The effect of resonance on transient microbubble response; response; experimental and theoretical observations

Nonlinear microbubble oscillations can be exploited by signal processing to provide contrast enhancement in ultrasound imaging. High-speed optical investigations have established that microbubbles resonate in-vitro, but resonance has not yet been established in-vivo. Here we aim to establish the acoustic signature of resonance, which will help identify its occurrence in-vivo and further optimize microbubble signal processing. Resonant microbubbles provide a transient acoustic amplitude response, which is a signature unique to resonance. Large numbers of acoustic signals from single lipid-shelled Definity® microbubbles have been measured using a calibrated micro-acoustic system and a unique transient characteristic of resonance has been identified in the onset of scatter. We present theoretical results based on the Mooney-Rivlin strain softening shell model to show that for realistic shell parameters pulse durations longer than those routinely used in clinical imaging pulses are required to reach a steady state microbubble resonance, indicating the majority of contrast enhancement arises from off-primary resonance microbubble scatter. The specificity of this response may generate higher sensitivity signal processing algorithms, and this should be investigated in the future in realistic vessel sizes to address in vivo requirements.