Chapter%2074:%20Biopotentials%20and%20Electrophysiology%20Measurement

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Chapter 74 Biopotentials and Electrophysiology
Measurement

• Teemu Rämö
• teemu.ramo_at_nokia.com

butler.cc.tut.fi/malmivuo/bem/bembook/
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Agenda

• 1st half
• Introduction to biopotentials
• Measurement methods
• Traditional ECG, EEG, EMG, EOG
• Novell VCG
• 2nd half
• Measurement considerations
• Electronics
• Electrodes
• Practices
• QA

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What are biopotentials

• Biopotential An electric potential that is
  measured between points in living cells, tissues,
  and organisms, and which accompanies all
  biochemical processes.
• Also describes the transfer of information
  between and within cells
• This book focuses strictly on the measurement of
  potentials

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Mechanism behind biopotentials 1/2

• Concentration of potassium (K) ions is 30-50
  times higher inside as compared to outside
• Sodium ion (Na) concentration is 10 times higher
  outside the membrane than inside
• In resting state the member is permeable only for
  potassium ions
• Potassium flows outwards leaving an equal number
  of negative ions inside
• Electrostatic attraction pulls potassium and
  chloride ions close to the membrane
• Electric field directed inward forms
• Electrostatic force vs. diffusional force
• Nernst equation
• Goldman-Hodgkin-Katz equation

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Mechanism behind biopotentials 2/2

• When membrane stimulation exceeds a threshold
  level of about 20 mV, so called action potential
  occurs
• Sodium and potassium ionic permeabilities of the
  membrane change
• Sodium ion permeability increases very rapidly at
  first, allowing sodium ions to flow from outside
  to inside, making the inside more positive
• The more slowly increasing potassium ion
  permeability allows potassium ions to flow from
  inside to outside, thus returning membrane
  potential to its resting value
• While at rest, the Na-K pump restores the ion
  concentrations to their original values
• The number of ions flowing through an open
  channel gt106/sec
• Body is an inhomogeneous volume conductor and
  these ion fluxes create measurable potentials on
  body surface

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Electrocardiography (ECG)

• Measures galvanically the electric activity of
  the heart
• Well known and traditional, first measurements
  byAugustus Waller using capillary electrometer
  (year 1887)
• Very widely used method in clinical environment
• Very high diagnostic value

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ECG basics

• Amplitude 1-5 mV
• Bandwidth 0.05-100 Hz
• Largest measurement error sources
• Motion artifacts
• 50/60 Hz powerline interference
• Typical applications
• Diagnosis of ischemia
• Arrhythmia
• Conduction defects

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12-Lead ECG measurement

• Most widely used ECG measurement setup in
  clinical environment
• Signal is measured non-invasively with 9
  electrodes
• Lots of measurement data and international
  reference databases
• Well-known measurement and diagnosis practices
• This particular method was adopted due to
  historical reasons, now it is already rather
  obsolete

Einthoven leads I, II III
Goldberger augmented leads VR, VL VF
Precordial leads V1-V6
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Why is 12-lead system obsolete?

• Over 90 of the hearts electric activity can be
  explained with a dipole source model
• Only 3 orthogonal components need to be measured,
  which makes 9 of the leads redundant
• The remaining percentage, i.e. nondipolar
  components, may have some clinical value
• This makes 8 truly independent and 4 redundant
  leads
• 12-lead system does, to some extend, enhance
  pattern recognition and gives the clinician a
  few more projections to choose from
• but.
• If there was no legacy problem with current
  systems, 12-lead system wouldve been discarded
  ages ago

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Electroencephalography (EEG)

• Measures the brains electric activity from the
  scalp
• Measured signal results from the activity of
  billions of neurons
• Amplitude 0.001-0.01 mV
• Bandwidth 0.5-40 Hz
• Errors
• Thermal RF noise
• 50/60 Hz power lines
• Blink artifacts and similar
• Typical applications
• Sleep studies
• Seizure detection
• Cortical mapping

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EEG measurement setup

• 10-20 Lead system is most widely clinically
  accepted
• Certain physiological featuresare used as
  reference points
• Allow localization of diagnostic features in the
  vicinity of the electrode
• Often a readily available wire or rubber mesh is
  used
• Brain research utilizes even 256 or 512 channel
  EEG hats

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Electromyography (EMG)

• Measures the electric activity of active muscle
  fibers
• Electrodes are always connected very close to the
  muscle group being measured
• Rectified and integrated EMG signal gives rough
  indication of the muscle activity
• Needle electrodes can be used to measure
  individual muscle fibers
• Amplitude 1-10 mV
• Bandwidth 20-2000 Hz
• Main sources of errors are 50/60 Hz and RF
  interference
• Applications muscle function, neuromuscular
  disease, prosthesis

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Electrooculography (EOG)

• Electric potentials are created as a result of
  the movement of the eyeballs
• Potential varies in proportion to the amplitude
  of the movement
• In many ways a challenging measurement with some
  clinical value
• Amplitude 0.01-0.1 mV
• Bandwidth DC-10 Hz
• Primary sources of error include skin potential
  and motion
• Applications eye position, sleep state,
  vestibulo-ocular reflex

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Vectorcardiogram (VCG or EVCG)

• Instead of displaying the scalar amplitude (ECG
  curve) the electric activation front is measured
  and displayed as a vector (dipole model,
  remember?)
• ? It has amplitude and direction
• Diagnosis is based on the curve that the point of
  this vector draws in 2 or 3 dimensions
• The information content of the VCG signal is
  roughly the same as 12-lead ECG system. The
  advantage comes from the way how this information
  is displayed
• A normal, scalar ECG curve can be formed from
  this vectro representation, although (for
  practical reasons) transformation can be quite
  complicated
• Plenty of different types of VCG systems are in
  use
• ? No legacy problem as such

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• Short break,
• Kahvia ja pullaa!

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The biopotential amplifier

• Small amplitudes, low frequencies, environmental
  and biological sources of interference etc.
• Essential requirements for measurement equipment
• High amplification
• High differential gain, low common mode gain ?
  high CMRR
• High input impedance
• Low Noise
• Stability against temperature and voltage
  fluctuations
• Electrical safety, isolation and defibrillation
  protection

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The Instrumentation Amplifier

• Potentially combines the best features desirable
  for biopotential measurements
• High differential gain, low common mode gain,
  high CMRR, high input resistance
• A key design component to almost all biopotential
  measurements!
• Simple and cheap, although high-quality OpAmps
  with high CMRR should be used

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Application-specific requirements

• ECG amplifier
• Lower corner frequency 0.05 Hz, upper 100Hz
• Safety and protection leakage current below
  safety standard limit of 10 uA
• Electrical isolation from the power line and the
  earth ground
• Protection against high defibrillation voltages
• EEG amplifier
• Gain must deal with microvolt or lower levels of
  signals
• Components must have low thermal and electronic
  noise _at_ the front end
• Otherwise similar to ECG
• EMG amplifier
• Slightly enhanced amplifier BW suffices
• Post-processing circuits are almost always needed
  (e.g. rectifier integrator)
• EOG amplifier
• High gain with very good low frequency (or even
  DC) response
• DC-drifting ? electrodes should be selected with
  great care
• Often active DC or drift cancellation or
  correction circuit may be necessary

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Electrical Interference Reduction

• Power line interference (50 or 60 Hz) is always
  around us
• Connects capacitively and causes common mode
  interference
• The common mode interference would be completely
  rejected by the instrumentation amplifier if the
  matching would be ideal
• Often a clever driven right leg circuit is used
  to further enhance CMRR
• ? Average of the VCM is inverted and driven back
  to the body via reference electrode

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Filtering

• Filtering should be included in the front end of
  the InstrAmp
• Transmitters, motors etc. cause also RF
  interference

Small inductorsor ferrite beadsin the lead
wiresblock HF frequencyEM interference
RF filtering withsmall capacitors
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50 or 60 Hz notch filter

• Sometimes it may be desirable to remove the power
  line interference
• Overlaps with the measurement bandwidth
• May distort the measurement result and have an
  affect on the diagnosis!
• Option often available with EEG EOG measuring
  instruments

Twin Tnotch filter
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Artifact reduction

• Electrode-skin interface is a major source of
  artifact
• Changes in the junction potential causes slow
  changes in the baseline
• Movement artifacts cause more sudden changes and
  artifacts
• Drifting in the baseline can be detected by
  discharging the high-pass capacitor in the
  amplifier to restore the baseline

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Electrical isolation

• Electrical isolation limits the possibility of
  passage of any leakage current from the
  instrument in use to the patient
• Such passage would be harmful if not fatal!

• Transformer
• Transformers are inherently high frequency AC
  devices
• Modulation and demodulation needed
• Optical isolation
• Optical signal is modulated in proportion to the
  electric signal and transmitted to the detector
• Typically pulse code modulated to circumvent the
  inherent nonlinearity of the LED-phototransistor
  combination

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Defibrillation Protection

• Measuring instruments can encounter very high
  voltages
• E.g. 15005000V shocks from defibrillator
• Front-end must be designed to withstand these
  high voltages

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Electrodes Basics

• High-quality biopotential measurements require
• Good amplifier design
• Use of good electrodes and their proper placement
  on the patient
• Good laboratory and clinical practices
• Electrodes should be chosen according to the
  application
• Basic electrode structure includes
• The body and casing
• Electrode made of high-conductivity material
• Wire connector
• Cavity or similar for electrolytic gel
• Adhesive rim
• The complexity of electrode design often neglected

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Electrodes - Basics

• Skin preparation by abrasion or cleansing
• Placement close to the source being measured
• Placement above bony structures where there is
  less muscle mass
• Distinguishing features of different electrodes
• How secure? The structure and the use of strong
  but less irritant adhesives
• How conductive? Use of noble metals vs. cheaper
  materials
• How prone to artifact? Use of low-junction-potenti
  al materials such as Ag-AgCl
• If electrolytic gel is used, how is it applied?
  High conductivity gels can help reduce the
  junction potentials and resistance but tend to be
  more allergenic or irritating

Baseline drift due to thechanges in
junctionpotential or motion artifacts?Choice of
electrodes
Muscle signalinterference? Placement
Electromagneticinterference? Shielding
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Ag-AgCl, Silver-Silver Chloride Electrodes

• The most commonly used electrode type
• Silver is interfaced with its salt
  silver-chloride
• Choice of materials helps to reduce junction
  potentials
• Junction potentials are the result of the
  dissimilar electrolytic interfaces
• Electrolytic gel enhances conductivity and also
  reduces junction potentials
• Typically based on sodium or potassium chloride,
  concentration in the order of 0.1 M weak enough
  to not irritate the skin
• The gel is typically soaked into a foam pad or
  applied directly in a pocket produced by
  electrode housing
• Relatively low-cost and general purpose electrode
• Particularly suited for ambulatory or long term
  use

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Gold Electrodes

• Very high conductivity ? suitable for low-noise
  meas.
• Inertness ? suitable for reusable electrodes
• Body forms cavity which is filled with
  electrolytic gel
• Compared to Ag-AgCL greater expense,
  higherjunction potentials and motion artifacts
• Often used in EEG, sometimes in EMG

Conductive polymer electrodes

• Made out of material that is simultaneously
  conductive and adhesive
• Polymer is made conductive by adding monovalent
  metallic ions
• Aluminum foil allows contact to external
  instrumentation
• No need for gel or other adhesive substance
• High resistivity makes unsuitable for low-noise
  meas.
• Not as good connection as with traditional
  electrodes

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Metal or carbon electrodes

• Other metals are seldom used as high-quality
  noblemetal electrodes or low-cost carbon or
  polymericelectrodes are so readily available
• Historical value. Bulky and awkward to use
• Carbon electrodes have high resistivity and are
  noisier but they are also flexibleand reusable
• Applications in electrical stimulation and
  impedance plethysmography

Needle electrodes

• Obviously invasive electrodes
• Used when measurements have to be taken from the
  organ itself
• Small signals such as motor unit potentials can
  be measured
• Needle is often a steel wire with hooked tip

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• Thats it,
• Now for QA

SQUID Superconducting Quantum Interference
Device