Temperature elevation in mouse liver due to ultrasonic heating calculated by FEM with bioheat transfer equation

The sound velocity in biological tissue depends on temperature. At body temperature, fat tissue has a negative temperature coefficient, while non-fat tissue has a positive one. Based on the properties, a method for tissue characterization using temperature-dependent sound velocity has been proposed. In this study, ultrasonic heating was analyzed using the finite element method based on the bioheat transfer equation, concentrating on heating time and temperature rise within the regulatory output limits of diagnostic ultrasound equipment. The volumetric heat capacity, attenuation coefficient, thermal diffusivity, and sound velocity were set to values for normal mouse liver: 3.99e+6 J/m3/°C, 0.3 dB/MHz, 0.5 W/m/°C, and 1600 m/s, respectively. Ultrasonic intensity distributions were calculated for a circular concave transducer (8.2 MHz, 10 mm, diameter, 15 mm focal length) and a ring-shaped transducer (5.2 MHz, 10 mm outer diameter, 4 mm inner, 15 mm focal length). These were applied to finite element simulations. For 5.2 MHz, 10 ms, and 1.6 MPa, the temperature rise was 0.051°C; for 8.2 MHz, 1.0 ms, and 3.27 MPa, it was 0.039°C. Nonlinear effects were not considered. The results showed that longer ultrasound exposure tends to produce greater temperature rises. [Work supported by AMED (JP24ym0126814).]