electroencephalograph machine
TRANSCRIPT
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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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