Single microbubbles vibrations measured with an “acoustical camera"

Abstract

Measuring and understanding the nonlinear vibrations of contrast agent microbubbles in response to an ultrasound wave is essential to optimize the technique employed by an ultrasound scanner to distinguish microbubbles from tissue. In this context, an acoustical method was developed to retrieve the radial response of single microbubbles to a pressure wave by means of a low-amplitude probing wave. If the frequency of the latter is much higher than the spherical resonance frequency of the microbubble (typically between 1 and 10 MHz), the relative amplitude modulation (induced by a pressure wave) in the signal scattered in response to the probing wave is quasi-equal to the radial strain (i.e., relative variation in radius) induced by the pressure wave. A reference response to the probing wave acquired before and after the transmission of the pressure wave allows us to reveal asymmetry in microbubble oscillations. Although efficient nonlinear wave interaction is well known in bubbly liquids, we demonstrated that such an “acoustical camera” can extract quantitative information (the radius as function of time) on single bubble vibrations by analyzing the nonlinear coupling between two ultrasound waves.