The long-term goal of this research is to improve characterization methods for HIFU transducers. The objective of this application is to test a nonlinear derating method that can predict the thermal effects of HIFU beams at high powers. The central hypothesis of this research is that temperature rise within the focal volume in tissues can be estimated by performing measurements in water and using a nonlinear derating method to quantify bioeffects in tissues. They plan to test the hypothesis and accomplish the objective by pursuing the two specific aims: 1) To derive algorithms for assessing the amplitudes of pressure modes in tissue, given the amplitudes in water. The working hypothesis is that a derating algorithm for converting pressure mode amplitudes from water to amplitudes in tissue will be accurate and that, when inserted into the expression for temperature modes, will enable assessment of temperature rise in a tissue phantom; and 2) To evaluate the derating approach with inverse heat-transfer method in tissue phantoms using thermocouples. The working hypothesis is that measurements in tissue phantoms, combined with inverse method, can validate the derating algorithm. The expected outcome of this study is a unique method for establishing the thermal efficacy and safety of the devices that is accurate, low-cost, and applicable at clinically relevant powers. The method is expected to have an important positive impact by providing a tool that will expedite the preclinical testing of new HIFU systems, benefitting both the ultrasound device industry and FDA reviewers.
For calculating the temperature rise by HIFU ablation, the tissue pressure harmonics and modal intensities need to be known. These can be derived from modal intensities and pressure harmonics in water. Derating factors as a function of radial positions and acoustic powers have been derived at the transducer focal plane to calculate the tissue modal intensities from water modal intensities. In order to use a single derating factor for all locations, the derived derating factors have been averaged over radial locations at the transducer focus. In addition, pressure harmonics in water have been derived by solving the plane wave equation in water. The derived pressure harmonics in water have been used in the tissue wave equation to estimate the tissue pressure harmonics. The results show that the derating factor at the transducer focal point reduces somewhat from 0.300 to 0.285 by increasing the power from 5 W to 60 W, respectively. The averaged derating factor at the transducer focus plane is also decreased from 2.19 to 2.15 with the raise in power from 5 W to 60 W. Furthermore, the tissue pressure harmonics from the direct solution of the wave equation in tissue have been compared with the derated pressure harmonics. There difference between the derived pressure harmonics by these two methods is found to be negligible. The results of recording the tissue phantom temperature rise for different transducer acoustic powers show that the location of the HIFU beam with respect to the transducer surface is increased with the increase in power from 5 W to 60 W. It can be concluded that the measured pressure harmonics and modal intensities in water can be used to estimate the pressure harmonics and subsequently the temperature rise in tissue. This unique interdisciplinary research links the field of bio-heat transfer, wave propagation, image analysis, and non-invasive diagnostic technique. By exposing the students in our on-going HIFU research program, students acquired knowledge of fundamental bioheat transfer mechanisms, the clinical importance of device characterization applicable to thermal therapy as well as understanding the regulatory aspects of device characterization. A better knowledge of the derating parameters for HIFU transducer is expected to enable the FDA to establish clearer requirements on ultrasound technology that involves establishing intensity and power levels as a function of beam focus. The research allowed students to be involved with multiple phases of the medical-device path to market. The experience acquired in this study of medical-device technology has been incorporated into the curriculum at University Cincinnati’s Transport in Engineering and Medicine lab. The PI was successful in recruiting woman undergraduate student for this research.
