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Biological Signal & Signal Processing

김준식

의료기기 이해를 위한 공학이론 (2014-2학기)

Contents

• 생체신호 (물리량) – 전기생리학적 신호 – 광학 신호 – 압력 신호 – 기타 (소리, 온도, 화학성분 등)

• 생체신호 (신체부위) – 뇌파, 근전도, 심전도 등

• 신호처리 – 시계열 신호처리 – 주파수 분석

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생체신호

• 종류: 열, 전기신호, 생화학물질, 소리, 빛 등

• 위치: 머리, 심장, 근육, 혈관 등

소리 초음파

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열 신호

압력신호

혈압 뇌압

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광학신호

심전도

전기생리학적 신호

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ECG (Electrocardiogram)

Apex

• www.getbodysmart.com

Systole & Diastole

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Rhythmical excitation of the heart

• Sinus Node (Sino-Artrial, S-A node) : 발진

– Self excitation

– Sinus nodal fiber

• Internodal pathways – 심방 근육을 수축(이완) 시키며

0.3m/s로 전달

• Atrio-Ventricular Node (A-V node) – 심방 신호보다 심실 신호가 약 1/6초

늦게 뛰도록 딜레이

• A-V bundle – 심방과 심실 사이에 전달

– One-way conduction : 분리, 절연

• Ventricular Perkinje System – 매우 빠른 속도(A-V Node의 150배,

심실 근육의 6배)로 신호를 심실 근육에 전달

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Einthoven, Willem

1860-1927, Dutch physiologist, b. Java, M.D.

Univ. Of Utrecht, 1885.

Professor at the Univ. of Leiden from 1886.

Measurement of electric currents developed by the heart

Invention of a string galvanometer

Electrocardiogram (EKG);

a graphic record of the action of the heart

The Nobel Prize in Physiology or Medicine 1924 "for his discovery of the mechanism of the electrocardiogram"

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Standard Limb Leads

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ECG Waveform

• P : atrial depol.

• QRS complex : ventricular depol., atrial repol.

• T : ventricular repol.

• U : T 이후에 간혹 관측

• PR : conduction delay at AV node

– P의 시작~QRS의 시작 (Q가 안 보이면 R)

– 심방의 excitation ~ 심실의 excitation

– 0.16 in normal

• ST : average duration of plateau regions of ventricular cells

• Q-T interval

– 심실의 수축이 지속되는 시간

– 0.35 in normal

– Q(안 보이면 R)~T의 끝

• HR

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Normal and abnormal Cardiac Rhythms

• 70 bpm

• Bradycardia

• Tachycardia

• Ectopic

• (b) first degree heart block (delay)

• Second degree heart block (2:1, 3:1)

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근전도

전기생리학적 신호

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Electromyogram (EMG, 근전도)

Single Motor Unit (SMU)

- 자의적인 노력으로 활성화시킬 수 있는 최소단위

- 구성 근육을 동시에 firing

- Bioelectric source in volume conductor

- Triphasic, 지속시간 3-15ms, 20-2000uV, 6-30 Hz

표면에서 측정시의 어려움

- 다양한 형태의 전극 : monopolar, bipolar, multipolar, needle type

• 단계적으로 더 많은 힘을 가했을 때의 파형. (c), (d)가 되면 개별 파형의 모양은 보기 힘들다. (점선: 10msec)

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Characteristics of the EMG Signal

• Amplitude (stochastic) ; 0~10 mVpp or 0~1.5 mVrms

• Frequency ; 0~500Hz (Dominant energy ; 50~150 Hz)

Electrode and Amplifier Design

• Differential amplification

– CMRR = 90 dB (generally), 120 dB (current tech.)

– Limit ; expensive, electrical instability, different phase from two sources

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Electrode and Amplifier Design

• Input impedance – Source impedance between the skin and the electrode = several k~Mohms

– Present ; 1012 ohms in parallel with 6pF

• Active electrode design – Differential amplifier

– Low output impedance cable effect will not generate significantly

• Filtering – 20~500 Hz (roll-off of 12 dB/oct)

• Electrode stability – Stable chemical reaction from sweating or humidity changes

EMG Signal Processing

• For several decades, integrated rectified signal is used

• RMS vs. AVR (average rectified) value of the EMG signal

– AVR ; similar integrated rectified signal

do not have a specific physical meaning

– RMS ; the power of the signal

has a clear physical meaning

• RMS value is preferred for most applications

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Applications of the EMG Signal

• To determine the activation timing of the muscle

– When the excitation to the muscle begins and ends

• To estimate the force produced by the muscle

• To obtain an index of the rate at which a muscle fatigues

– Through the analysis of the frequency spectrum of the signal

Definition

“..is the study of muscle function

through the inquiry of the electrical

signal the muscles emanate.”

Basmajian&DeLuca, Muscles Alive 1985,

page 1

Electromyography

...

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Origin of the EMG Signal

From: Kumar/Mital 1996, p. 61,

64

Muscle

Fibers

Nervous contraction command produces

a muscle action potential on the muscle

membranes

Muscle Contraction / Muscular Work

Generation of Muscle Action Potentials

From: Kumar/Mital 1996, p. 73

Bipolar Electrode

Configuration

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Motor Unit Recruitment and Frequency

Laurig 1983

Superposed Surface Signal:

Firing Frequency of Motor Units

Recruitment

of Motor Units

뇌파

전기생리학적 신호

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• CNS (Central Nervous System)

– Brain

• Brain Stem(뇌간) – Diencephalon(간뇌)

– Thalamus(시상)

• Cerebellum(소뇌)

• Cerebrum(대뇌)

– Spinal cord

• 감각, 운동, 판단, 기억…의 중추

EEG (ElectroEncephaloGraphy)

Cortex : 피질

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• Spontaneous EEG

• Evoked Potential

– Somatosensory Evoked Potential

– Auditory Evoked Potential : AEP, AER(Response)

– Visual Evoked Potential : VEP, VER(Response)

보고된 선행연구에 의하면 P300은 정보처리과정 중 자극에 대한 주의력, 자극인지, 기억탐색, 불확실감의 해소 등을 반영. 주의력, 기억력, 인지능력 등이 높을수록 P300의 진폭이 커지는 경향이 있으며, P300이 발생한 시점(Latency)이 빨라짐

Introduction of the anatomy and function of the brain

Hans Berger

1873(May 21)-1941

Born in Neuses near Coburg, Germany

Neurologist

Recorded the first human Electroencephalogram (EEG) in 1929.

Christened by Gibb in 1953,

the father of electroencephalography

founder of psychophysiology

First EEG recorded by Hans Berger, circa 1928.

.

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Neurophysiological Basis

– Extracellular & intracellular field potentials

Biological Source of MEG

propagation

Action potential Postsynaptic potential

synapse B (magnetic field)

Q (current)

Action potential Postsynaptic potential

Action potentials: - Fast : no/little temporal summation - Cancellation : fields diminish rapidly

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Wavelike Field Potential

E1, E2: intracellular electrodes in the afferent fiber E3, E4: membrane potentials of the dendritic elements E5: field potentials in the EEG and DC-EEG

Wave Generation Many neurons need to sum their activity in order to be detected by EEG electrodes. The timing of their activity is crucial. Synchronized neural activity produces larger signals.

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Generating Synchronicity

• Pacemaker • Mutual coordination

EEG

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EEG Potentials

• EEG potentials are good indicators of global brain state.

• EEG often displays rhythmic patterns at characteristic frequencies

EEG Rhythms

• Gamma: 20-60 Hz (“cognitive” frequency band)

• Beta: 14-20 Hz (activated cortex)

• Alpha: 8-13 Hz (quiet waking)

• Theta: 4-7 Hz (sleep stages)

• Delta: less than 4 Hz (sleep stages, specially “deep sleep”)

• Higher frequencies: active processing, relatively de-synchronized activity (alert wakefulness, dream sleep).

• Lower frequencies: strongly synchronized activity (nondreaming sleep, coma).

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The Clinical EEG

Montage

• An EEG voltage signal represents a difference between the voltages at two electrodes

• The display of the EEG for the reading encephalographer may be set up in one of several ways.

• The representation of the EEG channels is referred to as a montage.

• Bipolar and monopolar montage

International 10-20 system placement Modified combinatorial nomenclature

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• Epilepcy (간질)

uncontrolled excessive

brain activity

• Generalized Epilepsy

*Grand mal: clonic convulsion

*Petit mal

• Partial Epilepsy

The abnormal EEG

뇌자도

전기생리학적 신호

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MEG (Magnetoencephalography)

• 뇌신경의 전기적 활동에 의해 발생하는 자기장을 검출하는 장치

AMPERE'S RIGHT-HANDED SCREW RULE

FARADAY’ S LAW

History of Biomagnetism • 1963, first biomagnetic recording

by Baule and McFee (Syracuse, NY); MCG

• 1968, David Cohen (Illinois→ MIT & MGH) developed MEG

• 1969, Jim Zimmerman (MIT) invented SQUID

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The History of Magnetoencephalography

David Cohen, Ph.D.

Associate Professor of Radiology

at the Harvard Medical School and Massachusetts General Hospital

Inventor of magnetoencephalography,

first measured in 1968 at the Francis Bitter Magnet Laboratory at MIT

1980 1997

Whole-head sensors arrays which use 100 to 300 sensors

at different locations

1991 2005

The first MEG in Korea

MEG Procedure

current

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MEG vs. Other Functional Imaging

MEG/EEG

– Neuroelectricity

PET

– Hemodynamics

– Neurochemistry

fMRI

– Hemodynamics

Adapted from Churchland et al., Science, 1988

Temporal resolution (sec)

Spatial re

solu

tion (m

m)

0.001 0.01 0.1 1 10 100 1000

2

4

6

8

10

MEG

fMRI

CT

PET

MRI

EEG SPECT invasiveness

Difference between fMRI and MEG

• Temporal resolution

• Functional segregation

• Inhomogeneous spatial resolution

MEG (motor) fMRI (motor)

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Difference between fMRI and MEG

• Experimental paradigm

– Block, event-related

• Auditory stimulation

• Movement artifact

• Temporal connectivity

MEG vs. EEG

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MEG/EEG Signals

Cellular currents in an active neuron papulation

EEG: measuring potential differences on the scalp Secondary current (volume conduction)

MEG: measuring the extracranial magnetic field Primary current

Strength of Magnetic Signals

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Magnetically Shielded Room

• Two or Three high magnetic permeability layers

• One layer of aluminum

External magnetic field

Magnetic flux

시계열 분석

생체신호처리

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필터링

• 원하지 않는 성분을 제거하는 것

– 즉, 원하는 것만 취하고 싶을 때 사용

– 예: 정수기, 공기청정기, 진공청소기, 모기장, 체, 어망 등

생체신호의 필터링

– 원하지 않는 부위의 신호나 성분을 최소화하며 목적하는 신호를 얻어내는 과정 • 민감한 생체신호를 측정, 분석할 때 특히 중요

– 공간 • 근전도, 안전도를 신호를 최소화하며 뇌파분석

• 태아심음

– 시간 • 심장박동에 동기화된 MRI, CT 촬영

• 간질 모니터링

– 주파수 • 뇌파의 주파수 성분 (알파파, 베타파, …)

• 신호는 특정 주파수 대역 & 잡음은 주파수 전대역, 또는 그 반대

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Notch Filter

Notch filter (60 Hz)

Bandpass Filter

Bandpass filter (8~13 Hz)

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Lowpass Filter

Low pass filter (~40 Hz)

Highpass Filter

High pass filter (70 Hz ~)

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Random Nature of EMG

The raw EMG consists of an arbitrary superposition of motor

action potentials and cannot be reproduced in precise shape:

3 standardized contraction of the M. biceps br.

Need of Signal Processing steps to increase repeatability!

EMG Signal Processing: Rectification

Benefit: only positive values - mean, peak, area calculaton

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EMG Signal Processing: Smoothing

Deleting non reproducible amplitude spikes

Root Mean Square at 300ms

Averaging in time normalized cycles

Repetitive sequence of movements in ms =>

Time normalized cycle

0%

100%

Result:

The “typical” EMG

pattern of a given

movement

• Reduction of

variability

• Good

reproducibility

• Comparison plots

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Variability within trials

M. Tibialis Anterior:

Smoothed rectified EMG

Activation patterns in gait

5 Regular Gait Strides

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EMG Amplitude Normalization 1: MVC

Mic

row

olt

% M

VC

EMG in ratio to a Maximum Volontary Contraction = %

MVC

Rescaling of microvolts to percent of reference contraction

MVC

100%

Test Trials

EMG Amplitude Parameters

Peak

[µV]

Raw

Signal

Mean

[µV]

Area

[µV/sec]

Rectifie

dSignal

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Filtering

50 Hz low pass filtering 2~50 Hz band pass filtering

Filtering

50 Hz lowpass filter

N20m at the primary somatosensory cortex

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Averaging

• Emphasize the evoked brain response

• Diminish the random noise

• 10, 20, 30, 50, 100, Ref (200)

주파수 분석

생체신호처리

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푸리에 변환

• 수많은 주기를 가지는 신호의 합

• 푸리에 변환으로 어떤 주파수의 신호가 강하고 약한지 파악 할 수 있다.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-5

-4

-3

-2

-1

0

1

2

3

4

5

𝐺 𝑓 = 𝑓(𝑡)𝑒−𝑖2𝜋𝑓𝑡𝑑𝑡∞

−∞

EMG Frequency Spectrum

DeLuca, Knaflitz 1992: Surface Electromyography:

What’s new, page 24

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EMG Frequency Parameters

Peak power

Median frequency

Mean frequency

Total power

Frequency in Hertz

Increasing EMG Amplitude Due to Fatigue

From: Kumar/Mital 1996, a.a.O. , p. 79

Static Work Condition:

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DeLuca 1997: The use of surface electromyography in Biomechanics, page 157

• Muscle Fatigue Index

Typically in Static positions

-EMG Power Spectrum shifts to lower

Frequencies

Frequency Based Fatigue Analysis

뇌파의 파워 스펙트럼

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Time and Frequency Smoothing

• Fixed and varied length time windows

Simple Approach

• Sliding a fixed length window

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Ictal ECoGs

Significant clusters over thresholds ( > 20uV )

Spatial distribution pattern of ictal HFO zones

Park et al., Clinical Neurophysiology., 2011

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Thank You