A study on microbubble-assisted acoustic cavitation

The cavitation-induced damage was carefully avoided in the past for safety reasons. Nevertheless, cavitation based therapy has attracted considerable attention recently for its capability of causing less damage to surrounding normal tissue than conventional ultrasound thermal therapy. Our long-term goal is to integrate ultrasound diagnosis and therapy- to achieve localized cancer treatment by microbubble-assisted cavitation under safety regulations. This study is the first step to the long-term work. The inertial cavitation doses (ICDs) are obtained and studied at a constant mechanical index (MI). The MIs vary from 0.06 to 0.79. The ICDs induced by 1.5-MHz signals and 3-MHz signals with the same MIs and the same pulse lengths are not equal. Furthermore, the ICD induced by a signal cascading a 3-MHz signal with a 1.5-MHz signal is compared with those induced by a 3-MHz signal and a 1.5-MHz signal at constant MI. The three waveforms induce different ICDs even if they have the same MIs. This is probably because the formulation of MI was based on the cavitation activities of free gas bubbles, but encapsulated bubbles were used in our experiments. Another reason is that MI is not directly related to the degree and amount of cavitation as ICD. As a result, MI might not be the best index to quantify acoustic cavitation. Furthermore, the cascade waveforms induce greater ICDs than single-frequency waveforms under specific conditions. It is possible to induce acoustic cavitation more efficiently by properly designing the transmit waveforms. Finally, the use of MI as the only parameter to represent potential cavitation-induced tissue damage is also questionable and requires further investigation.