An integrated and dynamic imaging technique for simultaneously monitoring tissue damage formation and cavitation bubble activity during High Intensity Focused Ultrasound (HIFU) Therapy

High intensity focused ultrasound (HIFU) is a widely used and non-invasive surgery technique, which requires real-time monitoring for safety and efficiency. However, current imaging methods usually display information associated with tissue thermal lesions and cavitation bubble activities in a mixed fashion. This paper proposes an integrated and dynamic monitoring imaging technique that uses specific temporal sequences of HIFU exposures and radio frequency (RF) data acquisition, and post-treatment algorithms to separate the cavitation information from the tissue thermal lesion information and display them simultaneously. Two kinds of imaging pulse sequences synchronized with HIFU were designed: (1) Two consecutive frames with the same transmission waveforms were transmitted, and the two corresponding RF data frames were acquired before and between every two HIFU exposures. (2) A pulse inversion (PI) transmission technique was utilized, and the corresponding RF data frame was acquired, which was divided into a positive frame and a negative frame. By choosing the two RF data frames with positive pulse transmission before and during HIFU treatment, the integral differential images could be obtained. This can reflect whole changes of a treated area, including tissue damage formation and cavitation bubble activity. By choosing the two consecutive RF data frames or the PI-based frame between HIFU exposures at the same time, the transient differential images or PI-based second-/sub-harmonic images could be obtained, which could reflect cavitation bubble activity. The former was related to the movement, dissolution, and/or collapse of cavitation bubbles, and the latter was related to the non-linearity of cavitation bubbles. The in vitro experimental results from porcine muscle verified the validity of the proposed monitoring imaging method. This method could monitor tissue damage formation and cavitation bubble activity simultaneously during HIFU therapy, and it has the potential to be used on a low-cost ultrasound platform due to the simple algorithms.