1Department of Advanced Biomedical Engineering and Science, Graduate School of Medicine, Tokyo Women's Medical University, 2Central Research Laboratory, Hitachi, Ltd., 3Department of Bioengineering, School of Engineering, The University of Tokyo, 4Institute of Advanced Biomedical Engineering and Science, Tokyo Women's Medical University
Purpose: Development of a technique to confirm the focus position in advance is desired for safe and reliable treatment with high-intensity focused ultrasound (HIFU). The purpose of this study is to induce displacement by means of acoustic radiation force generated by a low-energy HIFU pulse to make displacement images using ultrasound, and visualize the focal spot. Subjects and Methods: The optimal interval between the end of exposure and observation of displacement may differ in the case of sensitivity and accuracy. Hence, we set the time interval between the end of exposure and imaging of the focal spot to 0.3 ms, 0.8 ms, and 1.3 ms to compare the predicted focal spot with the actual thermocoagulation spot by HIFU. The experiment sample was porcine liver. Results and Discussion: The sensitivity was sufficient at every time interval. On the other hand, accuracy decreased as the time interval increased. A thermocoagulation spot with a size of 1.1 mm×3.6 mm was predicted with accuracy of 0.8 mm at 0.3 ms, but due to a change in displacement distribution, the accuracy and precision decreased at 0.8 ms and 1.3 ms. This result was explained with shear wave propagation. It became clear that it was necessary to consider the speed of shear waves in order to visualize the focal spot correctly when configuring the conditions for making displacement images. Conclusion: Using focal spot imaging, the operator can predict the thermocoagulation spot in advance. This technique overcomes the issue of targeting the focal spot during HIFU exposure, and since it is a compact system using an ultrasonic device, use of HIFU treatment may become more widespread at medical institutions for a wider range of diseases.