Time and pressure dependence of acoustic signals radiated from microbubbles.

Encapsulated microbubbles are considered to be microsensors for in vivo blood pressure measurements in the cardiovascular system. To study the potential of this method, we developed a simulation and an experimental set-up that relate various characteristics of radiated acoustic signals from the microbubbles to the varying ambient pressure. Both the simulation and the experiment show that the radiated pressure from microbubbles generates a significant subharmonic component, which is modulated by changes in the ambient pressure. A time-dependent decrease of the steady-state radii within a population of microbubbles causes a phase reversal phenomenon, which explains the observed time delay in the build-up of the subharmonic modulation response. Additionally, we identify a frequency-capturing effect that indicates the termination of the nonlinear behavior of the microbubbles. Our research suggests that these subharmonic signals can be used for in vivo blood pressure measurements and highlights some of the considerations that need to be addressed in developing such techniques.