Extraction of temperature distribution of high intensity focused ultrasound: Nonlinear propagation

Background: The aim of the study is the extraction of acoustic pressure distribution in the target tissue layersbased on the nonlinear behavior of waves. The nonlinear behavior effect of high intensity focused ultrasound(HIFU) on the temperature distribution of the tissue was extracted and compared with the linear behavior.Materials and Methods: The acoustic pressure field was calculated using the Westervelt equation and wascoupled with Pennes thermal transfer equation. The simulations were performed for three layers of skin, fat,and muscle using Comsol software. The disagreement between two linear and nonlinear models was analyzedwith Kolmogorov–Smirnov test. The pressure and temperature distributions were calculated in nonlinearmodel by changing the acoustical parameters of the transducer including; intensity, effective radiation area,focal length and sonication time.Results: Model results were validated with experimental results with ۹۸% correlation coefficient (p<۰.۰۵).There are not significantly different between the pressure amplitude and temperature distribution in linearand nonlinear models at low intensity (p>۰.۰۵), but with increasing intensity to ۱۰ W/cm۲, in nonlinearmodel, maximum pressure and maximum temperature increased ۴۰% and ۲۰% compared with linear model.For input intensities of ۱.۵, ۲, ۸ and ۱۰ W/cm۲, the maximum pressure (at focal point) increased ۱۰, ۱۲, ۲۲,۴۰% and maximum temperature increased ۱, ۲,۱۲,۲۰% in nonlinear model compared to linear model. At ۰.۸and ۱.۵ cm۲ effective radiation area, the maximum temperature in nonlinear model increased from ۴۳ to ۷۹°C. By decreasing the focal lengths from ۱۰ to ۷.۵ mm, the maximum temperature increased from ۴۵ to ۸۷°C.Conclusion: It is concluded a change in the input parameters of the transducer, it can be very effective intreating. The results emphasize the effects of nonlinear propagation and acoustical radiation parameters toimprove the HIFU treatment.