Response of bubbles to diagnostic ultrasound: a unifying theoretical approach

TL;DR

This paper presents a unified theoretical framework for understanding how micrometer-sized bubbles scatter and emit sound in response to diagnostic ultrasound, revealing nonlinear effects that significantly enhance sound emission.

Contribution

It introduces a nonlinear oscillator model that unifies scattering and emission phenomena, recovering known formulas in the linear case and highlighting nonlinear effects.

Findings

Nonlinear oscillations can increase scattering and emission cross sections by orders of magnitude.

Most incident sound energy is converted into emitted sound during nonlinear bubble oscillations.

The model unifies scattering and emission physics, resolving apparent contradictions.

Abstract

The scattering of ultrasound from bubbles of micrometer-sized radius, such as used in contrast enhancers for ultrasound diagnostics, is studied. We show that sound scattering and ``active'' emission of sound from oscillating bubbles are not contradictory, but are just two different aspects derived from the same physics. Treating the bubble as a nonlinear oscillator, we arrive at general formulas for scattering and absorption cross sections. We show that several well-known formulas are recovered in the linear limit of this ansatz. In the case of strongly nonlinear oscillations, however, the cross sections can be larger than those for linear response by several orders of magnitude. The major part of the incident sound energy is then converted into emitted sound, unlike what happens in the linear case, where the absorption cross sections exceed the scattering cross sections.