Formation process of thermal damage in a target area of high intensity focused ultrasound and effectiveness analysis of B-ultrasound real-time monitoring

High intensity of focused ultrasound (HIFU) is an effective tumor therapy, taking advantage of the thermal effect and cavitation effect to generate thermal damage to the target tissue. However, inaccurate ultrasonic dose control may result in ineffective or excessive treatment. Thus, real-time monitoring of the thermal damage formation process is critical. To evaluate the effectiveness of real-time monitoring of B-ultrasound, ex-vivo bovine livers were irradiated by 1.155 MHz focused ultrasound with emission time T1 of 200 ms and interval time T2 of 200 ms. For orthogonal experiments, ultrasound was irradiated at sound power of 100 W, 125 W, and 150 W for 10 s, 20 s, and 40 s, respectively. B-ultrasound image sequences are collected using a 7.5 MHz linear array and compared with backscattered echo signals and thermal damage slices, respectively, to build relationships between B-mode ultrasound monitoring and thermal effect or cavitation effect. The experimental results demonstrated that the tissue ablation process caused by thermal effect cannot be effectively monitored using B-ultrasound, but the process caused by the cavitation effect can. The analysis revealed a strong temporal correlation between the appearance of bright spots in B-ultrasound images and the sudden increase of the scattered echo power spectrum, which were caused by a large number of micro-bubbles from cavitation. The damaged cavity structure of the slices and the development trend of micro-bubbles showed a strong spatial correlation. Furthermore, the sudden increase in the scattered echo signal shows the potential of early warning of cavitation, as it is 1.2–2.0 s ahead of the bright spot in the B-ultrasound image, laying the experimental basis for improving the effectiveness of B-ultrasound monitoring in clinical HIFU surgery