Using the correlation property of subharmonic response as an index of cavitation of microbubbles [ultrasonics contrast agent]

In this study, we try to specify whether the weak correlation value of two consecutive Doppler signals for the subharmonics (SH) is only related to the reduced observation time, since the SH are generated from the smaller bubbles, the radiation force effect also reduces the correlation value. Our numerical results indicate that most bubbles producing the SH undergo cavitation while the applied pressure is increased to make the SH significant. In addition, the numerical results show that there are a non-negligible amount of bubbles, which produce the SH and undergo cavitation simultaneously, yielding the spiky Doppler response. Such behavior is demonstrated in the experimental Doppler power spectrum for the SH. The inspection of this spectrum shows that it does not look like the broadened spectrum due to the radiation force. In addition, an experiment diminishing the radiation force effect showed that the correlation is still very weak. Finally, the rapidly decreased SH intensity under continuous insonification can give us clear evidence that most of the bubbles producing the SH would be destroyed while they are moving through the sample volume.