Biomedical Instrumentation Chapter 6 in Introduction to Biomedical Equipment Technology By Joseph Carr and John Brown
Signal Acquisition • Medical Instrumentation typically entails monitoring a signal off the body which is analog, converting it to an electrical signal, and digitizing it to be analyzed by the computer.
Types of Sensors: • Electrodes: acquire an electrical signal • Transducers: acquire a non-electrical signal (force, pressure, temp etc) and converts it to an electrical signal
Active vs Passive Sensors: • Active Sensor: • Requires an external AC or DC electrical source to power the device • Strain gauge, blood pressure sensor • Passive Sensor: • Provides it own energy or derives energy from phenomenon being studied • Thermocouple
Sensor Error Sources • Error: • Difference between measured value and true value.
5 Categories of Errors: • Insertion Error • Application Error • Characteristic Error • Dynamic Error • Environmental Error
Insertion Error: • Error occurring when inserting a sensor
Application Error: • Errors caused by Operator
Characteristic Error: • Errors inherent to Device
Dynamic Error: • Most instruments are calibrated in static conditions if you are reading a thermistor it takes time to change its value. If you read this value to quickly an error will result.
Environmental Error: • Errors caused by environment • heat, humidity
Sensor Terminology • Sensitivity: • Slope of output characteristic curve Δy/ Δx; • Minimum input of physical parameter will create a detectable output change • Blood pressure transducer may have a sensitivity of 10 uV/V/mmHg so you will see a 10 uV change for every V or mmHg applied to the system.
Output Output Input Input Which is more sensitive? The left side one because you’ll have a larger change in y for a given change in x
Ideal Curve Output Input Sensitivity Error Sensor Terminology • Sensitivity Error = Departure from ideal slope of a characteristic curve
Sensor Terminology • Range = Maximum and Minimum values of applied parameter that can be measured. • If an instrument can read up to 200 mmHg and the actual reading is 250 mmHg then you have exceeded the range of the instrument.
Sensor Terminology • Dynamic Range: total range of sensor for minimum to maximum. Ie if your instrument can measure from -10V to +10 V your dynamic range is 20V • Precision = Degree of reproducibility denoted as the range of one standard deviation σ • Resolution = smallest detectable incremental change of input parameter that can be detected
Xi Xo Accuracy • Accuracy = maximum difference that will exist between the actual value and the indicated value of the sensor
Offset Error • Offset error = output that will exist when it should be zero • The characteristic curve had the same sensitive slope but had a y intercept Output Output Input Input Offset Error Zero offset error
Linearity • Linearity = Extent to which actual measure curved or calibration curve departs from ideal curve.
Full Scale Input Ideal Measure Output Din(Max) Input Linearity • Nonlinearity (%) = (Din(Max) / INfs) * 100% • Nonlinearity is percentage of nonlinear • Din(max) = maximum input deviation • INfs = maximum full-scale input
Hysteresis • Hysteresis = measurement of how sensor changes with input parameter based on direction of change
Output = F(x) P F2 Input = x F1 B Q Hysteresis • The value B can be represented by 2 values of F(x), F1 and F2. If you are at point P then you reach B by the value F2. If you are at point Q then you reach B by value of F1.
F(t) Tolerance Band Tresponse 100% 70% Rising Response Time Time Ton Response Time • Response Time: Time required for a sensor output to change from previous state to final settle value within a tolerance band of correct new value denoted in red can be different in rising and decaying directions
F(t) Tolerance Band Tresponse 100% 70% Rising Response Time Time Ton Response Time • Time Constant: Depending on the source is defined as the amount of time to reach 0% to 70% of final value. Typically denoted for capacitors as T = R C (Resistance * Capacitance) denoted in Blue
Response Time • Convergence Eye Movement the inward turning of the eyes have a different response time than divergence eye movements the outward turning of the eyes which would be the decay response time Tdecay F(t) Decaying Response Time Toff Time
F(x)* = ax + bx2+cx4+ . . . +K F(x)* = ax + bx3+cx5+ . . . +K Output F(x) Output F(x) F(x) = mx + K F(x) = mx + K K K Input X Input X Dynamic Linearity Measure of a sensor’s ability to follow rapid changes in the input parameters. Difference between solid and dashed curves is the non- linearity as depicted by the higher order x terms
Asymmetric = F(x) != |F(-x)| where F(x)* is asymmetric around linear curve F(x) then F(x) = ax + bx2+cx4+ . . . +K offsetting for K or you could assume K = 0 Symmetrical = F(x) = |F(-x)| where F(x) * is symmetric around linear curve F(x) then F(x) = ax +bx3 + cx5 +. . . + K offsetting for K or you could assume K =0 Dynamic Linearity
Av Av = Vo/Vi 1.0 Frequency (w) radians per second Frequency Response of Ideal and Practical System • When you look at the frequency response of an instrument, ideally you want a wideband flat frequency response.
Av Av = Vo/Vi 1.0 0.707 FL FH Frequency (w) radians per second Frequency Response of Ideal and Practical System • In practice, you have attenuation of lower and higher frequencies • FL and FH are known as the –3 dB points in voltage systems.
Examples of Filters • Ideal Filter has sharp cutoffs and a flat pass band • Most filters attenuate upper and lower frequencies • Other filters attenuate upper and lower frequencies and are not flat in the pass band
Electrodes for Biophysical Sensing • Bioelectricity: naturally occurring current that exists because living organisms have ions in various quantities
Electrodes for Biophysical Sensing • Ionic Conduction: Migration of ions-positively and negatively charge molecules throughout a region. • Extremely nonlinear but if you limit the region can be considered linear
Electrodes for Biophysical Sensing • Electronic Conduction: Flow of electrons under the influence of an electrical field
Bioelectrodes • Bioelectrodes: class of sensors that transduce ionic conduction to electronic conduction so can process by electric circuits • Used to acquire ECG, EEG, EMG, etc.
Bioelectrodes • 3 Types of electrodes: • Surface (in vivo) outside body • Indwelling Macroelectrodes (in vivo) • Microelectrodes (in vitro) inside body
Bioelectrodes • Electrode Potentials: • Skin is electrolytic and can be modeled as electrolytic solutions Metal Electrode Electrolytic Solution where Skin is electrolytic and can be modeled as saline
Electrodes in Solution • Have metallic electrode immersed in electrolytic solution once metal probe is in electrolytic solution it: • Discharges metallic ions into solution • Some ions in solution combine with metallic electrodes • Charge gradient builds creating a potential difference or you have an electrode potential or ½ cell potential
Electrodes in Solution 2 cells A and B, A has 2 positive ions And B has 3 positive ions thus have a Potential difference of 3 –2 = 1 where B is more positive than A A ++ B +++
Electrodes • Two reactions take place at electrode/electrolyte interface: • Oxidizing Reaction: Metal -> electrons + metal ions • Reduction Reaction : Electrons + metal ions -> Metal
Vae Metal A Vbe Metal B Electrolytic Solution Electrodes • Electrode Double Layer: formed by 2 parallel layers of ions of opposite charge caused by ions migrating from 1 side of region or another; ionic differences are the source of the electrode potential or half-cell potential (Ve).
Electrodes • If metals are different you will have differential potential sometimes called an electrode offset potential. • Metal A = gold Vae = 1.50V and Metal B = silver Vbe = 0.8V then Vab = 1.5V – 0.8 V = 0.7V (Table 6-1 in book page 96) Vae Metal A Vbe Metal B Electrolytic Solution
Electrodes • Two general categories of material combinations: • Perfectly polarized or perfectly nonreversible electrode: no net transfer of charge across metal/electrolyte interface • Perfectly Nonpolarized or perfectly reversible electrode: unhindered transfer of charge between metal electrode and the electrode • Generally select a reversible electrode such as Ag-AgCl (silver-silver chloride)
Rt= internal resistance of body which is low Vd = Differential voltage Vd Rsa and Rsb = skin resistance at electrode A and B Electrode A C1a Vea + - Rsa Cellular Resistance R1a Rc - Vo Mass Tissue Resistance Rt Vd Cellular Potentials + Electrode B C1b Veb + - Rsb R1b Ionic Conduction Electronic Conduction • R1A and R1B = resistance of electrodes • C1A and C1B = capacitance of electrodes
Electrode Potentials cause recording Problems • ½ cell potential ~ 1.5 V while biopotentials are usually 1000 times less (ECG = 1-2 mV and EEG is 50 uV) thus have a tremendous difference between DC cell potential and biopotential • Strategies to overcome DC component • Differential DC amplifier to acquire signal thus the DC component will cancel out • Counter Offset-Voltage to cancel half-cell potential • AC couple input of amplifier (DC will not pass through) ie capacitively couple the signal into the circuit
Electrode Potentials cause recording Problems • Strategies to overcome DC component • Differential DC amplifier to acquire signal thus the DC component will cancel out • Counter Offset-Voltage to cancel half-cell potential • AC couple input of amplifier (DC will not pass through) • Capacitively couple the signal into the circuit
Binding Spot Pin-Tip Connector Shielded Wire Electrode Surface Medical Surface Electrodes • Typical Medical Surface Electrode: • Use conductive gel to reduce impedance between electrode and skin • Schematic:
Medical Surface Electrodes • Have an Ag-AgCl contact button at top of hollow column filled with gel • Gel filled column holds actual metallic electrode off surface of skin and decreases movement artifact • Typical ECG arrangement is to have 3 ECG electrodes (2 differentials signals and a reference electrode)
Problems with Surface Electrodes • Adhesive does not stick for a long time on sweaty skin • Can not put electrode on bony prominences • Movement or motion artifact significant problem with long term monitoring results in a gross change in potential • Electrode slippage if electrode slips then thickness of jelly changes abruptly which is reflected as a change in electrode impedance and electrode offset potential (slight change in potential)
Potential Solutions for Surface Electrodes Problems • Additional Tape • Rough surface electrode that digs past scaly outer layer of skin typically not comfortable for patients.
Other Types of Electrodes • Needle Electrodes: inserted into tissue immediately beneath skin by puncturing skin on an angle note infection is a problem. • Indwelling Electrodes: Inserted into layers beneath skin -> typically tiny exposed metallic contact at end of catheter usually threaded through patient’s vein to measure intracardiac ECG to measure high frequency characteristics such as signal at the bundle of His