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Basic Concepts

Basic Concepts. Of Medical Instrumentation. Block diagram of a generalized instrumentation system. The Bourdon Gauge. Block diagram of the pressure gauge based on Bourdon tube. Outputs. Signal. Signal. Data. Sensor. conditioning. processing. displays. Measurand. Feedback. Data.

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Basic Concepts

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  1. Basic Concepts Of Medical Instrumentation Dr. Bahauddin Karagozuglu

  2. Block diagram of a generalized instrumentation system Dr. Bahauddin Karagozuglu

  3. The Bourdon Gauge Dr. Bahauddin Karagozuglu

  4. Block diagram of the pressure gauge based on Bourdon tube Dr. Bahauddin Karagozuglu

  5. Outputs Signal Signal Data Sensor conditioning processing displays Measurand Feedback Data Data Effector storage communication A typical medical measurement system Dr. Bahauddin Karagozuglu

  6. Patient Instrument Patient Clinician Instrument Feedback with and without clinician Dr. Bahauddin Karagozuglu

  7. Abnormal readings Clinician Patient Instrument A patient monitors vital signs and notify a clinician if abnormalities occur Dr. Bahauddin Karagozuglu

  8. Detailed generalized medical measurement system Dr. Bahauddin Karagozuglu

  9. Alternative operational modes • Direct-indirect modes • Sampling and continuous modes • Generating and modulating sensors • Analog and digital modes • Real-time and delayed-time modes Dr. Bahauddin Karagozuglu

  10. Example to sampled data Complete blood count for a male subject. Dr. Bahauddin Karagozuglu

  11. Amplitude Amplitude Time Time Analog and digital signals Analog signals can have any amplitude value Digital signals have a limited number of amplitude values Dr. Bahauddin Karagozuglu

  12. Amplitude Amplitude Time Time Continuous and discrete-time signals Continuous signals have values at every instant of time Discrete-time signals are sampled periodically and do not provide values between these sampling times Dr. Bahauddin Karagozuglu

  13. Origins of common biological signal Dr. Bahauddin Karagozuglu

  14. Medical measurement constraints Dr. Bahauddin Karagozuglu

  15. Setting sensor specifications Sensor specifications for a blood pressure sensor are determined by a committee composed of individuals from academia, industry, hospitals, and government. Dr. Bahauddin Karagozuglu

  16. Specifications for ECG Specification values for an electrocardiograph are agreed upon by a committee. Dr. Bahauddin Karagozuglu

  17. Classification of biomedical instruments • Quantity sensed: pressure, flow, temperature etc. • Principle of transduction: resistive, inductive, capacitive, ultrasonic or electrochemical • Organ system studied: cardiovascular, pulmonary, nervous, and endocrine systems. • Clinical medical specialties: pediatrics, obstetrics, cardiology, or radiology. Dr. Bahauddin Karagozuglu

  18. Interfering and modifying inputs An interfering input may shift the baseline Original waveform A modifying input may change the gain Dr. Bahauddin Karagozuglu

  19. Simplified Electrocardiographic recording system Dr. Bahauddin Karagozuglu

  20. Compensation Techniques • Inherent insensitivity • Negative feedback • Signal filtering • Opposing inputs Dr. Bahauddin Karagozuglu

  21. Negative feedback y + Gd xd - Hf Dr. Bahauddin Karagozuglu

  22. Signal filtering Signals without noise are uncorrupted Interference superimposed on signals causes error. Frequency filters can be used to reduce noise and interference Dr. Bahauddin Karagozuglu

  23. Opposing inputs • Differential amplifier: v0 = Gd(vA- vB) • DC cancellation (bucking) An input signal with dc offset An input signal without dc offset Dr. Bahauddin Karagozuglu

  24. Accuracy Precision and reproducibility Resolution Statistical control Static sensitivity Zero drift Sensitivity drift Linearity Input ranges Input impedance Generalized Static Characteristics Dr. Bahauddin Karagozuglu

  25. Data points with Accuracy Accuracy: closeness with which an instrument reading approaches the true or accepted value of the variable (quantity) being measured. It is considered to be an indicator of the total error in the measurement without looking into the sources of errors. low accuracy Accuracy is often expressed in percentage high accuracy Dr. Bahauddin Karagozuglu

  26. Data points with Precision • A measure of the reproducibility of the measurements; i.e., given a fixed value of a variable, precision is a measure of the degree to which successive measurements differ from one another. low precision • Number of distinguishable alternatives. 2.434 V is more precise than 2.43 V. high precision Dr. Bahauddin Karagozuglu

  27. Resolution • The smallest change in measured value to which the instrument will respond. Statistical control: random variations in measured quantities are tolerable, Coulter counter example Dr. Bahauddin Karagozuglu

  28. Tolerance • Maximum deviation allowed from the conventional true value. • It is not possible to built a perfect system or make an exact measurement. All devices deviate from their ideal (design) characteristics and all measurements include uncertainties (doubts). • Hence, all devices include tolerances in their specifications. If the instrument is used for high-precision applications, the design tolerances must be small. • However, if a low degree of accuracy is acceptable, it is not economical to use expensive sensors and precise sensing components Dr. Bahauddin Karagozuglu

  29. Sensor Sensor signal signal Measurand Measurand Static sensitivity A low-sensitivity sensor has low gain A high sensitivity sensor has high gain Dr. Bahauddin Karagozuglu

  30. Static sensitivity constant over a limited range Dr. Bahauddin Karagozuglu

  31. Zero and sensitivity drifts Dr. Bahauddin Karagozuglu

  32. Output Output Input Input Linearity A nonlinear system does not fit a straight line A linear system fits the equation y = mx + b. Dr. Bahauddin Karagozuglu

  33. Output Output Input Input Calibration for linearity The one-point calibration may miss nonlinearity The two-point calibration may also miss nonlinearity Measuring instruments should be calibrated against a standard that has an accuracy 3 to 10 times better than the desired calibration accuracy Dr. Bahauddin Karagozuglu

  34. Hysteresis A hysteresis loop. The output curve obtained when increasing the measurand is different from the output obtained when decreasing the measurand. Dr. Bahauddin Karagozuglu

  35. Independent nonlinearity Dr. Bahauddin Karagozuglu

  36. Input ranges An input signal which exceeds the dynamic range The resulting amplified signal is saturated at 1 V Dr. Bahauddin Karagozuglu

  37. Input impedance Xd1 : effort variable System Output Xd2 : flow variable Dr. Bahauddin Karagozuglu

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