Electromagnetic Blood Flow Meter Dr. Erdem Topsakal, Advisor Brian McCalebb Taffa Porter Ky

1. Electromagnetic Blood Flow Meter Dr. Erdem Topsakal, Advisor Brian McCalebb Taffa Porter Kyle Eubanks Nashlie Sephus

2. Outline • Problem Statement • Solution • Introduction/Historical Information • Technical Constraints • Practical Constraints • Design Approaches/Tradeoffs - Testing Apparatus - Calibration - Probe - Electronics • Timeline • References

3. Problem • The electromagnetic blood flow meters were originally used by the University of Mississippi Medical Center. • Obtaining replacements is no longer possible. • Other commercially available flow meters proved inadequate.

4. Solution • Reproduce and improve the original probe • Reproduce similar results to the original meter • Minimum Cost • Dependable • Easy to use • Obtain accurate results

5. Electromagnetic Blood Flow Meter • What Are Flow Meters? • Electromagnetic blood flow meters measure blood flow in blood vessels • Consists of a probe connected to a flow sensor box

6. Electromagnetic Blood Flow Meter • Why They Are Used? • Offers quantitative data during surgical operations • Provides a functional assessment of newly joined vessels, grafts and organs • Used in prosthesis in conjunction with cardiovascular surgical procedures • Most accurate results through both acute and chronic implants

7. Electromagnetic Blood Flow Meter How Do They Work? • Faraday's Law of Magnetic Induction • Liquid acts as a conductor • Voltage is induced directly related to the average flow velocity • “The faster the flow rate, the higher the voltage” • Voltage is measured by sensing electrodes mounted in the meter tube • Voltage is then sent to the flow sensor box

8. Outline • Problem Statement • Solution • Introduction/Historical Information • Technical Constraints • Practical Constraints • Design Approaches/Tradeoffs - Testing Apparatus - Calibration - Probe - Electronics • Timeline • References

9. Technical Constraints

10. Measurement Accuracy • Affected by: - Stray magnetic fields detected by electrodes - Non-uniform magnetic field - Turbulent fluid flow - Non-homogenous fluid • Accurate to within ±1 cm/s from 0.1-1 m/s - 10% maximum error at 0.1 m/s - 1% maximum error at 1 m/s

11. Maximum Fluid Velocity • Fluid velocities in an aorta - 89±9.5 cm/s during heart contraction - 36±6.0 cm/s between contractions • Fluid velocity for flow meter - 1 m/s maximum • Importance of low maximum fluid velocity - Maximizes accuracy for low fluid velocities by allowing more precision in A/D conversion

12. Conductivity • Calibrated conductivity range • Conductivity of blood - 70 Siemens per centimeter - Varies by up to 20% based on flow rate • Acceptable conductivity range of flow meter - 60–80 Siemens per centimeter

13. Size • Probe Size • 22 mm inner diameter • Reasoning - Requested by sponsor - Diameter of aorta ranges from 21-35 mm - Larger aortas taper down to smaller diameters

14. Practical Constraints

15. Sustainability • Implanted for 2-3 months • Protection of wire leads, magnetic core, & wire coil • Maintenance of electrodes

16. Ethical • Designed for cow’s aorta only • Not approved or tested for human use

17. Outline • Problem Statement • Solution • Introduction/Historical Information • Technical Constraints • Practical Constraints • Design Approaches/Tradeoffs - Testing Apparatus - Calibration - Probe - Electronics • Timeline • References

18. Testing Apparatus

19. Testing Apparatus

20. Testing Apparatus

21. Testing Apparatus • Via Aqua 1800 - $25.00 - 480 GPH, variable flow rate - 3/4” connections - Saltwater safe • Acrylic tubing - $3.32 / 6ft. - 7/8” OD, 3/4” ID, 1/16” thickness - Insulating material

22. Testing Apparatus • Dialysis tubing - $5.25 / 10ft - Will be used in future testing - 22 mm diameter - 1 mil thickness - Closely replicates the conductivity of an aorta - May use multiple layers to adjust the conductivity or increase the water pressure it can withstand

23. Calibration • Calibration and error can be minimized by selecting the proper voltage source waveform • DC - Voltage drift from polarization of electrodes • Sinusoidal AC - Reduces voltage drift by changing polarity - Induces emf in electrodes from magnetic flux variation • Square-wave AC - Reduces voltage drift by changing polarity - Reduces emf in electrodes from magnetic flux variation

24. Calibration • Why does the output voltage of the probe not directly correlate to the fluid velocity? • Ideally, they are directly proportional • However, sources of error include: - Stray magnetic fields picked up by electrodes - Resistive and capacitive current leakage - Voltage loss through tubing - Voltage drift - Noise

25. Calibration • Fundamentals of Calibration • Under no flow conditions - Voltage observed is the combination of all sources of error since no voltage from fluid flow • Zero the output waveform - Cancels out all unwanted voltage contributions - Output now has a zero baseline voltage • Output voltage is now directly proportional to the fluid velocity

26. Calibration • Procedure - The most correct solution is to subtract the entire unwanted waveform - Requires unnecessary computations - Sampling occurs at the same place every cycle - Under no flow, calculate the average voltage at the desired sampling location for every cycle over some period - Subtracting this voltage for every sample gives the voltage contribution due to fluid velocity alone

27. Outline • Problem Statement • Solution • Introduction/Historical Information • Technical Constraints • Practical Constraints • Design Approaches/Tradeoffs - Testing Apparatus - Calibration - Probe - Electronics • Timeline • References

28. Probe Key components • Magnetic core • Magnet wire • Electrodes

29. Probe • Magnetic cores • Permeability - Describes how much a material is affected by magnetic fields and how strong a field it can generate • High permeability results in: - A stronger magnetic field

30. Probe • Magnetic cores

31. Probe • Magnet wire Key factors - Thin - Well-insulated - Close to range of 34 gauge (commonly used in industrial electromagnetic blood flow meters) - Availability - Low cost

32. Probe • Electrodes Desirable qualities • Low resistance - More sensitive • High capacitance - Reduces source impedance and signal attenuation

33. Electrodes - Capacitance Platinized Platinum Capacitance (µF) Frequency (Hz)

34. Electrodes - Resistance Resistance (kΩ) Platinized Platinum Frequency (Hz)

35. Electrodes material Electrodes

36. Electronics Flow sensor box

37. Design Approach Timeline

38. References [1] Shercliff, J.A., The Theory of Electromagentic Flow-Measurement. Cambridge: University Press, 1962,pp. 2-4, 125. [2] Flowmeter Directory. Flowmeter Directory. 2007. http://www.flowmeterdirectory.com/flowmeter_electromagnetic.html [3] Tavoularis, S., Measurement in Fluid Mechanics. Cambridge: University Press, 2005, pp. 217. [4] Omega Engineering. Omega Engineering. 2006. http://www.omega.com/techref/flowcontrol.html [5] EesiFlo. Eesiflo. 2007. http://www.eesiflo.com/applications.html [6] Braun, U., and vetJosef, F., “Duplex ultrasonography of the common carotid artery and external jugular vein of cows,” American Journal of Veterinary Research, vol. 66, no. 6, pp.962-965, 2005. [7] Khan, S. R., and Islam, M. N., “Studies on the prospect of bioprostheses by cow aortic valve for human use,” Bangladesh Med Res Counc Bull, vol. 17, no. 2, pp. 75-80, 1991. [8] Wyatt, D. G., and Phil, D., "Problems in the Measurement of Blood Flow by Magnetic Induction," Phys. Med. Biol., vol. 5, pp. 369-399, 1961.