Basic Principles of ElectroencephalographyBasic Principles of Electroencephalography& & MagnetoencephalographyMagnetoencephalography

PoPo--Lei Lee, PhDLei Lee, PhDDepartment of Electrical Engineering, Department of Electrical Engineering, National Central University, TAIWANNational Central University, TAIWAN

YungYung--Yang Lin, MD, PhDYang Lin, MD, PhDNational YangNational Yang--Ming UniversityMing University

Taipei Veterans General Hospital, TAIWANTaipei Veterans General Hospital, TAIWAN

Structure Function

Mentality

Brain, Body, & MindIntroduction

Presurgical Mapping of Vital Areas in the BrainfMRI + MEG/EEG

Tumor

Epileptic foci

Introduction

Visualization & Modulation of

Human Brain Information ProcessingIntroduction

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Integrated Brain Research Unit (IBRU)

MEG (Magnetoencephalography) EEG (Electroencephalography) 3T MRI

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Multimodality•MEG/EEG source localization

•Structure MRI

•Functional MRI

•Brain computer interface (BCI)

•MEG/EEG rhythm analyis

•Clinical studies

Introduction

Basics of electroencephalographyBasics of electroencephalography

In 1875, Richard In 1875, Richard CatonCatondiscovered the EEG discovered the EEG from the exposed from the exposed brains of rabbits and brains of rabbits and monkeys.monkeys.

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In 1925, Hans Berger In 1925, Hans Berger measured human brainmeasured human brain’’s s electrical activity on the electrical activity on the scalp.scalp.

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NeuroelectricNeuroelectric potentialspotentials

Electroencephalogram (EEG)Electroencephalogram (EEG)Evoked potential (EP)Evoked potential (EP)EventEvent--related potential (ERP)related potential (ERP)

Recording site: scalpRecording site: scalpcortexcortexwithin the depth of the brainwithin the depth of the brain

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Overview of signaling between neuronsOverview of signaling between neurons

PostPost--synaptic (synaptic (dendriticdendritic) potentials vs. ) potentials vs. axonal action potentials. axonal action potentials.

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Neural electrical activityNeural electrical activity

Currents of the action potentialCurrents of the action potential

Postsynaptic cellular currentsPostsynaptic cellular currents

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restingpotential

overshoot

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influ

x K +efflux

Action potential

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Postsynaptic cellular currentPostsynaptic cellular current

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Current dipoleCurrent dipole

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Action potential Action potential vsvs Postsynaptic currentPostsynaptic current

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DipolesDipolesThe dipoles make the major The dipoles make the major contribution to the scalp contribution to the scalp potential.potential.When neurons are activated, When neurons are activated, local currents are produced.local currents are produced.EEG measures the currents EEG measures the currents that flow during the excitations that flow during the excitations of the dendrites of many of the dendrites of many pyramidal neurons in the pyramidal neurons in the cerebral cortex.cerebral cortex.Potential differences are Potential differences are caused by summed caused by summed postsynaptic potentials from postsynaptic potentials from pyramidal cells that create pyramidal cells that create dipoles between soma and dipoles between soma and apical dendrites.apical dendrites.

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Necessary conditions: Necessary conditions: Aligned neurons and synchronous activityAligned neurons and synchronous activity

Neurons which are Neurons which are radiallyradiallysymmetric, randomly oriented or symmetric, randomly oriented or asynchronously activated do not asynchronously activated do not produce externally observable produce externally observable electric or magnetic fields.electric or magnetic fields.

Neurons which are nonNeurons which are non--radiallyradiallysymmetric, spatially aligned and symmetric, spatially aligned and synchronously activated add up synchronously activated add up to produce externally observable to produce externally observable electric or magnetic fields.electric or magnetic fields.

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Radial equivalent dipoleRadial equivalent dipole

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Tangential equivalent dipoleTangential equivalent dipole

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International 10International 10--20 system20 system

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EEG analysisEEG analysis

Visual analysis of spontaneous waveformsVisual analysis of spontaneous waveformsEpilepsy, infectious diseases, metabolic disorders, Epilepsy, infectious diseases, metabolic disorders, stroke, degenerative diseasesstroke, degenerative diseases

Spectral analysisSpectral analysisTopographic mapping, dipole modeling Topographic mapping, dipole modeling AEP, VEP, SEPAEP, VEP, SEPERD, ERSERD, ERSP300P300PolysomnographyPolysomnography

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Research and clinical applications Research and clinical applications of the EEGof the EEG

The greatest advantage of EEG is speed. EEG can The greatest advantage of EEG is speed. EEG can determine the relative strengths and positions of determine the relative strengths and positions of electrical activity in different brain regions. electrical activity in different brain regions.

Monitor alertness, coma and brain deathMonitor alertness, coma and brain deathLocate areas of damage following head injury, stroke, tumor, etcLocate areas of damage following head injury, stroke, tumor, etc..Test afferent pathways (by evoked potentials)Test afferent pathways (by evoked potentials)Monitor cognitive engagementMonitor cognitive engagementProduce biofeedback situationsProduce biofeedback situationsControl anesthesia depthControl anesthesia depthInvestigate epilepsy and locate seizure originInvestigate epilepsy and locate seizure originTest epilepsy drug effectsTest epilepsy drug effectsAssist in experimental cortical excision of epileptic focusAssist in experimental cortical excision of epileptic focusMonitor human and animal brain developmentMonitor human and animal brain developmentTest drugs for convulsive effectsTest drugs for convulsive effectsInvestigate sleep disorder and physiologyInvestigate sleep disorder and physiology

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Basics of Basics of magnetoencephalographymagnetoencephalography

Basics of Basics of magnetoencephalographymagnetoencephalography

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“MEG” is short for…Magnetoencephalography

MEG Stands for...● Magnetoencephalography– Magneto = magnetic– Encephalo = brain– Graphy = writing● MEG = the magnetic signals from the brain

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Neuromagnetic fieldsNeuromagnetic fields

MagnetoencephalogramMagnetoencephalogram (MEG)(MEG)Evoked fields (EF)Evoked fields (EF)EventEvent--related fields (ERF)related fields (ERF)

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Overview

● Part 1: Introduction to MEG– What it measures– How it works– MEG recording procedure– Ways to interpret

● Part 2: Clinical and research applications

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Part 1• What MEG measures

– Neuroscience– Physics behind MEG

• How does MEG work?– S.Q.U.I.D. technology– Shielded room

• MEG recording procedure• Ways to interpret MEG

– Dipole models– Distributed models– Other techniques

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Your body and brain are full of nerves which transmit and receive information electrically, producing “magnetic fields” that provide a window into their function

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The axon has bidirectional currents and therefore two opposing dipoles whose magnetic fields effectively cancel each other

The post-synapse has unidirectional current and therefore only has one dipole. The magnetic field is not canceled in this case

Origin of MEG signalWhat MEG measures : Neuroscience

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What MEG measures...

Theory:– MEG measures groups of post-synaptic currents of layer 5 cortical neurons firing synchronously

Practical:– MEG provides maps of where and when groups of certain types of neurons fire– This is a FUNCTIONAL measurement

Gray matter of cat cortex

Layer V

What MEG measures : Neuroscience

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Structure and functionWhat MEG measures : Neuroscience

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A current dipole can be thought of as a battery that has a positive and a negative pole. Electrical current flows from positive to negative. Because there are two poles, we call dipole. It sets up…

- Electric field

- Magnetic field

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Current dipoleWhat MEG measures : Physics

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Electrical currents generate magnetic fields

The magnetic field lines make a ring around the direction of the current flow

Magnetic field and right hand rule

Point right thumb in direction of current, and fingers curl around direction of magnetic field

What MEG measures : Physics

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Image your head was a sphere.Put a dipole in it.The dipole can be: Radial Tangential Partly radialPartly tangential

Dipole in a sphere

MEG will see the tangential dipoles in the brain. But not the radial ones

What MEG measures : Physics

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Tangential dipoles are tangential to the SKULL; NOTnecessarily to the brain

Thus dipoles which are radial to brain surface can be seen if they are tangential to skull

MEG is blind to…What MEG measures : Physics

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MEGMEG v.s. EEG in signal detectionv.s. EEG in signal detection

The scalp and skull, which distort the electric The scalp and skull, which distort the electric potentials, are transparent for magnetic fields. potentials, are transparent for magnetic fields. Thus, MEG can pick up neuronal activities Thus, MEG can pick up neuronal activities ‘‘directly through the skulldirectly through the skull’’..

MEG is mainly sensitive to MEG is mainly sensitive to tangential sourcestangential sources, , whereas EEG reflects all intracranial currents.whereas EEG reflects all intracranial currents.

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Urban Noise

Car at 50 m

Screwdriver at 5 m

Transistor, IC chip at 2 m

Human heartSkeletal musclesFetal heartHuman eyeHuman Brain (alpha)Human Brain (evoked response)SQUID System Noise level

B (Teslas)

Magnetic fieldsWhat MEG measures : Physics

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How does MEG work?

Two key technologies:

- S.Q.U.I.D. Sensors

- Shielded Room

Kamerlingh Onnes

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Faraday’s Law

• Make a loop of wire

• Pass magnetic field through this loop

• A current will be induced

- This is a magnetic flux to current converter

The voltage on the loop changes as a function of the magnetic flux passing through it.

How does MEG work : SQUID

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Superconductors? Why?

• To boost sensitivity

• Brain signal < 10-12 Tesla – VERY SMALL

• Superconductors require extreme cold – Use liquid helium (-2700C)

How does MEG work : SQUID

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• Remove extraneous magnetic fields

• Of interest:- Brain Response < 10-12 Tesla- VERY SMALL

• Extraneous:- Earth’s Magnetic Field ≈ 5*10-5 Tesla- TOO BIG

Shielded roomHow does MEG work : Shielded Room

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Properties of MEG

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3D digitizer• Place 4 coils and EOG electrodes

• Landmark: Nasion, RPA and LPA

MEG recording procedure

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MEG Recording•Whole-scalp 306 channel neuromagnetometer (VectorviewTM, Elekta Neuromag, Helsinki, Finland)

•Continuous and transient real-time data acquisition

•Optional 64 EEG channels•8 trigger lines•4 order low-pass filter: 30…1000 Hz•High-pass filter: down to DC•Head Position Indicator (Polhemus Isotrak II)

MEG recording procedure

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Ways to interpret MEG– Dipole models

– Distributed models

– Other techniques

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Data Y Current density J

Inverse problem(ill-posed)

Inverse problem(ill-posed)

Forward problem(well-posed)Y = K(J) + E

Forward problem(well-posed)Y = K(J) + E

Dipole modelsWays to interpret MEG: Dipole models

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Ways to interpret MEG: Dipole models

Auditory neuromagnetic responses

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Posterior

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MEG-MRI Coordination

LPA Nasion RPA

Ways to interpret MEG: Dipole models

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Source modelingWays to interpret MEG: Dipole models

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2 Dipoles modelWays to interpret MEG: Dipole models

Right dipole

Left dipole

Goodness(%)

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Fitting result

latency dipole location

dipole strength

goodness

Ways to interpret MEG: Dipole models

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Noise problems

Apply SSP

1-40 Hz Bandpass

Filter

2-40 Hz Bandpass

Filter

Ways to interpret MEG: Dipole models

External magnetic disturbances which have fairly uniform distribution but varying amplitudes can be suppressed by the signal space projection (SSP) technique. The measured signals span a signal space whose dimension equals the number of channels n being measured.

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Part 2

Clinical and research applications of MEG

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Clinical applicationsClinical applications

EpilepsyEpilepsyEnhanced identification of spike activityEnhanced identification of spike activityLocalization of spike focus and functional areasLocalization of spike focus and functional areasEvaluation of epileptic pathogenesisEvaluation of epileptic pathogenesisFunctional evaluation before and after treatment interventionFunctional evaluation before and after treatment interventionMaking a thorough evaluation possible for epilepsy patients (spiMaking a thorough evaluation possible for epilepsy patients (spike ke localization; relation to functional cortices; evaluation of localization; relation to functional cortices; evaluation of sensorimotorsensorimotor, auditory, visual, and cognitive functions), auditory, visual, and cognitive functions)

Functional mapping in patients with brain tumor Language lateralizationFunctional evaluation in various neurological diseases

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