|M.Sc Student||Ganor Yaniv|
|Subject||Analysis of Radiated Acoustic Signals from Micro-Bubbles:|
Effects of Changes in Ambient Pressure
|Department||Department of Agricultural Engineering||Supervisor||Professor Emeritus Eitan Kimmel|
Encapsulated micro-bubbles, which are used as ultrasound contrast agents in ultrasonography, may be considered for in-vivo blood pressure measurement in the cardiovascular system. For this goal, we develop a simulation and an experimental setup that relate the frequency response of radiated acoustic signals from the micro-bubbles to the varying ambient pressure. Two complementing methods are used: micro-bubbles modeling and simulation, and laboratory experiments. Simulations are used to gain an understanding of the expected response of micro-bubbles to acoustical drive and ambient pressure variations. The laboratory experiments are used to verify and confirm the simulation results, and examine methods for potential use of micro-bubbles for blood pressure measurement. The design and construction of the experimental setup, and the experimental procedures used, are based on results and conclusions from extensive simulation runs.
Our results show that the radiated pressure from micro-bubble generates a significant sub-harmonic component modulated by the ambient pressure, for pressure changes of up to 200mmHg. We found that a decrease in the micro-bubbles steady-state radii, attributed to diffusion, causes a 180º phase reversal between the radiated pressure modulation and the ambient pressure variation. This phase-reversal explains the observed time delay in the build-up of the subharmonic modulation response. Additionally, we identify a frequency-capturing effect that controls the frequency of the radiated pressure at the sub-harmonic vicinity, and defines an upper time limit on the usability of micro-bubbles for ambient pressure measurements. Our experiments, in which Optison™ micro-bubbles were used, support the simulation results and establish a potential use of Optison™ micro-bubbles for in-vivo blood pressure measurements.