Non-linear Acoustic Emissions from Therapeutically Driven Contrast Agent Microbubbles.

Area of Science:

• Acoustics
• Biomedical Engineering
• Fluid Dynamics

Background:

• Non-linear acoustic emissions from microbubbles are vital for monitoring ultrasound therapies.
• The precise mechanisms generating these emissions, especially under therapeutic conditions, remain unclear.

Purpose of the Study:

• To investigate the source of non-linear acoustic emissions from microbubbles under focused ultrasound.
• To elucidate the role of bubble collapse in generating these emissions at therapeutic pressure amplitudes.

Main Methods:

• Dual-perspective high-speed imaging (210,000 fps) and ultra-high-speed shadowgraphy (10 Mfps).
• Observation of microbubbles in a capillary exposed to 692 kHz focused ultrasound.
• Simultaneous acoustic emission collection using a calibrated needle hydrophone.

Main Results:

• Periodic bubble-collapse shock waves were identified as the dominant source of non-linear acoustic emissions.
• Subharmonics were observed at higher driving amplitudes, linked to bubble collapse.
• Variations in shock wave amplitude and timing contributed to broadband emissions.

Conclusions:

• Bubble-collapse shock waves significantly influence microbubble acoustic emissions.
• Findings have implications for in vivo cavitation detection and ultrasound therapy mechanisms.
• Challenges the conventional stable/inertial cavitation classification.