Post on 03-Jun-2022
2014-09-26
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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