The Luxembourg-Gorky effect for in vitro characterization of lipid-coated microbubbles

The Luxembourg-Gorky effect, defined as a transfer of amplitude-modulation from one wave to a second wave, is applied to lipid-coated microbubbles tailored for nonlinear contrast ultrasound imaging. This nonlinear effect is non-classical because not created in micro-homogeneous media. A simple experimental set-up allowing the dual-frequency dual-beam insoniflcation of single bubbles was developed to study the interaction between a high-frequency probing ultrasonic wave and an amplitude-modulated low-frequency wave. Low-amplitude acoustic pressures are employed for both waves to reduce acoustically induced non-reversible processes. The frequency of the probing wave lies above the resonance frequency of microbubbles such that the pressure amplitude scattered by a single bubble is proportional to its diameter. As a result the bubble radial oscillation induced by the low-frequency wave produces an amplitude modulation and a phase modulation in the high-frequency signal scattered by single bubbles. The analysis of the latter signal provides direct access to the LF radial motion of bubbles. Using the high-frequency signal recorded before and after the LF manipulation as a reference, we show how the Luxembourg-Gorky effect can be used to quantify the asymmetry in acoustical induced vibrations of lipid-coated microbubbles. Among 220 investigated single bubbles, 49% exhibited full compression-only behavior. This low-amplitude highly nonlinear behavior is of great interest for medical contrast ultrasound imaging. Further work will aim at developing an "single-bubble acoustical camera" able to derive the absolute value of the radial oscillation.