Stable and transient subharmonic emissions from isolated contrast agent microbubbles

Ultrasound contrast agents (UCAs) have been widely studied in recent years in order to improve and develop new, sophisticated imaging techniques for clinical applications. In order to improve the understanding of microbubble-ultrasound interactions, an acoustic dynamic characterization of UCA microbubble behavior was performed in this work using a high frame-rate acquiring and processing system. This equipment is connected to a commercial scanner that provides RF beam-formed data with a frame-rate of 30 Hz. Acquired RF sequences allows us to follow the dynamics of cavitation mechanisms in its temporal evolution during different insonifying conditions. The experimental setup allowed us to keep the bubbles free in a spatial region of the supporting medium, thus avoiding boundary effects that can alter the ultrasound field and the scattered echo from bubbles. The work focuses on the study of subharmonic emission from an isolated bubble of contrast agent. In particular, the acoustic pressure threshold for a subharmonic stable emission was evaluated for a subset of 50 microbubbles at 3.3 MHz and at 5 MHz of insonation frequencies. An unexpected second pressure threshold, which caused the standstill of the subharmonic emission, was detected at 3.3 MHz and 5 MHz excitation frequencies. A transient subharmonic emission, which is hypothesized as being related to the formation of new free gas bubbles, was detected during the ultrasound-induced destruction of microbubbles. An experimental procedure was devised in order to investigate these behaviors and several sequences of RF echo signals and the related spectra, acquired from an isolated bubble in different insonation conditions, are presented and discussed in this paper.