[Study of ultrasound attenuation during HIFU propagation in ox liver]

Ultrasound attenuation has been measured during HIFU propagation in fresh ox liver at different frequency using a radiation force method. The acoustic radiation force was measured before and after the prepared ox liver of various thicknesses (20 mm, 40 mm, 60 mm, 80 mm) which were put into degassed water and exposed to different transducer surface output acoustic powers at a room temperature (20 degrees C) with therapeutic transducers of various frequencies, and then the ultrasound attenuation coefficient was calculated. With the use of therapeutic transducer 4, the focus of the ultrasonic beam was set to be 20 mm, 40 mm or 60 mm deep into the tissue surface using B-mode ultrasound guidance. A single exposure was performed with focal intensity ISATA = 22.0 x 10(3) W/cm2(ISATA, in degassed water) and exposure time 5 sec. The sample was cut after treatment to measure the volume of coagulative necrosis. For a specific therapeutic transducer, the radiation force ratio in liver of constant thickness is independent of the transducer surface sound intensity and the area of the ultrasound window at the sample front face. The radiation force ratio and the volume of coagulative necrosis induced by HIFU are plotted as the functions of sample thickness (or exposed depth), in which it can be seen that these two parameters have the same exponential dependence on sample thickness. The ultrasound attenuation coefficient alpha in ox liver is shown to be frequency f dependent, it almost linearly increases with frequency t. This work shows that such a study is feasible, and offers experimental data that will be useful for future HIFU dosage studies.