Biomedical Instrumentation

1. Biomedical Instrumentation Sensors I

2. Instrumentation • Devices that can be used to make a measurement and give quantitative (or sometimes qualitative) results

3. Biomedical Instrumentation • Devices that can be used to make biological or medical measurement and give quantitative (or sometimes qualitative) results

4. Biomedical Instrumentation Examples

6. Future Biomedical Instrumentation • Tricorder (Star Trek) • Completely non-invasive • Internal and external measurements • Imaging • Intelligence to make diagnosis and suggest therapy

7. Display Storage Sensor Processor Basic Biomedical Instrument

8. Fundamental Rules of Biomedical Instrumentation • Minimum disturbance to the physiological system. • Maintain simplicity

9. Control And feedback Power source Sensor Perceptible output Primary Sensing element Variable Conversion element Signal processing Output display Measurand Data transmission Calibration Data storage signal Radiation, electric current, or other applied energy Generalized instrumentation system The sensor converts energy or information from the measurand to another form (usually electric). This signal is the processed and displayed so that humans can perceive the information. Elements and connections shown by dashed lines are optional for some applications.

10. Important Instrumentation Terms • Sensitivity—Change in output as a function of a change in input. • Stability—Consistency in output for a constant input. • Specificity—Ability to distinguish desired variable from other competing variables. • Accuracy—Difference between true value and measured value divided by the true value. • Precision—Number of distinguishable alternatives from which a given result is selected • Resolution—Smallest increment that can be measured with certainty. • Reproducibility—Same output for the same input.

11. Electrical Thermal Mechanical Chemical Optical Hydraulic Acoustic

12. Sensors

13. Types of Sensors • Physical • Chemical • Bioanalytical

14. Physical • Strain gauge • Accelerometer • Load cell (force) • Pressure sensor

15. Potentiometers

16. R2 R1 + - Linear and rotational position measurement • For example a potentiometer • voltage is a fraction of supply voltage is typically a few V • small gain required • may require buffering with an amplifier to prevent sensitivity to load impedance and long cables • i.e. to make into voltage source l x Potentiometer

17. RS + - Measurement techniques • Resistance varies in relation to a parameter • thermisters (temperature) • potentiometers (position) • use an amplifier to buffer against lower load impedances ?  R1 ? X sensor ? X

18. Wheatstone bridge R3 + - R3 R1 Rs R2 RS Measurement of small fractional changes • Many sensors vary their electrical characteristic by a small fraction over the range over which they are used • thermister - resistance varies by 4%/ºC • temperature IC varies by 1mA/K (i.e. 290-300 K = 3% change in i) • A differential amplifier can help to interface this to an ADC (or other device) to give a large fractional change R1 Vo + x1 - R2

19. 10 k Vref ADC sensor RS For example:an electronic medical thermometer • Employs a thermister to measure temperature in range 35-40 ºC. • < 0.1 ºC sensitivity required • 8-bit ADC • Thermister RS=10kW at 35 ºC • Temperature coefficient • 4% per ºC • At 40 ºC RS=10k x 1.045=12.166kW • Approximately linear 35 ºCto 40 ºC • Voltage at 40 ºC: Vmax=0.549V • Variation is 0.049/0.549=9% • 9% of ADC dynamic range is used • 23 counts for 5 ºC =0.22 ºC/count

20. RS 10 k ADC sensor • Or in graphical format • only levels 0-232 are unused: 0.22 • For this application • voltage across thermister is high • ADC has high input impedance • ADC is close to thermister • Thermister voltage can be applied directly to ADC without amplification • But the poor use of the ADC dynamic range means that the required resolution (0.1 ºC ) is not achieved...

21. 10k RS • A solution is to subtract the bias voltage with a differential amplifier (slide 93) • The voltage range is unchanged, but the voltage at the ADC input at the minimum temperature of 35 ºC is now zero volts • Using a differential amplifier with a gain of 10 with a Vref of 0.5V is now 0.49V, making good use of the ADC dynamic range VS 0.5VS 10k V’o Vref Vo + x10 ADC - VS/2 10k

22. Strain Gage • When external forces are applied to a stationary object, stress and strain are the result. • Stress is defined as the object's internal resisting forces, and • strain is defined as the displacement and deformation that occur.

23. Stress • Stress can be calculated stress can be calculated by dividing the force (F) applied by the unit area (A):

25. Strain is defined as the amount of deformation per unit length of an object when a load is applied. • Strain is calculated by dividing the total deformation of the original length by the original length (L):

26. L DL

27. The ideal strain gage is • small in size and mass, • low in cost, • easily attached, and • highly sensitive to strain but insensitive to ambient or process temperature variations.

29. Typical bonded strain-gage units Arrows above units show direction of maximal sensitivity to strain.

31. Accelerometers • Accelerometers are devices that produce voltage signals in proportion to the acceleration experienced.

32. The most general approach to acceleration measurement is to take advantage of Newton's law, which states that any mass that undergoes an acceleration is responding to a force given by F = ma.

33. Piezoelectric accelerometers are on the market, and are primarily offered for vibration measurement. • For moderate signals (milli-gs), fairly small devices with simple circuits are quite sufficient, so these devices can be in the 10-100 dollar range.

34. Pressure sensors • Aside from some fairly exotic approaches, pressure sensors all operate on the basis of the same principle: the detection of a physical force which arises due to pressure.

35. Flow • There are three basic approaches to the measurement of flow. • The first of these categories involves the use of thermal effects to measure fluid motion.

36. In general, this approach uses a heat source to deposit heat into the fluid, and a thermometer to measure the temperature of the fluid. • If the heat source is upstream of the sensor, flow increases heat transport and causes the sensor temperature to increase. • Another possible arrangement is to heat a thermistor with a fixed power, and measure its temperature. • In this case, fluid flow acts to cool the thermometer.

37. A slightly more complicated approach relies on Bernoulli's Equation, which is:

38. This roughly states that the Pressure + the kinetic energy density + the gravitational potential energy density is a constant throughout a fluid.

39. This principle is applied by measuring pressure at a pair of points in a fluid. • When water flows through a pipe with a varying diameter, the total flow rate in each region is a constant (since the fluid must all get through the tube). • Therefore, changes in tube diameter are compensated for by changes in fluid velocity. • By measuring the pressure in regions with different diameter, it is possible to measure fluid velocity.

40. The last technique for flow measurement is based on measurement of Doppler effects in sound transport.

41. Since sound is carried by pressure waves in a medium (the fluid), its transport laterally across a channel is affected by the motion of the fluid. • It is possible to measure the change in sound frequency due to fluid motion (direct Doppler effect, or listen for changes in the travel time from transmitter to receiver. • High sensitivity techniques generally measure frequency shifts, since excellent accuracy may be obtained by use of analog or digital signal processing techniques to measure small frequency shifts.

44. Chemical • Oxygen electrode • Glass electrode (pH) • Ion-selective electrode • CO2 sensor

45. The human nasal sensing apparatus contains a remarkably flexible and sensitive detection capability.

46. You are capable of detecting and distinguishing thousands of different smells with almost instantaneous recognition. • Odor detection is made very complicated because of the lack of actual uniqueness in the chemical basis of most smells. • There is no "garlic molecule" that is distinct from the "enchilada molecule"; yet you can easily distinguish these smells.

47. If a chemical sensing application requires detection of a particular molecule, several techniques are available. • These techniques are based on the unique properties of particular molecules.

48. Molecules may be recognized by their mass (or mass spectra...). • A mass spectrometers are used to detect and distinguish molecules.

49. Gas chromatography instruments are available. • In these instruments, the varying molecular diffusivities are used to detect specific molecules.