Transcript Chapter 74: Biopotentials and Electrophysiology Measurement

Chapter 74:
Biopotentials and
Electrophysiology
Measurement
Teemu Rämö
[email protected]
m
butler.cc.tut.fi/~malmivuo/bem/bembook/
Agenda
• 1st half
• Introduction to biopotentials
• Measurement methods
• Traditional: ECG, EEG, EMG, EOG
• Novell: VCG
• 2nd half
• Measurement considerations
• Electronics
• Electrodes
• Practices
• Q&A
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
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
Vm  70... 100 mV
 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:
RT ci ,k
Vk  
ln
zk F co,k
• Goldman-Hodgkin-Katz equation:
RT PK ci , K  PNa ci , Na  PCl ci ,Cl
Vm  
ln
zk F PK ci , K  PNa ci , Na  PCl ci ,Cl
Vm  70... 100 mV
Mechanism behind biopotentials 2/2
•
When membrane stimulation exceeds a threshold
level of about 20 mV, so called action potential
occurs:
1. Sodium and potassium ionic permeabilities of the
membrane change
2. Sodium ion permeability increases very rapidly at first,
allowing sodium ions to flow from outside to inside,
making the inside more positive
3. The more slowly increasing potassium ion
permeability allows potassium ions to flow from inside
to outside, thus returning membrane potential to its
resting value
4. While at rest, the Na-K pump restores the ion
concentrations to their original values
•
•
The number of ions flowing through an open
channel >106/sec
Body is an inhomogeneous volume conductor and
these ion fluxes create measurable potentials on
body surface
Electrocardiography (ECG)
• Measures galvanically the electric activity of the heart
• Well known and traditional, first measurements by
Augustus Waller using capillary electrometer (year 1887)
• Very widely used method in clinical environment
• Very high diagnostic value
2. Ventricular
depolarization
1. Atrial
depolarization
3. Ventricular repolarization
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
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
Why is 12-lead system obsolete?
• Over 90% of the heart’s 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 would’ve been
discarded ages ago
Electroencephalography (EEG)
• Measures the brain’s 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
EEG measurement setup
• 10-20 Lead system is most
widely clinically accepted
• Certain physiological features
are 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
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
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
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
Short break,
Kahvia ja pullaa!
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
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
G1  1  2
R2
R1
G2  
R4
R3
CMRR fine tuning
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 @ 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
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
VCM  iD R0
VCM
iD R0

R
1 2 2
R1
Filtering
• Filtering should be included in the front end of the InstrAmp
• Transmitters, motors etc. cause also RF interference
Small inductors
or ferrite beads
High-pass
filter
RF
filtering
with
in
the
lead
wires
to reject
DC
drifting
smallHF
capacitors
block
frequency
EM interference
Low-pass filtering
at several stages
is recommended to
attenuate residual
RF interference
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
Determines
notch
frequency
Twin T
notch filter
Notch
tuning
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
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!
1. Transformer
•
Transformers are inherently high frequency
AC devices
•
Modulation and demodulation needed
2. 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
Defibrillation Protection
• Measuring instruments can encounter very high voltages
• E.g. 1500…5000V shocks from defibrillator
• Front-end must be designed to withstand these high voltages
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)
2. Diodes or Zener diodes
protect against high
voltages
Discharge @ 0.7-15V
(note: even isolation
components such as
transformers and
optical isolators need
these spark gaps)
Discharge @ ~100V
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
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-potential 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 the
changes in junction
potential or motion artifacts
Choice of electrodes
Electromagnetic
interference
 Shielding
Muscle signal
interference
 Placement
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
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, higher
junction 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
Metal or carbon electrodes
• Other metals are seldom used as high-quality noble
metal electrodes or low-cost carbon or polymeric
electrodes 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
That’s it,
Now for Q&A
SQUID = Superconducting Quantum Interference Device