentialC Central

ECG ElectrocardiographyEEG ElectroencephalographyEMG Electromyography

EP EvokedPotentialF Frontal

O Occipital

P PosteriorRef Reference

SNR SignaltoNoiseRatioSP SynapticPotential

T Temporal

Pp. Voltagepeak-peak


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1.IntroductionAnelectroencephalogram(EEG)machineisadeviceusedtocreateadiagramofthe

electricalactivityofthebrain.Themachineisusedbothformedicaldiagnosis(see

section3.1),aswellasresearchapplication(seesection3.2).Onthefigurebelow,

thevoltageandfrequencybandoftheEEGiscomparedwiththeotherbiopotential

signals; thereare significantoverlapsbetween the different signals, and thiswill

leadtobio-artifactsartifactscollectedbythesensorfromabiopotentialsignalthat

isnotintendedfor.

Figure1:Frequencyandvoltagerangeofsomeofthemostcommonbiopotentialsignals(Webster,2010).

1.1.HistoryBy the end of the nineteenth century, scientists were trying to study and

measuretheelectricalpropertyofthebrain.In1875,RichardCatton,aBritish

surgeonfromLiverpool,measuredtheelectricalpotentialoftheexposedcortex

of rabbits and monkeys brain. In 1902, Hans Berger, studied the correlation

between the brain activity and the physiological phenomena (such as sleep,epilepsy,anesthesia).HeusedtheLippmanncapillaryelectrometerandEdelman

galvanometer to record the electrical pulses. However, none produced a

satisfactory result. Itwas finally in1924whereDr.Bergermade the firstEEG

recordingofahumansbrain.


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Figure2:FirstrecordingofEEGperformedbyDr.Bergerin1924(Wallis,2007).

It took another five-years of research by Edgar Douglas Adrian and B. C. H.

Matthews,whoreproducedBergersresults,beforeEEGwasaccepted.In1936,

WalterGrayusedalargenumberofelectrodespastedontothescalptocreatea

map of the brain activity. Areas around a tumor showed abnormal electrical

activity.


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2.SourceofEEGActivityNeurons transmit information throughout the body electrically through the

diffusion ofcalcium,sodiumandpotassium ionsacross the cellmembrane. These

electrical pulses (called action potentials, AP) travel down an axon. At the axon

terminal, the AP causes a synaptic potential (SP) that will be detected by the

dendritesofanotherneuron.SPhaslowervoltageamplitudethanAP,butproducea

currentthathaslargerdistributionthatcanbedetectedonthescalp.

Figure3:Theconnectionoftwoneurons.TheEEGmachinemeasurestheSPproduced(Adeli&Ghosh-Dastidar,

2010).

Undernormalconditions,actionsofthechemicaltransmittersatthepostsynaptic

neuronscontribute very little totheoverallbiopotentialenergyonthe scalp.The

firingoftheseSPareasynchronousandinrandomdirections.Consequently,thenet

resultofthespatialandtemporal influenceofthepotentialenergyonthescalpis

smallandnegligible.Whenapersonperformsaspecifictask,thepartofbrainthatis

associatedwith,firestheSPsynchronouslyanduniformly.Thesepotentials(called

evokedpotentials EP1) have relativelyhigh amplitudes and can bedetectedwith

electrodes.ItisimportanttonotethattheelectricalactivitymonitoredbytheEEG

isthepostsynapticpulseandtheelectrodecollectstheperpendicularcomponentof

thesignal.

1Evokepotentialsarepotentialsduetoastimulus


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TheEEG isdescribedaseither rhythmicor transients.Therhythmicactivitiesare

divided intobandsbasedon the frequencyandare foundatdifferent part of the

humanbrain.Tablebelowsummarizes themain four frequencybands.Thereare

alsothetwomorewavesknownasgammaandmuthatarenotdiscussedinthis

report.

Table1:Comparisonbetweenthealpha,beta,delta,andthetawavegroupoftheEEG(Webster,2010)

Group Frequency(Hz) Location Normally Pathologically

Delta Below3.5 Frontal in adults,

posteriorinchildren

-Sleepingadults

-Babies

-Diffuselesion

-Deepmidline

Theta 4-7 Location not related

toataskathand

-Youngchildren

-Drowsinessandarousal

inadults-Idling

- Focal subcortical

lesions

-Someinstancesofhydrocephalus

Alpha 8-13 Posterior region ofhead

-Relaxing/reflecting-Blinking

-Inhibitioncontrol

-Coma

Beta 13-40 Evenly distributedon both sides, more

towardsthefrontal

-Alert-Active/busy/thinking

- Benzodiazepines(BZs)

Figure4:ThefourmajortypesofEEGwaves(Webster,2010).

2.1.AlphaWaveThesewavesarefoundinvirtuallyall-normalpeopleataquite,transientstate

(i.e.restingstateofthecerebral).Theyusuallyoccurontheoccipitalregion,but


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canalsobefromthefrontalandposteriorofthescalpaswell.Theamplitudeof

these waves ranges form 20 to 200V. When a person sleeps, alpha waves

completely disappear. The alpha waves are also replaced by asynchronous

waves of higher frequency due to sensation or when the person is directed

towardsaspecificactivity.Inthefigurebelow,onecanobservethattheactionof

closingandopeningtheeyecompletelyreplacesthealphawaves.

Figure5:Theeffectofopeningandclosingtheeyereplacesthealphawavesbyasynchronoushigherfrequency

waves(Webster,2010).

2.2.BetaWaveAlthoughthesewavesnormallyoccurbetween13-30Hz,duringintensemental

activity,theycangoashighas50Hz.Theyareusuallyrecordedfromthefrontal

andparietalregionofthescalp.Therearetwotypesofbetawaves:betaIand

betaII.BetaIactsjustasalphawaves.BetaIIontheotherhand,appearduring

intense CNS activity. Thus, beta I is elicited bymental activity, and beta II is

inhibitedbyit.

2.3.DeltaWaveTheseincludetheveryslowEEGwaves.Theyonlyoccurduringdeepsleepor

seriousorganicbraindieses.Theyoccurinthecortex,completelyindependent

ofthelowerregionofthebrain.

2.4.ThetaWaveThesewavesoccurattheparietalandtemporalregionofthescalp.Theyoccur

whenapersonbecomesfrustrated,orwhenanelementofpleasantryisremoved

fromtheperson.


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3.UsagesofEEGAccordingtoR.D.Bickford(Bickford,1987),EEGsignalsareusedto

1. Monitoralertness,comaandbraindeath2. Locateareasofdamagefollowingheadinjury,stroke,tumor,etc.3. Testafferentpathways(byevokedpotentials)4. Monitorcognitiveengagement(alpharhythm)5. Producebiofeedbacksituations,alpha,etc.6. Controlanesthesiadepth(servoanesthesia)27. Investigateepilepsyandlocateseizureorigin8. Testepilepsydrugeffects9. Assistinexperimentalcorticalexcisionofepilepticfocus10.Monitorhumanandanimalbraindevelopment11.Testdrugsforconvulsiveeffects12.Investigatesleepdisorderandphysiology.3.1.ClinicalUsageAroutineEEGrecordingcanbeasshortas20mintoaslongascoupleofdays.It

usuallyinvolvestheattachmentof8+electrodestothescalp,dependingonthe

testandthelevelofresolutionrequired.EEGdeviceswerethefirstmethodfor

detecting tumors.However,MRIandCTscanshavereplacedtheEEGmachine

fordetectionoftumors.

TheEEGrecording isalsoused todistinguishbetweenepileptic seizuresfrom

the other types (psychogenic non-epileptic seizures). It can also be used to

differentiatebetweendeliriumandcatatonia.SincetheEEGwavesareameasure

ofbrainactivity,thenitcanbeusedtoassessbraindeath/coma.ApatientwithsleepdisordercanalsobestationedatthehospitalandbeconnectedtoanEEG

foruptoaweektorecordandanalyzetheirsleeppattern.

2ThisismyresearchthesiswithDr.GuyDumont.TheBISindex,whichisaspectral

analysisofEEG,isusedasthedepthofhypnosis(DOH)signal.Aimofthethesisisdesignanadaptiveclosedloopcontrolofthe(DOH).


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3.2.ResearchUsageEEG is extensively used in neuroscience, cognitive science, and anesthetic

studies.EventhoughEEGhasbeenreplacedbyMRIandCTscans,itisstillused

forcaseswherehighspatialresolutionof


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4.EEGMachineAnEEGmachinecansimplybebrokendownintofourcomponents:theelectrode,

amplifiers,acomputercontrolmodule,andadisplaydevice.Oneoftheadvantages

oftheEEG isthatfor its simplicity,it offers substantial insight intobrainactivity.

Generally,theelectrodesaremanufacturedusingGermansilver:analloymadeupof

copper,nickelandzinc.Germansilverissoftenoughtogrind,andhardenoughnot

tobreak.Alternatively,stainlesssteelissometimesusedforitscorrosionresistant

property. However, since it is a harder material, manufacturing process ismore

challengingandcostly.

Figure6:AtypicalEEGmachine.Theinstrumentontherightistheamplifier/filter(takenfrom

http://emgneedleelectrode.com)

4.1.ElectrodeThereare5basicstypesofelectrodes:

1. Disposable(pre-gelled)2. Reusable3. Headbandandelectrodecap4. Saline-basedelectrode5. Needleelectrode(insertedintothebrain)


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Thefirstfourelectrodesareknownassurfaceelectrodesastheyareattachedto

thescalp.Thelastoneisasubduralelectrode,asthescalpispuncturedandthe

electrodeisplacedintothebraindirectly.

Figure7:Asubduralelectrode

Figure8:Asurfaceelectrode

For amultichannelmontage3, the electrodecap isrecommendedas itreduces

the setup time.Skinpreparationdiffers slightly, but cleaning the oil and dead

skinisrecommended.Forsomeelectrodetypes(speciallythereusableGermany

silverelectrodes),aconductivegelisneeded.

3AmontageisthefinalresultoffiltrationanddecompositionoftheEEGsignal


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Figure9:Anelectrodecap(Teplan,2002)

TheEEGsignal issimply thedifferenceofvoltagebetweentwoelectrodes.To

make the comparison of the signal betweendifferent institutes and hospitals

easier, the CanadianDr. Herpert Jasper has defined an international standard

configuration.CalledtheStandardInternational10/20,thisconfigurationshows

theplacementoftheelectrodesonthepatientshead.Eachelectrodeshouldbe

placed within 10% of the standard. There is also a standard sequence of

measurements (Rahey, 2007). Before the introduction of this standard, EEG

readings were limited in accuracy, as even the same doctor would place the

electrodesatdifferentlocationsoneverypatient.Iftheelectrodesarenotplaced

on the rightposition, then the perpendicular components of the signal to the

electrodescouldbeminimized,leadingtoaweakersignal.

The scalp is divided into regions: F (frontal), C (central), P (posterior), O

(occipital), andT (temporal).The lettersare followedbyoddnumbers for the

leftsideofthebrain,andevennumbersforrightsideofthebrain.Leftandrightsideisdenotedfromthepatientspointofview.


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Figure10:TheStandardInternational10/20configuration(Kropotov,2005)

A higher resolution EEG can be obtained byusing the Standard International

10/10configurationshownbelow.

Figure11:TheStandardInternational10/10configuration(Kropotov,2005)

High input impedanceleadstoincreased signal distortion,whichreducesSNR

andtherefore,theactualsignalbeinglost.TopreventasmallSNR,anelectrode

input impedance of


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With modern EEG instruments, the choice of ground electrode is minimal.

Forhead(Fpz)ornearearlocationiscommonpractice,butlegsandwristsare

sometimes used as well. There are also several different reference electrode

placementmentionedinliterature.Thereferenceelectrodecouldbechosenas

the linkedear, tip of nose, C7, cortex, and etc. Theonly constraint is that the

referenceelectrodemustbeatanelectricallyneutrallocation.Eachchoiceofthe

locationhasitsownadvantageanddisadvantages.

4.2.AmplifiersandFiltersTheEEGmachineusesdifferentialamplifierstoproduceeachchannel.Oneach

input of the differential amplifier, one electrode is connected; the different

betweenthesetwoelectrodesiscalledtheEEGchannel.Inthediagrambelow,theinput1isconnectedtothepositiveterminal,andinput2isconnectedtothe

negativeterminal.

Figure12:AnEEGchannelisthedifferencebetweentwoelectrodesvoltage(EBME.co.ukLtd,2008)

Eachofthechannelsisthensubtractedfromthegroundelectrodetoremovethe

background noise and the low-frequency compartment (signal from the

biopotentialfromotherorgans).Wheninput1ismorenegativetheninput2,the

deflectionisup.


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Figure13:DifferentialamplifierproducingtheEEGchannel(EBME.co.ukLtd,2008)

There are three manners in which the electrodes are connected: common

referencederivation,averagereference,orbipolar.Thedifferencebetweentwochannelsisalwaysthedifferencebetweenthetwoelectrodes;irrelevanttothe

manner the channels were produced. This can easily be seen as followed.

Assumecommon-referencederivationisused.Denotechannel1asF1-Refand

channel2asF4-Ref.Thenchannel1minuschannel2hasthereferencesignal

cancelledandproducesF1-F4.

Figure14:Calculationshowingwhythedifferencebetweentwochannelsisalwaysthedifferencebetweenthe

twoelectrodes,irrelevanttothemannerthechannelswereproduced.

In the common-reference derivation, each amplifier records the difference

between each electrode and the common reference. The reference signal is

usuallytheelectrodebytheears.


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Figure15:(A)Bipolarand(B)commonreference.NotethatthemontageoftheEEGchanneldependsonthe

measurementlocation(Malmivuo&Plonsey,1995)

Intheaverage-referencederivation,theaverage-referencesignalisdenotedas

theaverageofallelectrodes.Theamplifierthenrecordsthedifferencebetween

each individualelectrodeandtheaveragereference. TheEEGmachineallows

theoperatortoremoveanyofthechannelstonotbeincludedinthecalculation

oftheaveragereferencesignal.

In bipolar derivation, each amplifier records the difference between two

electrodes that are opposite of each other. For instance, F3-C3 or C3-P3 are

eitheroppositeortransposeofeachother.ThiswillleadtomaximumSNR,asit

willbediscussedinsection5.1.

Figure16:Theamplifier/filtercomponentoftheEEGmachine.Allelectrodesplusthereferenceandtheground

signalsareinsertedintothismachine.Theoperatorcanusethecomputertoconfigurethedifferentialmanner

andviewthemontageonthemonitor(PicturecourtesyoftheMitSarMedical).

The EEG amplifiers input is equipped with diodes in order to prevent the

voltage to travel backwards, towards the scalp. This is a safety feature and

preventsshocktothepatientincaseofadevicemalfunction.


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The basic requirements that all biomedical amplifiers must have are (Nagel,

1995):

Thephysiologicalprocesstobemonitoredshouldnotbeinfluencedinanywaybytheamplifier

Themeasuredsignalshouldnotbedistorted Theamplifiershouldprovidethebestpossibleseparationofsignal

andinterferences

Theamplifierhastoofferprotectionofthepatientfromanyhazardofelectricalshock

Theamplifieritselfhastobeprotectedagainstdamagesthatmightresultfromhighinputvoltagesastheyoccurduringtheapplication

ofdefibrillatorsorelectrosurgicalinstrumentation

The EEG signal recorded by the electrode consists of five signals: the

desired biopotential, undesired biopotential, power line signal interferenceof

60Hz (the AC power line frequency), interference produced because of bad

connectionbetweentheelectrodeandthescalp,andnoise.

For isolating the electrically noisy environment, a high amplifier input

impedanceofatleast100Misimplemented.Theamplifieralsorequireshaving

ahighcommon-moderejection-ratio(CMMR)ofatleast100dB(Teplan,2002).

The amplification gain is1,000to100,0004(Nagel,1995). Theoptimalgain is

onethatmaximizestheSNR.TherawscalpEEGsignalisabout10-100Vand

10-20mVwhensubduralelectrodesareused.

Filtersareimplementedintheamplification-integratedunittoremovenoiseand

unwantedartifacts.Ahighpassfilterisusedtoremovethelowfrequencynoise

4Thehigherthegaindoesnotmeanthebetterthesignalwillbe.Thereareafew

parameters(suchassamplingrate,theA/Dconversion,thenoise)thatwilldeterminetheoptimalgain


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that remains in the signal after the subtracting the voltage from the ground5

(Teplan, 2002). The cutoff frequency of this high-pass filter is between0.1

0.7Hz(Bronzino,1995).A low-pass filter isused toensurethe signalis band-

limited.Thecutoff frequency isthehighestfrequencyofthesignal,whichfalls

between40Hz(betawave)andhalfthesamplingrate(whichisusually512Hz)

(Bronzino, 1995). If higher frequencies survive, then according to Nyquist

theorem,aliasingcouldoccur.

4.3.A/DConvertorModernEEGmachinesaredigitalandthusrequireA/Dconversions.Asampling

rate of 512Hz is common in clinical practices, and 20kHz in research

applications(Fisch,1999).Abilitytoresolvea0.5Visrecommended(Brunet&

Young,2000).TheresolutionofA/D(R)issimplytheratiobetweenthevoltage

range(VR)and2raisedtothepowerofnumberofbits(b).

=

2!

Afterthedataisdigitalized,theycangounderfurtherdigitalfiltration,according

totheobjectiveofthetestbeingperformed.LinearfilteringsuchasFIRorIIRto

morenovelnon-linearfilteringmethodsisused.Clinicalmachineshavea12bit

A/Dconvertor(Teplan,2002),butresearchconvertorsmightbelargerifhigher

resolutionisrequired.

4.4.ComputerProgram/MonitoringOncethedata isdigitalized,a computerprogramisusedtomonitortheresult.

Computersoftwareisusuallycapableofswitchingbetweendifferentmontages,

performing advancedmathematical calculation (including chaos analysis) and

exports the data. They are equipped to auto detect certain dieses and/or

characteristicsofthesignal.

5Breathingforinstance,causesalow-frequencycomponent


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Figure17:AsamplescreenshotoftheMitSarprogramusedtomonitorEEG.

4.5.CalibrationRoutine calibration is required to model the circuitry, and connection wires

noise as well as determining the exact amplification gain. Usually a known

impulse (sine or triangle) is generated on all of the inputs of themain EEG

amplifier. Thus, the generated signal passes through the entire EEGmachine,

withtheexceptionoftheelectrodes.Theoutputcanberecordedandcompared

withtheinputtodeterminetheamplificationgain,andtomodelthenoise.This

noiseshouldmainlybefromtheamplifiercircuitandtheA/Dconverter.Noise

valueshouldbeconsistentwiththemanufacturerspec,andisusually0.3-2VPP

(Teplan,2002).

Noisecanalsobedeterminedbyshort-circuitingtheinputsoftheamplifier,or

byplacingtheelectrodesintoasalt-bath,andthenmeasuringtheoutput.Since

inputsignaliszero,thentheoutputsignalisthenoisemodel.

Once the noise ismodeled, the number ofuseful bits can easily becomputed.

Takingtheratioof theaveragegeneratedEEGsignal amplitudeoverthenoise

amplitude,thenearestexponentof2isthenumberofusefulbits.Forinstance,if

theratiois50V/1V,thenthereare5usefulbits.


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There are commercial calibrators (known as performance checker), which

generate the requiredcalibratingsignal.More advancedEEGmachineshavea

built-in calibrators. An example of such a device is the D179 Performance

CheckerbyDigitimer.D179has32outputswith1ground/commonsocketand2

referencesockets.TheoutputoftheEEGmachineisconnectedtotheD179,and

thePerformanceCheckerwillautomaticallycomparetheinputtotheoutputand

modelthenoiseandtheamplificationgain.

Figure18:D179PerformanceCheckerusedtocalibrateanEEGmachine

4.6.TechnicalSpecificationThe technical specification of an EEG machine can change depending on the

usageofthedevice.Generally,forresearchpurposes,thedeviceisofahigher

quality.Thefollowingspecificationsaretakenfrom(Ananthi,2006).

AtypicalEEGmachineformedicaldevicesrequirementfornumberofelectrodes

varies significantly depending on the case. For a simple diagnostic, it might

requireonly8,whereastherequirementmightgoashighas256.Foranesthetic

patients,only4arerequired.

The frequency response of the system should be anywhere between 0.05 to

130Hz. The system should have a variable high-pass filter centered at


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frequencies between 0.1-5Hz, a low-pass filter centered at 50-256Hz. There

shouldalsobea50-60HzACfiltertoremovethepowerlinenoise.

ItrequiresaCMMRofatleast100dBwithamplifierinputimpedanceofatleast

100Mandelectroderesistancelessthan5k.

BelowisthespecificationofBWIIEEGmachine:

25channel:22EEGchannels,2Ref,1ground Calibrationonboard:0.5Hz,7Hz,15Hz(squareandsinewave) CMMR~125dB Low/highpassfilter0.0016to70Hz A/D12bitconversion

As a comparison, the government of India requires the following minimum

requirementfromanEEGmachine

32channel Frequencyresponsebetween0.05to70Hz Low-passfilterof15Hz,30Hz,and70Hz High-passfilterof0.1Hz,0.3Hz,1.5Hz,3Hz,5Hz) CMMR>100dB Amplifierinputresistance>10M


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5.EEGSignal5.1.SensitivityDistributionofEEGSignalAnimportantquestionwhenperformingEEGisthesensitivityofthesignal;in

otherwords,howdeepintothebraincanthesignalsbegeneratingfromandstill

bedetected by the EEG instrument? This problem is of no significancewhen

subduralelectrodesareused;itisamatterofdiscussionforsurfaceelectrodes.

RushandDriscollperformedthefirstsuchstudiesin1969.Intheirstudy,the

patients head is modeled as a perfect sphere an assumptiononly partially

valid.Theyplacetwoelectrodesonthepatientshead;oneproducesa current

whiletheothermeasuresthecurrent.Theirresults,whilepioneerinnature,had

limitedpracticality.

In 1987,PuikkonenandMalvimuousedthesamemodelasRushandDriscoll,

butpresentedtheresultswithleadfieldcurrentflowlines.Thedirectionofthe

sensitivity is then simply normal to the field current flow lines and the

magnitude of it is seen from the density of these field lines (Puikkonen &

Malmivuo, 1987). A limitation of this theorem is that it is a 2D model,

representinga3Ddistribution.Thefieldlinesthusbreaktodenoteachangein

the third dimension (see Figure 19. Note how the blue lines suddenly break,

denotingamovementinthethirddimension).

Inthe followingdiagrams, the twoelectrodesare separatedbyanglesof 180,

120,60,40,and20.Inthediagrams,thesolidbluelinesarethefieldcurrent

flow lines, the dotted black lines are the isosensitivity lines, and the green

shaded region isknown asHalf-sensitivity volume the volume atwhich the

fieldcurrentdensityisatleast50%ofitsmaximumvalue.Thelocationofthe

two electrodes isvisible from the highly dense fieldcurrent lines. The sphere

itselfhasthreeshades:theouteristheskin,themiddleistheskullandtheinner


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isthecerebralregion.Astheelectrodesmovecloserandclosertoeachother,the

current flowsmore inthe skin region, thusdecreasing the access tothe brain

itself. The noise also increases as a result (Suihko, Malmivuo, & Eskola). The

conclusionoftheirworkthensuggestsusingelectrodesthatareasfarfromeach

otheraspossibletoachieveoptimalquality(Malmivuo&Plonsey,1995).

Figure19:TheRush-Discrollmodel.Thesensitivitytothebrainregiondecreasesastheelectrodesmovecloser

toeachother(Malmivuo&Plonsey,1995)

5.2.BehavioroftheEEGSignalDigitaldecompositionisperformedonthedigitalizedEEGsignaltoseparatethe

differentEEGwavesfromfordetailedanalysis.Digitaldecompositioncouldbea

simpleband-passfilter.However,sincetheseEEGwavesoverlap infrequency

domain, then advanced Fourier and spectral analysis is required. The signal

needs to go through 3-level of decomposition (shown in figure below), to

successfullyextractthewaves(Adeli&Ghosh-Dastidar,2010).


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Figure20:SummaryofthesignaldecompositionshowinghowdifferentwavesareextractedfromtheEEG

signal(Adeli&Ghosh-Dastidar,2010).

WiththeseparatedEEGwavelets,itispossibletoperformdiagnostics.Belowisa

samplewaveletofanactualEEGsignal.

Figure21:AnexampleofadecomposedEEGsignalintowavelets(Malmivuo&Plonsey,1995).


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5.3.BasicPrinciplesofEEGDiagnosticsThe EEG signal is related to the consciousness lever of an individual. With

increaseactivity,thesignalfrequencyincreasesandamplitudedecreases.With

eyes closed, alphawaves dominate until the personwalls asleep, which then

results in a decrease of the wave. During REM sleep, the signal frequency

increases.Indeepsleep,amplitudeishighandfrequencyislow.Thesignalsof

theseexamplesareshownbelow.

Figure22:EEGsignalofanawake,lightsleep,REMsleepanddeepsleeppersonalongwithapersonincoma

(Malmivuo&Plonsey,1995).


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6.Risks&HazardsEEG machines are non-invasive devices and are risk-free. EEG with subdural

electrodesurprisinglyhasminimaldamageonthebrainif the rightneedle size is

selected. In the case of an epilepsy study, the seizure is sometimes intentionally

induced.However, these triggersare done undermaximummedical care and are

verycontrolled.

TheEEGamplifiersinputisequippedwithdiodesinordertopreventthevoltageto

travelbackwards,towardsthescalp.Thisisasafetyfeatureandpreventsshockto

thepatientincaseofadevicemalfunction.


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7.ArtifactsArtifactsaresignalswithhighamplitudesanddifferentshapesascomparedtothe

desired biopotential signal. They can either be patient artifacts (unwanted

physiological biopotentials), or technical artifacts (power line noise, and

interferencesignals).

7.1.PatientArtifactsThesearethesignalsthatarepickedupfromthescalp,butareoriginatedfrom

the non-cerebral compartment. Many physiological signals can arise from the

patientsbody.Anybodymovements,suchasblinkingtheeye,cancauseaspike

intheEEGsignal.EMG(signalfromskeletalmuscle),andECG(signalfromthe

heart)canalsobepickedbytheEEGelectrodeandcancontributetotheartifact.

7.2.TechnicalArtifactsThesearethesignalsthathaveoriginatedfromtheoutsideofthepatientsbody.

Movement by the patient can cause the electrode to move, and create an

electrode-skin interference. The electrode impedance can sometimes fluctuate

and cause signal distortion. Poor grounding can also cause significant 60Hz

artifacts.

7.3.ArtifactCorrectionTocorrectthepatientartifacts,additionalelectrodestomonitoreyemovement,

theEMGandECGmayberequired.Recently,usingFourieranalysis,signalshave

beendecomposedandthecomponentsrelatedtootherbiopotentialshavebeen

weightedout.Forthetechnicalartifacts,bettergrounding,insulationandhigher

qualityelectrodescanbeusedtominimizethenoisepickedupbythesystem.


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Conclusion

Electroencephalographybelongstoabioelectricalimagingtoolsetsusedwidelyin

bothclinicalaswellasresearchenvironments.EEGmachinemeasureschangesin

the electrical potentials created as the results of excitation of the postsynaptic

neurons.Thesignalitself iscomposedof fourprimarilywaves, denoted asalpha,

beta,theta,anddelta.

EEGmachine isa non-invasivemethod tomeasure the EEG signal (with the rare

occasionwhensubduralelectrodesareused)andcausesnopainandhasminimal

risktothepatient.

TheEEGmachineitselfconsistsofanelectrode(fromeitherGermansilveroractual

silver)coatedwithconductivegel,anamplifierwithagainof1,000100,000with

an input impedance of at least 100dB, analog high-pass filter to remove low

frequencynoisewithacutofffrequencycenteredat0.1-0.7Hz,analoglow-passfilter

toband-limit the signal andremovehigh frequencynoisewitha cutoff frequency

centered at 50Hz to half sampling frequency, at least 12bit A/D convertor, EEG

softwarecapable toperformdigital filtering andwavelet decomposition tocreate

therequiredmontages.

Many believe thatEEGmachinewill lead to awide range of discoveries in basic

brainfunctionalityaswellasbetterunderstandingofneurologicaldiseases.


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