The major difference between this system and a conventional instrumentation system is:
The source of the signals (measurand) is a living tissue or energy is applied to living tissue.
Measurand
Physical quantity, property, or condition that is being measured by the
system.
* most important issue : accessibility
- internal (blood pressure), on body surface (ECG, EEG)
- emanate from the body (infra-red radiation)
- derived from a tissue sample (blood or biopsy)
Medically important measurands
• Biopotentials (ECG, EEG, EMG, EOG, etc.)
• Pressure, flow, dimensions (imaging)
• Displacement (velocity dx/dt, acceleration d2x/d2t, and force =
md2x/d2t)
• Impedance, temperature and chemical concentration
The measurand may be localized to a specific organ or anatomical
Sensor
* The transducer or sensor should only respond to the form of energy present in the measurand to the exclusion of all others!
* The sensor should interface with the living system to minimize the energy extracted and being minimally invasive!
Signal conditioning
Usually the sensor output can not directly drive the display, therefore
signal processing or conditioning is required
Examples of signal processing:
1. Impedance matching
2. Amplification
3. Filtering
4. Mathematical mapping
5. Linearizing
6. Analog-to-digital conversion (ADC)
7. Digital-to-analog conversion (DAC)
8. Signal averaging to reduce noise (i.e. evoked response)
9. Transformation (time domain
frequency domain)
10. Compensation for undesirable sensor characteristics
11. Etc.
Output displays
Examples of output displays:
1. Numerical
2. Graphical
3. Discrete
4. Continuous
5. Permanent or temporary
• Most displays rely on our vision, but auditory sense is also sometimes used (for example, Doppler ultrasonic signals)
• User controls and output displays should conform to human factors engineering guidelines for the design of medical devices
Auxiliary Elements
*Calibration signal with the properties of the measurand should be applied to the sensor input or as early in the signal processing chain as possible
**Many forms of feedback (automatic or manual) may be required to elicit the measurand, to adjust the sensor and signal conditioner and to direct the flow of output (display, storage, transmission)
***Data storage for signal conditioning or examination of alarm conditions or implementation of different processing algorithms
**** Data communication transmission of patient data to remote display at nurse’s station and medical center
Operation Modes
1. Direct and Indirect Modes• Direct: Measurand directly to sensor
- readily accessible or
- acceptable invasive procedure
For example: direct blood pressure measurement
• Indirect: measurand not accessible- Use another measurand with known relation to the desired one
- Use some form of energy or material that interacts with the
desired measurand to generate a new accessible one
For example:
Cardiac output (volume of blood pumped/min by the heart)
- Measurements of respiration & blood gas concentration
- Dye dilution
- Morphology of internal organs determined from X-rays
2. Sampling or Continuous Modes
• Sampling: Parameters that change slowly do not require continuous measurements
For example: body temperature, ionic concentrations, etc.
• Continuous: Parameters that change fast enough to require continuous measurements
For example: ECG, EEG, EMG, respiratory gas flow, etc.
Note: Frequency content of the measurand, the objective of the measurement, the condition of the patient and the potential liability of the physician influence how often data should be acquired
3. Generating and Modulating Sensors
• Generating: Produce output from energy taken directly from measurand
For example: photovoltaic cell (output voltage related to irradiation)
• Modulating: Measurand changes flow of energy from an external source that affects the output of a sensor
For example: photoconductive cell (apply external power to the sensor to measure changes in resistance with irradiation)
4. Analog and Digital Modes
• Analog: Continuous (parameter takes on any value within the dynamic range)
For example: Parameters that change fast enough to require continuous measurements: ECG, EEG, EMG, respiratory gas flow, etc.
• Digital: Discrete (parameter takes on a finite number of different values)
* Most sensors are analog (i.e., strain gages, thermistors, etc.)
* Very few sensors are digital in nature (i.e., shaft encoders)
5. Real-time and delayed-time Modes
Real-time: Sensors must acquire signals as they actually occur
• Output is not always displayed immediately, because some types of signal processing (i.e. averaging, transformations, etc) require considerable amount of date before production of final results
Delayed-time Often acceptable (short delays) unless urgent feedback & control depend on output
• Cell cultures provide an example where several days of delay may be required before an output is obtained!
The source of the signals (measurand) is a living tissue or energy is applied to living tissue.
Measurand
Physical quantity, property, or condition that is being measured by the
system.
* most important issue : accessibility
- internal (blood pressure), on body surface (ECG, EEG)
- emanate from the body (infra-red radiation)
- derived from a tissue sample (blood or biopsy)
Medically important measurands
• Biopotentials (ECG, EEG, EMG, EOG, etc.)
• Pressure, flow, dimensions (imaging)
• Displacement (velocity dx/dt, acceleration d2x/d2t, and force =
md2x/d2t)
• Impedance, temperature and chemical concentration
The measurand may be localized to a specific organ or anatomical
Sensor
* The transducer or sensor should only respond to the form of energy present in the measurand to the exclusion of all others!
* The sensor should interface with the living system to minimize the energy extracted and being minimally invasive!
Signal conditioning
Usually the sensor output can not directly drive the display, therefore
signal processing or conditioning is required
Examples of signal processing:
1. Impedance matching
2. Amplification
3. Filtering
4. Mathematical mapping
5. Linearizing
6. Analog-to-digital conversion (ADC)
7. Digital-to-analog conversion (DAC)
8. Signal averaging to reduce noise (i.e. evoked response)
9. Transformation (time domain
frequency domain)
10. Compensation for undesirable sensor characteristics
11. Etc.
Output displays
Examples of output displays:
1. Numerical
2. Graphical
3. Discrete
4. Continuous
5. Permanent or temporary
• Most displays rely on our vision, but auditory sense is also sometimes used (for example, Doppler ultrasonic signals)
• User controls and output displays should conform to human factors engineering guidelines for the design of medical devices
Auxiliary Elements
*Calibration signal with the properties of the measurand should be applied to the sensor input or as early in the signal processing chain as possible
**Many forms of feedback (automatic or manual) may be required to elicit the measurand, to adjust the sensor and signal conditioner and to direct the flow of output (display, storage, transmission)
***Data storage for signal conditioning or examination of alarm conditions or implementation of different processing algorithms
**** Data communication transmission of patient data to remote display at nurse’s station and medical center
Operation Modes
1. Direct and Indirect Modes• Direct: Measurand directly to sensor
- readily accessible or
- acceptable invasive procedure
For example: direct blood pressure measurement
• Indirect: measurand not accessible- Use another measurand with known relation to the desired one
- Use some form of energy or material that interacts with the
desired measurand to generate a new accessible one
For example:
Cardiac output (volume of blood pumped/min by the heart)
- Measurements of respiration & blood gas concentration
- Dye dilution
- Morphology of internal organs determined from X-rays
2. Sampling or Continuous Modes
• Sampling: Parameters that change slowly do not require continuous measurements
For example: body temperature, ionic concentrations, etc.
• Continuous: Parameters that change fast enough to require continuous measurements
For example: ECG, EEG, EMG, respiratory gas flow, etc.
Note: Frequency content of the measurand, the objective of the measurement, the condition of the patient and the potential liability of the physician influence how often data should be acquired
3. Generating and Modulating Sensors
• Generating: Produce output from energy taken directly from measurand
For example: photovoltaic cell (output voltage related to irradiation)
• Modulating: Measurand changes flow of energy from an external source that affects the output of a sensor
For example: photoconductive cell (apply external power to the sensor to measure changes in resistance with irradiation)
4. Analog and Digital Modes
• Analog: Continuous (parameter takes on any value within the dynamic range)
For example: Parameters that change fast enough to require continuous measurements: ECG, EEG, EMG, respiratory gas flow, etc.
• Digital: Discrete (parameter takes on a finite number of different values)
* Most sensors are analog (i.e., strain gages, thermistors, etc.)
* Very few sensors are digital in nature (i.e., shaft encoders)
5. Real-time and delayed-time Modes
Real-time: Sensors must acquire signals as they actually occur
• Output is not always displayed immediately, because some types of signal processing (i.e. averaging, transformations, etc) require considerable amount of date before production of final results
Delayed-time Often acceptable (short delays) unless urgent feedback & control depend on output
• Cell cultures provide an example where several days of delay may be required before an output is obtained!
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