Threshold of Linear and Non-Linear Behavior of High Intensity Focused Ultrasound (HIFU) in Skin, Fat, and Muscle Tissue Using Computer Simulation

Introduction: In this study, the ultrasound-tissue interactions to obtain the resulting treatment thermal plans of high-intensity focused ultrasound (HIFU) are simulated. Material and Methods: The simulations were performed for three layers of skin, fat, and muscle using Comsol software (version 5.3). The acoustic pressure field was calculated using the Westervelt equation and was coupled with Pennes thermal transfer equation to obtain thermal distribution. The pressure field was calculated and compared in two linear and non-linear models. Results: By increasing the input sound intensity, the non-linear behavior becomes more pronounced and higher harmonics of the fundamental sound have appeared and increased the pressure, and temperature at the focal point. At input intensities of 1.5, 2, 5, 8, 10, 20, and 30 W/cm2, the maximum acoustic pressure in the non-linear model compared to the linear model was 10, 11, 15, 22, 40, 47, 65, and 85%, respectively. The maximum temperature in the non-linear model increased by 9, 10, 12, 20, 22, 24, 31, and 45% compared to the linear model. Model results were validated with experimental results with a 95% correlation coefficient. The results of the input intensity 1.5, 2 and 5 W/cm2 were acceptable (p<0.05) and from input sound intensity 8 W/cm2 to above, there was a significant difference between the data (p<0.05). Also, maximum pressure and maximum temperature in the non-linear model are 20% more than in the linear model. Conclusion: In the non-linear propagation model, the resulting thermal pattern changed significantly with the change of the input sound intensity. © 2022