Translated Abstract
In recent years, high intensity focused ultrasound (HIFU) which generates thermal, mechanical and cavitation effects is widely applicated as a solution of non-invasive tumor treatment. Therefore，it is a key point in this research field that how to enhance the therapeutic efficiency using cavitation effect.The optic investigation based on ICCD could observe the dynamic behaviors of microbubbles and microbubble clouds under different spatial and temporal conditions, and provide the theoretical basis to the further application of focused ultrasound effectively.
In the first step, this paper discusses the dynamic behavior of a single cavitation bubble in a HIFU acoustic field discribed by using the Rayleigh-Plesset equation. After analyzing the numerical simulation of cavitation threshold, bubble collapse time, physical activities and resonant frequency, we concluded that: as the water has a temperature of 25 C, once the sound intensity reaches its cavitation threshold, a cavitation bubble is formed in the solution. Its collapse time is in direct propotion to the sound intensity, but is limit to the period of the sound wave. The physical activities of a single bubble in acoustic field depends on the sound pressure, initial bubble radius and gas contained in the bubble etc., a high sound pressure, a large radius and existence of inert gas may lead the cavitation effect more acute.
In this study, we tried to use the superoxide anion (O2-) released in the collapse of cavitation bubbles to oxidate alkaline luminol solution, which issues a blue and violet light. The ICCD camera can be used to obtain the distribution of the cavitation bubbles in the HIFU acoustic field. In this study, based on the improved sonoluminescence system, we observed respectively the sonoluminescence phenomenon in free HIFU acoustic field, interface of organization and the participation of ultrasound contrast agent. The results show that: in a certain sound power, collapse of bubble didn’t appear in the focal field because of the ultrasonic radiation force and the collapse time. The luminous phenomena appear only in the anterior and posterior areas. Luminous intensity is proportional to the sound power of HIFU within a certain range. Moreover, growth rate of the intensity becomes greater and the light location is also further from transducer as the sound power increases. There is a strong reflection of the sound wave by biological tissue surface, which creates the standing wave field and thus an uneven distribution of cavitation bubbles. This causes the sound ECL appearing like bright and dark stripes. Participation of ultrasound contrast agents increases the amount of cavitation bubble in solution, which makes cavitation significantly enhanced and more superoxide anion released. So the light intensity is greater than in the free field.
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