Bio Signal Measurement Flashcards
What are polarizable electrodes?
- No charge across the electrodeelectrolyte interface when current is applied - Current is displacement current - Electrode behaves like a capacitor - Overpotential is due to concentration Vc - Example: Pt - Pt electrodes are used as stimulating electrodes as there is no resistive current
What are non-polarizable electrodes?
-Current passes freely across the electrode-electrolyte interface. - Requires resistive energy to make transition - Perfectly non-polarizable electrodes have no overpotentials π π = 0 - In reality, π π = π π + π π΄, which is much lesser than π π = π π +π πΆ + π π΄ - Example: Ag-AgCl (approximately) - Ag-AgCl electrodes are commonly used in recording as they have a stable potential
What can relative motion between electrode and skin cause?
disturbs the charge layer and gives rise to
voltage.
Whats the stratum Corneum Layer of the skin?
has
high electrical resistance,
hinders charge movement and acts as a capacitor.
Preventive Measures for electrode and skin interface
Limit motion - Remove the stratum corneum - Signal Filtering : High pass filtering can help reduce the capacitance effect of the stratum corneum
Methods of removing the Stratum COrneum
- Soap and water
- Rubbing with isopropyl alcohol
- Abrading with sandpaper
- Using scotch tape
- Scratching with a needle
If blood appears when removing the stratum corneum what does that mean?
That would be the dermis layer - Caused by the complete removal of the epidermis - This will compromise the protective function
Whats the epidermis?
Protective layer
Whats the dermis?
contains the blood
vessels, nerves, and hair follicles.
What happens if theres movement of electrodes during measurement?
-When a polarizable electrode is in contact with the electrolytic gel, a double layer of charge forms at the interface.
β’ Movement of the electrode will disturb the distribution of the charge
and causes a temporary change in the πΈβπ
- The motion artifact is not as pronounced for nonpolarizable electrodes.
- Signal due to motion artifact is low frequency, so it can be filtered out if the biological signal has mostly high frequency content. (like EMG)
- When the signals have a lot of low frequency content (EEG, ECG, EOG) non-polarizable electrodes should be used.
What are types of electrodes?
- Surface Vs Percutaneous
- Non polarizable Vs Polarizable
- Reusable Vs Disposable
- Microelectrodes
Whats the line borne interface?
β’ Capacitive coupling of the body
- To the domestic supply
- To ground
β’ Current flow through body
- Too small to feel
- Safe
β’ Develops potential, Ecm
- Common mode
β’ Common signal is much larger
than the signalWhats a monopolar electrode configuration?
β’ It is not always true that the
inactive region will remain inactive
β’ Will include some power line noise
HAS AN ACTIVE AND INACTOVE CHANNEL
WHats a bipolar electrode configuration?
β’ Focuses just on the area of
interest
β’ Rejects 60 Hz noise
What IA are well suited for bimedical applications?
β’ Ultra high impedance (in the order of 1015 Ξ©) ensures that the electrodes are not loaded (Zout β 100 MΞ©)
β’ High and stable linear gains. A single external resistor can be used to
vary the gain 10-100X
β’ Very high CMRR β allows the measurement of small signals that are
contaminated with high common-mode voltages.
Problem with single op-amp implementation
- Want high input impedance and large Ad
- but impedence and difference gain are inversely proportional
Why do ECG, EEG, EMG, and EOG signals need to be filtered?
- Amplitudes are small
- Match instrumentation bandwidth to that of the desired signal
- To improve SNR of measurement
- Prevent aliasing when we are digitising the signal
Why do we filter?
Filtering clearly improves the signal quality
4 Types of filters
- low pass
- high pass
- bandpass
- band stop
whats a cut-off frequency?
Cut-off frequency delimits the pass and stop band - No attenuation below fc - Infinite attenuation above fc - Abrupt transition between the pass and the stop band (zero width for the transition band, or infinite slope at transition)
What are passband requirements?
β’ Ideally, we want the signal shape to be maintained at the output while still improving the SNR. β’ This requires that the time delay for each component of the signal (within the passband) is the same. Aka β Constant group delay
Whats a butterworth filter
- Attempts to give an ideal amplitude response
- Flat passband and a sharp transition
- But phase response suffers
Whats a bessel filter
- Ideal phase response, with a constant group delay in the pass band
- But the amplitude response suffers- pronounced slope in the passband
whats a tchebyshev filter?
- Ideal transition response
- Both the amplitude and the phase response suffers
What do we want from a filter?
1) Brick wall like amplitude response
2) Linear phase response
3) Steep transition between pass and
stopband
Butter Worth Magnitude Response
- -3dB point is fc β corner/cut off frequency
- Maximally flat response in the pass band
- Gain in stop band β 0 β’ As nβ, the transition region narrows
- Attenuation slope β -20n dB/decade
- As n β, the amplitude response looks more like the ideal brick wall response.
LOOK AT THE THREE FILTERS IN LECTURE 11&12
Comparison of BW, Bessel, and Tchebyshev
β’ BW β flat magnitude response
β’ Bessel β Linear phase response
β’ Tchebyshev β sharp transition
region
Why do we convert to digital formats?
β’ Offline
- Diagnostics
- Biomedical research
β’ Online (real time)
- Control of assistive devices like prosthesis and exoskeletons
- diagnosticsTwo steps of ADC
- Sampling -> continuous to discrete time
* Takes snapshots T seconds apart
* Called Pulse Amplitude modulation - Digitisation -> continuous to discrete amplitude
* Called Pulse code modulation
Frequency of Sampling Routine
β’ Fh - the highest frequency of the input signal
β’ Fs β sampling frequency
β’ Fourier transform of the sampled signal gives a repeated spectrum
(repeats every fs)
How are sampled signals recovered?
-Use a lowpass filter where fc=fh
β’ To avoid spectral overlapping, the fsshould at least be 2fh
β’ This is the Nyquist sampling criterion
β’ Practically, we always take fs >2fh
What is Aliasing in the frequency domain?
- Aliasing = confusion
- Aliasing happens when the signal is under sampled.
- Spectral folding β gives a distorted signal upon recovery.
What is aliasing in the time domain?
- If under sampled, the recovered signal doesnβt look anything like the original.
- Band-limiting the signal prior to sampling can prevent aliasing βAnti aliasing filters
- There is a trade off between aliasing error, filter order and sampling frequency.
What is digitization/Quantization?
β’ Uses a number of discrete amplitude levels to represent a continuous amplitude range.
β’ Pulse code modulation
- Finite number of code words (Q)
- Number of symbols = m
- Code word length = n
- Number of possible codewords
π = π^n
SNR of ADC
β’ SNR = Signal power/ Noise power
β’ SNR due to Quantization using the ADC
- SNR for Gaussian input=6N-4.7 dB
- SNR for sinusoidal input=6.02N+1.76 dB
Quantization Error (RMS) quantisation error noise=q/sqrt(12)
quantisation error noise=q/sqrt(12)
WHats ENOB?
-Effective number of bits β’ To calculate ENOB β’ Measure input signal power β’ Measure Noise power ( difference between power of the full frequency spectrum and the input signal power) β’ Calculate the SNR based on measurement β’ πΈπππ΅ =(πππ ππππ π’πππ β1.76)/6.02dB
Whats electromyography?
The discipline that deals with the detection, acquisition, processing,
analysis and use of the electrical signals that emanate from
contracting muscles.
EMG- plot of myoelectric voltage vs. time
Uses of an EMG?
β’ Index of muscle activity
- EMG gives an indication of the output force of a given muscle
- EMG signal amplitude increases for increasing output force
- Activity relative to MVC (Maximum Voluntary contraction)
β’ Muscle Fatigue index
- Fatigue is a specific phenomenon in which the muscle force
decreases due to prolonged activity
- Fatigue causes a shift in the frequency content in the EMG, and also
reduces the conduction velocity within the fibre
Applications of EMG
β’ Assessment of muscle pathologies
- Lower back pain
- Distinguishing between Parkinson tremors and essential tremors
β’ Control of powered Limb prostheses
What is SFAP?
Single Fibre Action potential
β’ Electrodes measure the change
in the dipole
What is an SFAP Train -SFAPT
- The time between the firings Ti is not equal
- Ti is a random variable with mean of 20ms to 200ms (depending on the force output), and a variation (SD ~ 10-20% of the mean)
- Ti can thus be modelled as a gaussian random variable
What is a motor unit (SFAP)?
β group of muscle fibres with a common nerve axon
β’ All fibres in a motor unit fire synchronously
β’ Innervation Zone
β’ SFAP for each fibre will depend
on:
- Position of the end plate
- Velocity of the action potential
What is MUAP?
- MUAP = Collection of all fibre signals
- Larger in magnitude
- Large time duration
- Waveform is different from SFA
What does MUAP depend on?
β’ Relative geometry of electrode and muscle fibres in the motor unit in its
vicinity
β’ Position of detection surface relative to the innervation zone
β’ Size of the muscle fibres (amplitude is proportional to the diameter of the fibre)
β’ Number of muscle fibres in the motor unit
How does a motor unit sustain muscle contraction?
- To sustain a muscle contraction, the motor units must be activated repeatedly.
- The resulting sequence of MUAP of a single Motor unit is called MUAPT.
- The MUAP within a MUAPT will remain constant if the geometric relationship between the electrode and the active muscle fibres remains the same
What is an inter pulse interval (MUAP)
time between adjacent MUAP
-Mathematically, can be expressed as a sequence of Dirac delta impulses convoluted with a filter representing the shape of the MUAP
MES-Myoelectric Signal- EMG SIGNAL
β’ EMG signal = linear summation of the MUAPTS (from a number of motor units)
Time Domain EMG SIgnal Analysis Techniques?
- Rectification (half or full wave)
- Mean absolute value
- Integration
- Root mean square
Frequency DOmain EMG Signal Analysis Techniques
- Power spectrum density of signal (from FFT)
- Useful measures of the PDS are median frequency, mean frequency and the bandwidth of the spectrum
- Median frequency is less sensitive to noise
- Frequency spectrum parameters are appropriate for analysing signals that are stationary or almost stationary.
What is an ECG?
β’ ECG = surface level measurement of the electrical activity of cardiac muscles.
β’ Also known as EKG
β’ Cardiac electrogram β when electrodes are placed on or in the heart
to measure the electrical activity.
β’ Self excitation property (donβt need a nerve to initiate the firing)
β’ The muscle fibres fire more or less synchronously
What are pacemaker cells?
- Those groups of cells that have the ability to pace the heart.
- 3 such groups
- SA node, AV node, Bundle of His & Purkinjee fibre
Electrical Activity of Heart
- Rest phase β atria fill with blood. There is no electrical activity.
- The SA node depolarizes spontaneously, and waves of
depolarization move from the SA node across the atria β causing a
mechanical contraction pushing the blood into the ventricles - AV node then delays the depolarization spread to ensure that the ventricles are filled
- Purkinjee fibres conduct the depolarization wave very rapidly β causing the ventricles to contractfrom the bottom upwards. (Ventricular depolarization)
- Ventricles repolarize (T wave) and return to the rest phase.
What doe clinicians observe in an ECG?
the shape, duration, amplitude, etc of the ECG as
compared to the normal ECG to detect diseases.
What is an ECG lead?
ECG lead β is a graphical description of the electrical activity and is
computed by analysing electrical currents detected by several
electrodes.
What is the standard 12 lead configuration?
12 lead display β has 12 curves derived from 10 electrodes that are placed
on standard location.
- 6 leads come from 4electrodes placed on the limbs.
- 6 leads come from the 6 electrodes placed on the chest.
What are the standard electrode leads for on a person?
β’ All the limb leads use the electrode on the Right leg to be the ground electrode.
- Standard electrode leads are
(1) RA β LA
(2) RA β LL
(3) LA - LL
Augmented Limb Leads
β’ Augmented limb leads use stand limb electrode configuration, but
the electrodes are connected through resistors to a common node.
β’ This decreases the noise level and increases the size of the
apparent signal.
Einthovenβs triangle
- Leads I, II and III form the original leads constructed by Wilhelm Einthoven.
- Their spatial orientation forms a triangle in the chest.
β’ Based on Kirchoffβs current law:
πΏπππ πΌ + πΏπππ πΌπΌπΌ = πΏπππ πΌπΌ
only need info from two leads
ECG SIgnal SPecifications
β’ Bandwidth: 0.1 β 100 Hz β’ Gains: between 100 to 5000 (usually much smaller than EMG as the signal is larger) β’ CMRR: 100 dB or above β’ Input resistance: >100 MΞ©
Markers for heart attacks
ST segment acts as marker
What is an EEG?
β’ Electrical activity of the brain as
measured from the scalp.
What is an ECoG?
(Electrocorticogram)β the
electrodes are placed on the
exposed surface of the brain
(cortex)
What is a depth recording in the brain?
Depth recording β when
electrodes are advanced into the
brain.
Standard EEG electrode placement?
β10β and β20β refer to the % distance
between the adjacent electrodes (of the
front to back or the right to left distance
on the skull)
EEG Frequency? COmparison to EMG and ECG
- Gamma has the highest bandwidth β only between 25 to about 40Hz.
- Frequency content of EEG is much lower than that in EMG or ECG.
EEG Monitoring Applications?
-sleep studies - (Study brain waves in relation to sleep)
β’ We go through different stages of sleep as we sleep through the night,
and each stage plays a different role in the proper brain function.
Eg: Stage 3 & 4 β body recuperates,
Stage 2 βmemory consolidation.
What is a muscle force twitch?
β’ The action potential in a muscle fibre is the signal for the fibre to
contract or generate a force.
β’ Muscle fibre contraction occurs when specialized, regularly arranged proteins β actin and myosin β form linkages (called cross bridges) and slide past each other.
What is the force twitch response?
β’ The muscle twitch lasts much longer than the action potential.
How can the force developed by a muscle be graded?
- Action potential in one axon, generates a twitch in all the muscle fibres that it innervates.
- (1) If more axons are activated, then more force is produced by the twitch.
- (2) If the same axon is stimulated more frequently, the twitches fuse into a `tetanic contraction.
What does a muscle contraction depend on?
β number of axons that are stimulated, and the frequency of stimulation of each axon.
Firing rate and force of twitch
The higher the firing rate, the higher the steady-state force. (up to
saturation β called tetanic contraction)
What is Hennemanβs size principle?
- The size principle states that as more force is needed, motor units are recruited in a precise order according to the magnitude of their force output.
- Small units are recruited first βexhibiting task appropriate recruitment
Why is Henneman;s Size Principle Beneficial?
- Minimises amount of fatigue experienced by using fatigue resistant first, and only using the fatigable fibres when high forces are needed.
- The relative change in force produced by additional recruitment remains relatively constant.
Is there some characteristics we can measure in each segment to see if it changes as we
hold a contraction?
Mean Frequency and conduction velocity both decrease when a person is fatigued.
Atlantic Clinic for Upper Limb Prosthetics
- Most advanced upper-limb fitting clinic in Canada
- Myoeelctric/powered upper limb prostheses
- Custom prosthetic devices
- > 120 clients
Myoelectric Control - concept
β’ Electrical activity from a residual limb is amplified, processed and
used to control the flow of electricity from a battery to a motor, which
operates an artificial limb.
Myoelectric Control β Design aspects
β’ Must be comfortable -> weight should be minimised
β’ Work reliably
β’ Have a natural appearance during rest and during functional activities
β’ Should replace not only the mechanical function, but also the
cutaneous and the kinaesthetic sensation.
β’ Clinical acceptance β ease of fitting, adjustment and training
β’ Versatility β usually achieved through modular design
Early Developments of myoelectric control(1)
β’ On/Off control
- One or two control sites
- Voluntary open and automatic closing/ or voluntary opening and
closing
-device was not portable, before invention of transistors
Benefits to Proportional Control/Direct control
- Mapping individual EMG signalsto individual functions of a prosthetic device
- Good performance
- Intuitive for user
Cons to Proportional Control/Direct control
- Usually not a physiologically appropriate mapping
- limited number of control sites
- Control of limited number of DOF
