Visualization of the scattering of focused ultrasonic waves at solid-fluid interfaces

While high-intensity focused ultrasound (HIFU) is already being used for the ablation of tissue near the skin, such as in the case of prostate cancer, targeting tissue deeper inside the body remains challenging due to the increased obstruction and scattering of ultrasonic waves. In this work, the partial and complete obstruction of the ultrasonic beam path from a HIFU transducer operating at 670 kHz by bone phantom is imaged in laboratory experiments to visualize wave transmission and reflection at solid-fluid interfaces. Ultrasonic wave-scattering under such conditions has scarcely been the focus of previous ultrasound visualization studies. Thus, this work provides a qualitative visual reference for focused waves scattering at water-bone interfaces. A diffraction-based shadowgraph technique is used for the ultrasound visualization. The ultrasonic waves are imaged in water with no obstruction, with varying partial obstruction, and with complete obstruction by a thin fiber-filled epoxy plate mimicking bone tissue. Experimental findings are compared to those obtained through finite element simulations, showing good agreement. Furthermore, it is found that in certain partial obstruction cases, the waves scatter in such a way that the destructive interference between the transmitted waves lead to a significantly reduced maximum pressure at the focal point. Overall, the results of this study can provide a visual framework for future research in the field of therapeutic ultrasound.