Passive Cavitation Detection

1. Passive Cavitation Detection By Jeremy Bredfeldt ECE 298/299 – Senior Thesis Bioacoustics Research Laboratory University of Illinois

2. Table of Contents • 1. Purpose • 2. Cavitation Theory • 3. Experimental Method • 4. Results • 5. Conclusions

3. 1. Purpose The purpose of this work is to use a passive cavitation detection device to help determine the role played by cavitation in the pathogenesis of ultrasound-induced lung hemorrhage.

4. Ultrasound • Ultrasound is used in therapy and diagnostics • Therapeutic ultrasound: desired change in tissue (20-100MPa); tissue heating (controllable, understood), cavitation (difficult to measure and control) • Diagnostic ultrasound: no change in tissue is desired (.2-20MPa)

5. Ultrasound-induced lung hemorrhage Clinical levels of diagnostic ultrasound have been shown to cause hemorrhage in lung tissue of a variety of small mammals.

6. What causes ultrasound-induced lung hemorrhage? • Gas in lung plays a vital role • Mechanism is non-thermal • Mechanical mechanisms- Cavitation or non-cavitation? • For more than a decade, responsibility for lung damage has been placed on cavitation.

7. Cavitation or non-cavitation? • There is no direct evidence that shows that ultrasound-induced lung damage is caused by cavitation. • On the contrary, support is growing that implies cavitation is not the cause for ultrasound-induced lung damage.

8. 2. Cavitation Theory • What is cavitation? • Very broadly: cavity (bubble) formation in a liquid • More specifically: resonance or distortion of pre-existing cavities in a liquid in response to an acoustic field; this includes the extensive growth and collapse of microscopic bubbles in response to excitation by an acoustic field

9. Cavitation and lung-hemorrhage • Small air bubbles on the surface of the lung have been thought to serve as cavitation nuclei. • When these bubbles cavitate in the presence of an ultrasonic field, shock waves, free radicals, high temperatures, and microjets have been thought to cause tissue damage, resulting in hemorrhage.

10. How can cavitation be detected passively? • When a bubble oscillates, acoustic energy is emitted • At low drive power, bubbles pulsate linearly, emitting energy at the isonation frequency and higher harmonics • At high drive power, bubbles collapse emitting more broad-band energy and reflections of harmonics is depressed

11. 3. Experimental Methods • Detector block diagram • Transducers • Amplifiers • Data Acquisition and Processing

12. Passive Cavitation Detector

13. Transducers • Source • Center Frequency = 2.87MHz • Focal Length = 3.81cm • Diameter = 1.905cm • Receiver • Center Frequency = 13.0MHz • Focal Length = 3.81cm • Diameter = 1.27cm

14. Amplifiers • Clamped Diplexer • Allowed source transducer to be used in pulse echo mode for alignment purposes • Power Amplifier • Fc = 2.82MHz, PRF=10Hz, cycles=3 • Receiver Amplifier • Band pass filter from 1MHz – 45MHz • Gain=30dB, 40dB, and 50dB

15. Data Acquisition and Processing • Lecroy 9354TM set to sequence mode • Sampling rate maxed out at 50Ms/sec • Received 100 waveforms then stopped • Scope explorer software used to digitize 100 waveforms at a time • Matlab used to display and process data

16. 4. Results • Transducer alignment • Signals from solid targets • Signals from large air bubbles • Signals from microscopic Optison bubbles • Animal exposures - signals from pleural surface

17. Transducer Alignment

18. Signals from solid targets • Reflection received off of 381um steel piano wire target, placed vertically in beam field • Waveform shown corresponds to • MaxPc=40.76MPa (RS=35) • MaxPr=14.58MPa (RS=35)

21. Signals from large air bubbles • Bubbles were slowly injected into beam field using a syringe, evenly spanning the ten second scan • Bubbles were roughly estimated at having 3mm diameter • Waveforms correspond to: • MaxPc=40.76MPa (RS=35) • MaxPr=14.58MPa (RS=35)

24. Signals from microscopic bubbles • Optison® characteristics • Human serum albumin shells • Octafluoropropane gas • Average diameter = 2.0-4.5um • .4mL Optison® suspended in approx. 4.2e7 cubic mm degassed water ~= 6 microbubbles/cubic mm (focal region can be approximated as 1 cubic mm)

27. Animal exposures – Signals from pleural surface • 6 Rats were exposed • 3 exposed to all pressure levels, received waveform was recorded after each • 3 exposed to lowest and highest pressure levels only, received waveforms recorded • Lesions were scored and photographed • Difficult alignment procedure! 

28. Exposed to all pressure levels –only the highest pressure level shown here

29. Exposed to all pressure levels –only the highest pressure level shown here

30. Exposed to all pressure levels –only the highest pressure level shown here

31. Exposed to lowest and highest pressure levels only –highest pressure level shown here

32. Exposed to lowest and highest pressure levels only –highest pressure level shown here

33. Exposed to lowest and highest pressure levels only –highest pressure level shown here

34. 5. Conclusions • Correlation between received signal and lung hemorrhage is postitive • Is cavitation occuring on lung surface? • Conclusion: Cavitation is not occuring • 1. Not significantly more broad-banded • 2. Contains strong harmonics

35. Where to go from here? • Look at more contrast agent signals received through detector to get a more conclusive characterization of cavitation • Correlate findings to previous cavitation work by replicating experimental setups and comparing results to simulations of bubble resonation

36. 6. Acknowledgements • Special thanks to: Dr. O’Brien Rita Miller Jim Blue Bill Zierfuss Members of BRL

37. Questions? Thank you!