Biopotentials and  Electrophysiology  Measurement
Agenda <ul><li>1 st  half </li></ul><ul><ul><li>Introduction to biopotentials </li></ul></ul><ul><ul><li>Measurement metho...
What are biopotentials <ul><li>Biopotential: An electric potential that is measured between points in living cells, tissue...
Mechanism behind biopotentials 1/2 <ul><li>Concentration of potassium (K + ) ions is 30-50 times higher inside as compared...
Mechanism behind biopotentials 2/2 <ul><li>When membrane stimulation exceeds a threshold level of about 20 mV, so called a...
Electrocardiography (ECG) <ul><li>Measures galvanically the electric activity of the heart </li></ul><ul><li>Well known an...
ECG basics <ul><li>Amplitude: 1-5 mV </li></ul><ul><li>Bandwidth: 0.05-100 Hz </li></ul><ul><li>Largest measurement error ...
12-Lead ECG measurement <ul><li>Most widely used ECG measurement setup in clinical environment </li></ul><ul><li>Signal is...
Why is 12-lead system obsolete? <ul><li>Over 90% of the heart’s electric activity can be explained  with a dipole source m...
Electroencephalography (EEG)  <ul><li>Measures the brain’s electric  activity from the scalp </li></ul><ul><li>Measured si...
EEG measurement setup <ul><li>10-20 Lead system is most  widely clinically accepted </li></ul><ul><li>Certain physiologica...
Electromyography (EMG) <ul><li>Measures the electric activity of active muscle fibers </li></ul><ul><li>Electrodes are alw...
Electrooculography (EOG) <ul><li>Electric potentials are created as a result of the movement of the eyeballs </li></ul><ul...
Vectorcardiogram (VCG or EVCG) <ul><li>Instead of displaying the scalar amplitude (ECG curve) the electric activation fron...
 
The biopotential amplifier <ul><li>Small amplitudes, low frequencies, environmental and biological sources of interference...
The Instrumentation Amplifier <ul><li>Potentially combines the best features desirable for biopotential measurements </li>...
Application-specific requirements <ul><li>ECG amplifier </li></ul><ul><ul><li>Lower corner frequency 0.05 Hz, upper 100Hz ...
Electrical Interference Reduction <ul><li>Power line interference (50 or 60 Hz) is always around us </li></ul><ul><li>Conn...
Filtering <ul><li>Filtering should be included in the front end of the InstrAmp </li></ul><ul><li>Transmitters, motors etc...
50 or 60 Hz notch filter <ul><li>Sometimes it may be desirable to remove the power line interference </li></ul><ul><li>Ove...
Artifact reduction <ul><li>Electrode-skin interface is a major source of artifact </li></ul><ul><ul><li>Changes in the jun...
Electrical isolation <ul><li>Electrical isolation limits the possibility of passage of any leakage current from the instru...
Defibrillation Protection <ul><li>Measuring instruments can encounter very high voltages </li></ul><ul><li>E.g. 1500…5000V...
Electrodes – Basics <ul><li>High-quality biopotential measurements require </li></ul><ul><ul><li>Good amplifier design </l...
Electrodes - Basics <ul><li>Skin preparation by abrasion or cleansing </li></ul><ul><li>Placement close to the source bein...
Ag-AgCl, Silver-Silver Chloride Electrodes <ul><li>The most commonly used electrode type </li></ul><ul><li>Silver is inter...
Gold Electrodes <ul><li>Very high conductivity    suitable for low-noise meas. </li></ul><ul><li>Inertness    suitable f...
Metal or carbon electrodes <ul><li>Other metals are seldom used as high-quality noble metal electrodes or low-cost  carbon...
That’s it, Now for Q&A SQUID = Superconducting Quantum Interference Device
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Biopotentials

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Biopotentials

  1. 1. Biopotentials and Electrophysiology Measurement
  2. 2. Agenda <ul><li>1 st half </li></ul><ul><ul><li>Introduction to biopotentials </li></ul></ul><ul><ul><li>Measurement methods </li></ul></ul><ul><ul><ul><li>Traditional: ECG, EEG, EMG, EOG </li></ul></ul></ul><ul><ul><ul><li>Novell: VCG </li></ul></ul></ul><ul><li>2 nd half </li></ul><ul><ul><li>Measurement considerations </li></ul></ul><ul><ul><ul><li>Electronics </li></ul></ul></ul><ul><ul><ul><li>Electrodes </li></ul></ul></ul><ul><ul><ul><li>Practices </li></ul></ul></ul><ul><ul><li>Q&A </li></ul></ul>
  3. 3. What are biopotentials <ul><li>Biopotential: An electric potential that is measured between points in living cells, tissues, and organisms, and which accompanies all biochemical processes. </li></ul><ul><li>Also describes the transfer of information between and within cells </li></ul><ul><li>This book focuses strictly on the measurement of potentials </li></ul>
  4. 4. Mechanism behind biopotentials 1/2 <ul><li>Concentration of potassium (K + ) ions is 30-50 times higher inside as compared to outside </li></ul><ul><li>Sodium ion (Na + ) concentration is 10 times higher outside the membrane than inside </li></ul><ul><li>In resting state the member is permeable only for potassium ions </li></ul><ul><ul><li>Potassium flows outwards leaving an equal number of negative ions inside </li></ul></ul><ul><ul><li>Electrostatic attraction pulls potassium and chloride ions close to the membrane </li></ul></ul><ul><ul><li>Electric field directed inward forms </li></ul></ul><ul><ul><li>Electrostatic force vs. diffusional force </li></ul></ul><ul><li>Nernst equation: </li></ul><ul><li>Goldman-Hodgkin-Katz equation: </li></ul>
  5. 5. Mechanism behind biopotentials 2/2 <ul><li>When membrane stimulation exceeds a threshold level of about 20 mV, so called action potential occurs: </li></ul><ul><ul><li>Sodium and potassium ionic permeabilities of the membrane change </li></ul></ul><ul><ul><li>Sodium ion permeability increases very rapidly at first, allowing sodium ions to flow from outside to inside, making the inside more positive </li></ul></ul><ul><ul><li>The more slowly increasing potassium ion permeability allows potassium ions to flow from inside to outside, thus returning membrane potential to its resting value </li></ul></ul><ul><ul><li>While at rest, the Na-K pump restores the ion concentrations to their original values </li></ul></ul><ul><li>The number of ions flowing through an open channel >10 6 /sec </li></ul><ul><li>Body is an inhomogeneous volume conductor and these ion fluxes create measurable potentials on body surface </li></ul>
  6. 6. Electrocardiography (ECG) <ul><li>Measures galvanically the electric activity of the heart </li></ul><ul><li>Well known and traditional, first measurements by Augustus Waller using capillary electrometer (year 1887) </li></ul><ul><li>Very widely used method in clinical environment </li></ul><ul><li>Very high diagnostic value </li></ul>1. Atrial depolarization 2. Ventricular depolarization 3. Ventricular repolarization
  7. 7. ECG basics <ul><li>Amplitude: 1-5 mV </li></ul><ul><li>Bandwidth: 0.05-100 Hz </li></ul><ul><li>Largest measurement error sources: </li></ul><ul><ul><li>Motion artifacts </li></ul></ul><ul><ul><li>50/60 Hz powerline interference </li></ul></ul><ul><li>Typical applications: </li></ul><ul><ul><li>Diagnosis of ischemia </li></ul></ul><ul><ul><li>Arrhythmia </li></ul></ul><ul><ul><li>Conduction defects </li></ul></ul>
  8. 8. 12-Lead ECG measurement <ul><li>Most widely used ECG measurement setup in clinical environment </li></ul><ul><li>Signal is measured non-invasively with 9 electrodes </li></ul><ul><li>Lots of measurement data and international reference databases </li></ul><ul><li>Well-known measurement and diagnosis practices </li></ul><ul><li>This particular method was adopted due to historical reasons, now it is already rather obsolete </li></ul>Einthoven leads: I, II & III Goldberger augmented leads: V R , V L & V F Precordial leads: V 1 -V 6
  9. 9. Why is 12-lead system obsolete? <ul><li>Over 90% of the heart’s electric activity can be explained with a dipole source model </li></ul><ul><ul><li>Only 3 orthogonal components need to be measured, which makes 9 of the leads redundant </li></ul></ul><ul><li>The remaining percentage, i.e. nondipolar components, may have some clinical value </li></ul><ul><ul><li>This makes 8 truly independent and 4 redundant leads </li></ul></ul><ul><li>12-lead system does, to some extend, enhance pattern recognition and gives the clinician a few more projections to choose from </li></ul><ul><li>… but…. </li></ul><ul><li>If there was no legacy problem with current systems, 12-lead system would’ve been discarded ages ago </li></ul>
  10. 10. Electroencephalography (EEG) <ul><li>Measures the brain’s electric activity from the scalp </li></ul><ul><li>Measured signal results from the activity of billions of neurons </li></ul><ul><li>Amplitude: 0.001-0.01 mV </li></ul><ul><li>Bandwidth: 0.5-40 Hz </li></ul><ul><li>Errors: </li></ul><ul><ul><li>Thermal RF noise </li></ul></ul><ul><ul><li>50/60 Hz power lines </li></ul></ul><ul><ul><li>Blink artifacts and similar </li></ul></ul><ul><li>Typical applications: </li></ul><ul><ul><li>Sleep studies </li></ul></ul><ul><ul><li>Seizure detection </li></ul></ul><ul><ul><li>Cortical mapping </li></ul></ul>
  11. 11. EEG measurement setup <ul><li>10-20 Lead system is most widely clinically accepted </li></ul><ul><li>Certain physiological features are used as reference points </li></ul><ul><li>Allow localization of diagnostic features in the vicinity of the electrode </li></ul><ul><li>Often a readily available wire or rubber mesh is used </li></ul><ul><li>Brain research utilizes even 256 or 512 channel EEG hats </li></ul>
  12. 12. Electromyography (EMG) <ul><li>Measures the electric activity of active muscle fibers </li></ul><ul><li>Electrodes are always connected very close to the muscle group being measured </li></ul><ul><li>Rectified and integrated EMG signal gives rough indication of the muscle activity </li></ul><ul><li>Needle electrodes can be used to measure individual muscle fibers </li></ul><ul><li>Amplitude: 1-10 mV </li></ul><ul><li>Bandwidth: 20-2000 Hz </li></ul><ul><li>Main sources of errors are 50/60 Hz and RF interference </li></ul><ul><li>Applications: muscle function, neuromuscular disease, prosthesis </li></ul>
  13. 13. Electrooculography (EOG) <ul><li>Electric potentials are created as a result of the movement of the eyeballs </li></ul><ul><li>Potential varies in proportion to the amplitude of the movement </li></ul><ul><li>In many ways a challenging measurement with some clinical value </li></ul><ul><li>Amplitude: 0.01-0.1 mV </li></ul><ul><li>Bandwidth: DC-10 Hz </li></ul><ul><li>Primary sources of error include skin potential and motion </li></ul><ul><li>Applications: eye position, sleep state, vestibulo-ocular reflex </li></ul>
  14. 14. Vectorcardiogram (VCG or EVCG) <ul><li>Instead of displaying the scalar amplitude (ECG curve) the electric activation front is measured and displayed as a vector (dipole model, remember?) </li></ul><ul><ul><li> It has amplitude and direction </li></ul></ul><ul><li>Diagnosis is based on the curve that the point of this vector draws in 2 or 3 dimensions </li></ul><ul><li>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 </li></ul><ul><li>A normal, scalar ECG curve can be formed from this vectro representation, although (for practical reasons) transformation can be quite complicated </li></ul><ul><li>Plenty of different types of VCG systems are in use </li></ul><ul><ul><li> No legacy problem as such </li></ul></ul>
  15. 16. The biopotential amplifier <ul><li>Small amplitudes, low frequencies, environmental and biological sources of interference etc. </li></ul><ul><li>Essential requirements for measurement equipment: </li></ul><ul><ul><li>High amplification </li></ul></ul><ul><ul><ul><li>High differential gain, low common mode gain  high CMRR </li></ul></ul></ul><ul><ul><li>High input impedance </li></ul></ul><ul><ul><li>Low Noise </li></ul></ul><ul><ul><li>Stability against temperature and voltage fluctuations </li></ul></ul><ul><ul><li>Electrical safety, isolation and defibrillation protection </li></ul></ul>
  16. 17. The Instrumentation Amplifier <ul><li>Potentially combines the best features desirable for biopotential measurements </li></ul><ul><ul><li>High differential gain, low common mode gain, high CMRR, high input resistance </li></ul></ul><ul><li>A key design component to almost all biopotential measurements! </li></ul><ul><li>Simple and cheap, although high-quality OpAmps with high CMRR should be used </li></ul>CMRR fine tuning
  17. 18. Application-specific requirements <ul><li>ECG amplifier </li></ul><ul><ul><li>Lower corner frequency 0.05 Hz, upper 100Hz </li></ul></ul><ul><ul><li>Safety and protection: leakage current below safety standard limit of 10 uA </li></ul></ul><ul><ul><li>Electrical isolation from the power line and the earth ground </li></ul></ul><ul><ul><li>Protection against high defibrillation voltages </li></ul></ul><ul><li>EEG amplifier </li></ul><ul><ul><li>Gain must deal with microvolt or lower levels of signals </li></ul></ul><ul><ul><li>Components must have low thermal and electronic noise @ the front end </li></ul></ul><ul><ul><li>Otherwise similar to ECG </li></ul></ul><ul><li>EMG amplifier </li></ul><ul><ul><li>Slightly enhanced amplifier BW suffices </li></ul></ul><ul><ul><li>Post-processing circuits are almost always needed (e.g. rectifier + integrator) </li></ul></ul><ul><li>EOG amplifier </li></ul><ul><ul><li>High gain with very good low frequency (or even DC) response </li></ul></ul><ul><ul><li>DC-drifting  electrodes should be selected with great care </li></ul></ul><ul><ul><li>Often active DC or drift cancellation or correction circuit may be necessary </li></ul></ul>
  18. 19. Electrical Interference Reduction <ul><li>Power line interference (50 or 60 Hz) is always around us </li></ul><ul><li>Connects capacitively and causes common mode interference </li></ul><ul><li>The common mode interference would be completely rejected by the instrumentation amplifier if the matching would be ideal </li></ul><ul><li>Often a clever “driven right leg circuit” is used to further enhance CMRR </li></ul><ul><ul><li> Average of the V CM is inverted and driven back to the body via reference electrode </li></ul></ul>
  19. 20. Filtering <ul><li>Filtering should be included in the front end of the InstrAmp </li></ul><ul><li>Transmitters, motors etc. cause also RF interference </li></ul>Small inductors or ferrite beads in the lead wires block HF frequency EM interference RF filtering with small capacitors High-pass filter to reject DC drifting Low-pass filtering at several stages is recommended to attenuate residual RF interference
  20. 21. 50 or 60 Hz notch filter <ul><li>Sometimes it may be desirable to remove the power line interference </li></ul><ul><li>Overlaps with the measurement bandwidth </li></ul><ul><ul><li>May distort the measurement result and have an affect on the diagnosis! </li></ul></ul><ul><li>Option often available with EEG & EOG measuring instruments </li></ul>Twin T notch filter Determines notch frequency Notch tuning
  21. 22. Artifact reduction <ul><li>Electrode-skin interface is a major source of artifact </li></ul><ul><ul><li>Changes in the junction potential causes slow changes in the baseline </li></ul></ul><ul><ul><li>Movement artifacts cause more sudden changes and artifacts </li></ul></ul><ul><li>Drifting in the baseline can be detected by discharging the high-pass capacitor in the amplifier to restore the baseline </li></ul>
  22. 23. Electrical isolation <ul><li>Electrical isolation limits the possibility of passage of any leakage current from the instrument in use to the patient </li></ul><ul><li>Such passage would be harmful if not fatal! </li></ul><ul><li>Transformer </li></ul><ul><ul><li>Transformers are inherently high frequency AC devices </li></ul></ul><ul><ul><li>Modulation and demodulation needed </li></ul></ul><ul><li>Optical isolation </li></ul><ul><ul><li>Optical signal is modulated in proportion to the electric signal and transmitted to the detector </li></ul></ul><ul><ul><li>Typically pulse code modulated to circumvent the inherent nonlinearity of the LED-phototransistor combination </li></ul></ul>
  23. 24. Defibrillation Protection <ul><li>Measuring instruments can encounter very high voltages </li></ul><ul><li>E.g. 1500…5000V shocks from defibrillator </li></ul><ul><li>Front-end must be designed to withstand these high voltages </li></ul>1. Resistors in the input leads limit the current 3. Protection against much higher voltages is achieved with low-pressure gas discharge tubes (e.g. neon lamps) (note: even isolation components such as transformers and optical isolators need these spark gaps) Discharge @ ~100V 2. Diodes or Zener diodes protect against high voltages Discharge @ 0.7-15V
  24. 25. Electrodes – Basics <ul><li>High-quality biopotential measurements require </li></ul><ul><ul><li>Good amplifier design </li></ul></ul><ul><ul><li>Use of good electrodes and their proper placement on the patient </li></ul></ul><ul><ul><li>Good laboratory and clinical practices </li></ul></ul><ul><li>Electrodes should be chosen according to the application </li></ul><ul><li>Basic electrode structure includes: </li></ul><ul><ul><li>The body and casing </li></ul></ul><ul><ul><li>Electrode made of high-conductivity material </li></ul></ul><ul><ul><li>Wire connector </li></ul></ul><ul><ul><li>Cavity or similar for electrolytic gel </li></ul></ul><ul><ul><li>Adhesive rim </li></ul></ul><ul><li>The complexity of electrode design often neglected </li></ul>
  25. 26. Electrodes - Basics <ul><li>Skin preparation by abrasion or cleansing </li></ul><ul><li>Placement close to the source being measured </li></ul><ul><li>Placement above bony structures where there is less muscle mass </li></ul><ul><li>Distinguishing features of different electrodes: </li></ul><ul><ul><li>How secure? The structure and the use of strong but less irritant adhesives </li></ul></ul><ul><ul><li>How conductive? Use of noble metals vs. cheaper materials </li></ul></ul><ul><ul><li>How prone to artifact? Use of low-junction-potential materials such as Ag-AgCl </li></ul></ul><ul><ul><li>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 </li></ul></ul>Baseline drift due to the changes in junction potential or motion artifacts  Choice of electrodes Muscle signal interference  Placement Electromagnetic interference  Shielding
  26. 27. Ag-AgCl, Silver-Silver Chloride Electrodes <ul><li>The most commonly used electrode type </li></ul><ul><li>Silver is interfaced with its salt silver-chloride </li></ul><ul><li>Choice of materials helps to reduce junction potentials </li></ul><ul><ul><li>Junction potentials are the result of the dissimilar electrolytic interfaces </li></ul></ul><ul><li>Electrolytic gel enhances conductivity and also reduces junction potentials </li></ul><ul><ul><li>Typically based on sodium or potassium chloride, concentration in the order of 0.1 M weak enough to not irritate the skin </li></ul></ul><ul><li>The gel is typically soaked into a foam pad or applied directly in a pocket produced by electrode housing </li></ul><ul><li>Relatively low-cost and general purpose electrode </li></ul><ul><li>Particularly suited for ambulatory or long term use </li></ul>
  27. 28. Gold Electrodes <ul><li>Very high conductivity  suitable for low-noise meas. </li></ul><ul><li>Inertness  suitable for reusable electrodes </li></ul><ul><li>Body forms cavity which is filled with electrolytic gel </li></ul><ul><li>Compared to Ag-AgCL: greater expense, higher junction potentials and motion artifacts </li></ul><ul><li>Often used in EEG, sometimes in EMG </li></ul>Conductive polymer electrodes <ul><li>Made out of material that is simultaneously conductive and adhesive </li></ul><ul><li>Polymer is made conductive by adding monovalent metallic ions </li></ul><ul><li>Aluminum foil allows contact to external instrumentation </li></ul><ul><li>No need for gel or other adhesive substance </li></ul><ul><li>High resistivity makes unsuitable for low-noise meas. </li></ul><ul><li>Not as good connection as with traditional electrodes </li></ul>
  28. 29. Metal or carbon electrodes <ul><li>Other metals are seldom used as high-quality noble metal electrodes or low-cost carbon or polymeric electrodes are so readily available </li></ul><ul><li>Historical value. Bulky and awkward to use </li></ul><ul><li>Carbon electrodes have high resistivity and are noisier but they are also flexibleand reusable </li></ul><ul><li>Applications in electrical stimulation and impedance plethysmography </li></ul>Needle electrodes <ul><li>Obviously invasive electrodes </li></ul><ul><li>Used when measurements have to be taken from the organ itself </li></ul><ul><li>Small signals such as motor unit potentials can be measured </li></ul><ul><li>Needle is often a steel wire with hooked tip </li></ul>
  29. 30. That’s it, Now for Q&A SQUID = Superconducting Quantum Interference Device

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