Chapter 1: Basic concepts of medical instruments Flashcards
Describe the generalize instrumentation system
- Sensor converts info from measurand
- Signal created is processed and displayed
- Info is perceived
Measurand
physical quantity, property, or condition that the system measures.
Give examples of medical measurands
biopotential, pressure, flow, dimensions, displacement, impedance, temperature, chemical concentration
Sensor
- aka Transducer
- Sensors convert physical measurand to an electric output
- Must only respond to specific measurand and exclude all others
Describe how a sensor should interact with a living system
- must minimize energy extracted
- must be minimally invasive
Signal Conditioning
Since sensor output cannot be directly displayed, signals must be conditioned.
- amplified
- Filtered
- Conversion to other signal types (like digital)
Output Display
- Results of a measurement process must be displayed that is perceivable.
- Form of display depends on presentation desired, particular measurand, and how info will be used.
Auxiliary Elements
- data transmission
- calibrations
- data storage
- control
- feedback
Calibration signal
-Should be applied to sensor input as early in signal-processing chain as possible.
Direct-Indirect Modes
When desired measurand isn’t accessible, another measurand that is related to the desired one can be used to generate a new measurand.
Give an example of a direct-indirect mode
Cardiac output (volume of blood pumped per minute by the heart) is determined by measurements of respiration and blood gas concentration/ or from dye dilutions
Sampling and Continuous Modes
- Measurements that change very slowly may be sampled infrequently (like body temperature)
- Measurements may require continuous monitoring (like respiratory gas flow)
Generating Sensors
Produce their signal output from energy taken directly from the measureand
Modulating Sensors
Use the measurand to alter the flow of energy from an external source in a way that affects the output of the sensor
Analog Signal
- signal that is continuous and able to take on any value within the dynamic range.
- most signals operate in this mode
- require calibration often
Digital Signal. List advantages/disadvantages
- signal is able to take on only a finite number of different values.
- advantages: greater accuracy, repeatability, reliability, immunity to noise, periodic calibrations not required as often
Biomedical instrumentation Classifications
- quantity that is sensed
- principle of transduction
- organ system
- clinical medicine specialties
Desired inputs
measurands that the instrument is designed to isolate
Interfering inputs
quantities that inadvertently affect the instrument as a consequence of the principles used to acquire and process the desired inputs.
Modifying inputs
undesired quantities that indirectly affect the output by altering the performance of the instrument itself
Give example of two possibly interfering inputs
- stray magnetic fields
2. capacitively coupled noise
Give example of two possible modifying inputs
- orientation of patient cables
2. changes in electrode-skin impedance
How can most interfering and modifying inputs be reduced or eliminated?
-altering design of essential instrument components or by adding new components designed to offset the undesired inputs
transducer
converts a signal from one form of energy to another
What are two types of sensor mechanics?
- direct
2. indirect
What are 2 types of signal conversion?
- Single
2. Dynamic
What types of signal measurement?
- static
- Dynamic
- Pulsed
& additionally invasive or non-invasive
Direct sensor measurement
measure value is proportional to an electrical signal
Indirect sensor measurement
-derive the final measurement by solving the mathematical governing equations for the system and the sensor
Draw schematic of multiple conversion sensor and give an example.
physiological variable->intermediate variable->sensor->electrical signal.
example) serum glucose>hydrogen peroxide>H2O2 sensor>electrical signal
Draw schematic of single conversion sensor and give an example
physiological variable>sensor>electrical signal
example) body temp>thermistor>electrical resistance
static signals
change very little with time. can be approximated as a zero order system
dynamic signals
result in a significant change with time
Pulse signals
periodic with time. Usually like to count the pulses and correlated with the phenomena
Resolution
The smallest incremental quantity that can be measured with certainty
Reproducability
the ability of an instrument to give the same output for equal inputs applied over some period of time
Biological signals
- generally dynamic and stochasitic in nature
- difficult to measure because they undergo very discrete changes and there is much interferance
Accuracy
- Average closer to “true” value
- The difference between the true value and the measured value divided by the true value.
- Expressed as percentage of reading
Precision
- “Repeatibility”
- A measurement that expresses the number of distiguishable alternatives from which a given result is selected
Sensitivity
- An absolute quantity used in analog instruments
- smallest absolute amount of change that can be detected by a measurement
Resolution
The smallest incremental quantity that can be distinguished with certainty-usually A/D
Types of Instrument error
- Systematic/bias
2. Random
Systematic/bias error
can be removed by calibration
Random errors
can be removed by taking multiple measurements and averaging the results
What is the acceptable error?
- depends on the application
- high accuracy/precision generally means high cost
- anticipate the future (make higher quality stuff)
Give 4 examples of physical sensors
- Strain gauge/load cell
- Flow sensor
- Thermistor
- Pressure sensor
- Metal resistance thermometer (RTD)
Give 2 examples of bioanalytical sensors
- Glucose sensor
2. Lactate sensor
Give 4 examples of chemical sensors
- oxygen electrode
- glass electrode
- ion-selective electrode
- CO2 sensor
Actuator
A device that converts an electric signal to a physical output
Resistor sensor
- Converts the measurand to electric signal by changing its resistance
- Deformation of the sensor material is common mechanism to change the resistance
- such as strain gage
Conductance
(1/resistance)
Strain gage
- aka load cell
- a device whose electrical resistance varies in proportion to the amount of strain applied on the device
Wheatstone Bridge
- a circuit that is typically used to detect the resistance change of the resistor sensor
- contain four resistors arranged in a diamond
Linear Variable Differential Transformer
- LVDT
- when core is in null position (the center) the output voltage is zero.
- when core moves up the secondary coil gets more flux and so has larger voltage.
Capacitive Sensor
capacitance between two parallel plates of area A separated by distance x
-possible to measure displacement by altering either A or x
Piezoelectric Sensor
- an electric charge generator with high impedance and a small intrinsic capacitor
- requires a charge amplifier to amplify the electric signal from the piezoelectric sensor.
- amplifier must have extremly high input impedance to avoid extracting too much energy from sensor
Thermocouple
- based on discovery that electromotive force (emf) exists across a junction of two dissimilar metals
- two effects
- 1st effect: due to contact of two unlike metals and the junction temp
- 2nd effect: due to temp gradient along each single conductor
- one junction in water and icebath at zero temp
- other junction for temp measurement
Thermistor
- semiconductor made of cermaic materials that are thermal resistors with high negative temp coefficient.
- high resistance
- typically used for very high temperatures
RTD
Resistance temperature detectors
- made of coils or films or metals (usually platinum)
- resistance of metal increases when heated and decreases when cooled.
- very accurate
- very stable
- low resistance
- typically use an OpAmp type of circuit
Photoelectric optical sensors
- photo-emissive sensors
- photoconductive sensors (LDRs)
- photovoltaic sensors
Absorption/flourescence Optical sensors
-different dyes show peaks of different values at different concentrations when absorbance is plotted against wavelength
Phenol Red
- an optical sensor through absorbance
- a pH sensitive reversible dye whole relative absorbance is used to measure pH
HPTS
-an irreversable fluorescent dye used to measure pH
Direct Optical Biosensor
- detects changes in light intensity and signifies changes in measurant like mass or concentration
- ex: pulse or blood level
Fiber Optics
- amount of light transmitted is proportional to temperature
- sensor is extremely sensitive
- Most of light is trapped in the core
- if cladding is temperature sensitive, it might allow some light to leak through.
Pulse oximetry
- a spectrophotometric device
- detects and calculates differential absorption of light by oxygenated and reduced hemoglobin to ger sO2.
- two light sources and a photodetector
Potentiometric sensor
- an electrochemical sensor
- involve measurement of emf of a cell at zero current.
- proportional to the log of the concentration of the substance being determined
Amperometric Sensor
- an increasing/decreasing potential is applied to the cell until oxidation/reduction of the substance to be analyzed occurs and there is a sharp rise/fall in the current to give a peak current.
- peak current= concentration of electroactive material
Conductometric sensor
- an electrochemical sensor
- Normally results in a change in the electrical conductivity of the solution, which can be measured electrically
Chemical Biosensors
-produce an electrical output which is proportional to the concentration of biological analytes
pO2 electrode
- clark electrode
- cathode is a noble metal
- Dissolved O2 diffuses from the blood and is reduced
- voltage kept constant
- current is produced and measured directly
Describe the twisted wires technique
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