Biopotentials.ppt

Chapter 74: Biopotentials and Electrophysiology MeasurementTeemu [email protected]

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

Agenda1st halfIntroduction to biopotentialsMeasurement methodsTraditional: ECG, EEG, EMG, EOGNovell: VCG2nd halfMeasurement considerationsElectronicsElectrodesPracticesQ&A

What are biopotentialsBiopotential: 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 cellsThis book focuses strictly on the measurement of potentials

Mechanism behind biopotentials 1/2Concentration of potassium (K+) ions is 30-50 times higher inside as compared to outsideSodium ion (Na+) concentration is 10 times higher outside the membrane than insideIn resting state the member is permeable only for potassium ions

Potassium flows outwards leaving an equal number of negative ions insideElectrostatic attraction pulls potassium and chloride ions close to the membraneElectric field directed inward formsElectrostatic force vs. diffusional forceNernst equation:

Goldman-Hodgkin-Katz equation:

Mechanism behind biopotentials 2/2When membrane stimulation exceeds a threshold level of about 20 mV, so called action potential occurs:

Sodium and potassium ionic permeabilities of the membrane changeSodium ion permeability increases very rapidly at first, allowing sodium ions to flow from outside to inside, making the inside more positiveThe more slowly increasing potassium ion permeability allows potassium ions to flow from inside to outside, thus returning membrane potential to its resting valueWhile at rest, the Na-K pump restores the ion concentrations to their original valuesThe number of ions flowing through an open channel >106/secBody is an inhomogeneous volume conductor and these ion fluxes create measurable potentials on body surface

Electrocardiography (ECG)Measures galvanically the electric activity of the heartWell known and traditional, first measurements byAugustus Waller using capillary electrometer (year 1887)Very widely used method in clinical environmentVery high diagnostic value

ECG basicsAmplitude:1-5 mVBandwidth:0.05-100 Hz

Largest measurement error sources:Motion artifacts50/60 Hz powerline interference

Typical applications:Diagnosis of ischemiaArrhythmiaConduction defects

12-Lead ECG measurementMost widely used ECG measurement setup in clinical environmentSignal is measured non-invasively with 9 electrodesLots of measurement data and international reference databasesWell-known measurement and diagnosis practicesThis particular method was adopted due to historical reasons, now it is already rather obsolete

Einthoven leads: I, II & IIIGoldberger augmented leads: VR, VL & VF Precordial leads: V1-V6

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 redundantThe remaining percentage, i.e. nondipolar components, may have some clinical value

This makes 8 truly independent and 4 redundant leads12-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

Electroencephalography (EEG) Measures the brains electric activity from the scalpMeasured signal results from the activity of billions of neurons

Amplitude:0.001-0.01 mVBandwidth:0.5-40 Hz

Errors:Thermal RF noise50/60 Hz power linesBlink artifacts and similar

Typical applications:Sleep studiesSeizure detectionCortical mapping

EEG measurement setup10-20 Lead system is most widely clinically acceptedCertain physiological featuresare used as reference pointsAllow localization of diagnostic features in the vicinity of the electrodeOften a readily available wire or rubber mesh is usedBrain research utilizes even 256 or 512 channel EEG hats

Electromyography (EMG)Measures the electric activity of active muscle fibersElectrodes are always connected very close to the muscle group being measuredRectified and integrated EMG signal gives rough indication of the muscle activityNeedle electrodes can be used to measure individual muscle fibers

Amplitude:1-10 mVBandwidth:20-2000 Hz

Main sources of errors are 50/60 Hz and RF interference

Applications: muscle function, neuromuscular disease, prosthesis

Electrooculography (EOG)Electric potentials are created as a result of the movement of the eyeballsPotential varies in proportion to the amplitude of the movementIn many ways a challenging measurement with some clinical value

Amplitude: 0.01-0.1 mVBandwidth:DC-10 Hz

Primary sources of error include skin potential and motion

Applications: eye position, sleep state, vestibulo-ocular reflex

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 directionDiagnosis is based on the curve that the point of this vector draws in 2 or 3 dimensionsThe 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 displayedA 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

Short break,Kahvia ja pullaa!

The biopotential amplifierSmall amplitudes, low frequencies, environmental and biological sources of interference etc.Essential requirements for measurement equipment:High amplificationHigh differential gain, low common mode gain high CMRRHigh input impedanceLow NoiseStability against temperature and voltage fluctuationsElectrical safety, isolation and defibrillation protection

The Instrumentation AmplifierPotentially combines the best features desirable for biopotential measurementsHigh differential gain, low common mode gain, high CMRR, high input resistanceA key design component to almost all biopotential measurements!Simple and cheap, although high-quality OpAmps with high CMRR should be used

Application-specific requirementsECG amplifierLower corner frequency 0.05 Hz, upper 100HzSafety and protection: leakage current below safety standard limit of 10 uAElectrical isolation from the power line and the earth groundProtection against high defibrillation voltagesEEG amplifierGain must deal with microvolt or lower levels of signalsComponents must have low thermal and electronic noise @ the front endOtherwise similar to ECGEMG amplifierSlightly enhanced amplifier BW sufficesPost-processing circuits are almost always needed (e.g. rectifier + integrator)EOG amplifierHigh gain with very good low frequency (or even DC) responseDC-drifting electrodes should be selected with great careOften active DC or drift cancellation or correction circuit may be necessary

Electrical Interference ReductionPower line interference (50 or 60 Hz) is always around usConnects capacitively and causes common mode interferenceThe common mode interference would be completely rejected by the instrumentation amplifier if the matching would be idealOften 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

FilteringFiltering should be included in the front end of the InstrAmpTransmitters, motors etc. cause also RF interference

Small inductorsor ferrite beadsin the lead wiresblock HF frequencyEM interferenceRF filtering withsmall capacitors

50 or 60 Hz notch filterSometimes it may be desirable to remove the power line interferenceOverlaps 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

Artifact reductionElectrode-skin interface is a major source of artifactChanges in the junction potential causes slow changes in the baselineMovement artifacts cause more sudden changes and artifactsDrifting in the baseline can be detected by discharging the high-pass capacitor in the amplifier to restore the baseline

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

TransformerTransformers are inherently high frequency AC devicesModulation and demodulation needed

Optical isolationOptical signal is modulated in proportion to the electric signal and transmitted to the detectorTypically pulse code modulated to circumvent the inherent nonlinearity of the LED-phototransistor combination

Defibrillation ProtectionMeasuring instruments can encounter very high voltagesE.g. 15005000V shocks from defibrillatorFront-end must be designed to withstand these high voltages

Electrodes BasicsHigh-quality biopotential measurements requireGood amplifier designUse of good electrodes and their proper placement on the patientGood laboratory and clinical practicesElectrodes should be chosen according to the applicationBasic electrode structure includes:The body and casingElectrode made of high-conductivity materialWire connectorCavity or similar for electrolytic gelAdhesive rimThe complexity of electrode design often neglected

Electrodes - BasicsSkin preparation by abrasion or cleansingPlacement close to the source being measuredPlacement above bony structures where there is less muscle massDistinguishing features of different electrodes:How secure? The structure and the use of strong but less irritant adhesivesHow conductive? Use of noble metals vs. cheaper materialsHow prone to artifact? Use of low-junction-potential materials such as Ag-AgClIf 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 artifactsChoice of electrodes Muscle signalinterference Placement Electromagneticinterference Shielding

Ag-AgCl, Silver-Silver Chloride ElectrodesThe most commonly used electrode typeSilver is interfaced with its salt silver-chlorideChoice of materials helps to reduce junction potentialsJunction potentials are the result of the dissimilar electrolytic interfacesElectrolytic gel enhances conductivity and also reduces junction potentialsTypically based on sodium or potassium chloride, concentration in the order of 0.1 M weak enough to not irritate the skinThe gel is typically soaked into a foam pad or applied directly in a pocket produced by electrode housingRelatively low-cost and general purpose electrodeParticularly suited for ambulatory or long term use

Gold ElectrodesVery high conductivity suitable for low-noise meas.Inertness suitable for reusable electrodesBody forms cavity which is filled with electrolytic gelCompared to Ag-AgCL: greater expense, higherjunction potentials and motion artifactsOften used in EEG, sometimes in EMG

Conductive polymer electrodesMade out of material that is simultaneously conductive and adhesivePolymer is made conductive by adding monovalent metallic ionsAluminum foil allows contact to external instrumentationNo need for gel or other adhesive substanceHigh resistivity makes unsuitable for low-noise meas.Not as good connection as with traditional electrodes

Metal or carbon electrodesOther metals are seldom used as high-quality noblemetal electrodes or low-cost carbon or polymericelectrodes are so readily availableHistorical value. Bulky and awkward to useCarbon electrodes have high resistivity and are noisier but they are also flexibleand reusableApplications in electrical stimulation and impedance plethysmography

Needle electrodesObviously invasive electrodesUsed when measurements have to be taken from the organ itselfSmall signals such as motor unit potentials can be measuredNeedle is often a steel wire with hooked tip

Thats it,Now for Q&ASQUID = Superconducting Quantum Interference Device