The relationship between bubble concentration and the acoustic emission energy of separate frequency bands.

Krit Sujarittam,James J Choi

doi:10.1121/10.0009394

Abstract

This letter presents the relationship between bubble concentration and the energy ratio of low to high frequency bands of their acoustic emissions. Two sensors, placed perpendicular and concentric to a transmitter, captured the emissions from sonicated microbubbles. Emissions from different bubbles arrived at the perpendicular sensor with small time differences. Low frequencies with periods longer than the time differences interfered constructively, while higher frequencies interfered both constructively and destructively. The low-frequency (2nd-3rd harmonics) to high-frequency (7th-12th harmonics) energy ratio increased with the bubble concentration. The relationship was not observed with the concentric sensor, where the time differences were larger.

Highlights

In microbubble-mediated ultrasound therapies, cavitation—volumetric oscillations of a bubble—is responsible for producing both therapeutic and adverse biological effects
The purpose of this study was to evaluate whether the energy ratio of low to high frequency content of acoustic emissions from bubbles changed with bubble concentration
Most cycles contained only one secondary compressional peak with a very small amplitude. This resulted in a periodic waveform where two compressional peaks and one rarefactional peak appear on every acoustic period

Summary

Introduction

In microbubble-mediated ultrasound therapies, cavitation—volumetric oscillations of a bubble—is responsible for producing both therapeutic and adverse biological effects. Acoustic-based methods can estimate the concentration of intravenously injected preformed microbubbles using acoustic pressures and frequencies relevant to imaging procedures (Marsh et al, 1998; Mari et al, 2007; Lampaskis and Averkiou, 2010; Sciallero et al, 2011; Leithem et al, 2012). An acoustic-based method that uses therapeutically relevant pressures and frequencies is not available. In such a regime, bubbles are expected to act very differently, such as fracturing, dissolution, and undergoing inertial cavitation (Chomas et al, 2001; Lindsey et al, 2015; Shi et al, 2000; Ilovitsh et al, 2018). The concentration of cavitation events during a treatment is often unknown

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