Subharmonics are not an infallible safety metric for cavitation dynamics

There has been a substantial increase in the range of applications for focused ultrasound in recent years with multiple clinical trials reporting promising results. Reliable treatment monitoring protocols will be essential for future clinical development of these therapies. Acoustic emissions provide a valuable source of data for real time monitoring, especially for procedures in which cavitation plays an important role. Currently, however, there is a lack of consensus as to whether specific frequency components (e.g., broadband noise or subharmonics) are more or less valuable as indicators of cavitation. Our aim in this study was to investigate the relationship between cavitation bubble dynamics and corresponding acoustic emissions from single bubbles, with a specific focus on the conditions producing subharmonic emissions.

We used a combination of acoustic measurements and ultra-high speed imaging captured simultaneously from single phosphoplipid-coated microbubbles generated using a microfluidic device to control their size and separation.

Contrary to much of the literature, we found that there were no well-defined thresholds in any of the frequency components commonly used in cavitation monitoring (harmonics, subharmonics, ultraharmonics or broadband noise) at a driving frequency of 0.5 MHz within a therapeutically relevant range of pressures (0.12-1.2 MPa), nor was there any clear relationship between changes in the vigor of cavitation bubble dynamics and the corresponding subharmonic emissions. In addition, the magnitude of the subharmonic signal power was typically an order of magnitude lower than the local broadband noise, even after compensating for receiver sensitivity.

These results suggest that caution is needed in utilizing simple metrics such as the presence or absence of a subharmonic signal as an indicator of cavitation; especially in applications where off-target cavitation represents a safety concern.