A 2D-FEM Model of Nonlinear Ultrasound Propagation in Trans-cranial MRgFUS Technique

Magnetic Resonance guided Focused Ultrasound (MRgFUS) is a non-invasive technique based on the thermal ablation of a target using high intensity focused ultrasound. MRgFUS treatment applied to brain is challenging due to the skull presence that attenuates ultrasound, leading to heating effects in bone region. In this study, we simulate trans-cranial nonlinear ultrasound propagation considering the detailed structure of bone tissue. We developed a 2D Finite Element (FE) model that mimics the propagation of focused ultrasound through skin, skull and brain tissue. The skull is represented as a three-layered system with two cortical tables packing a layer of trabecular bone. We assume that the space between the concave transducer and tissue is filled by water. Nonlinear ultrasound propagation is determined through Westervelt equation. To control reflection, absorbing layers have been implemented on the boundaries of the domains. The solution of the pressure equation is subsequently coupled with Pennes bioheat equation to determine the temperature distribution in the tissue region. The acoustic pressure, acoustic intensity and temperature distribution are achieved from FE simulation. Highest values of acoustic pressure occur in the focal area and in the bone tissue region. Ablative temperatures, i.e. superior to 55 °C, are achieved in the target zone and at the cortical-trabecular interface. The thermal response in the focal region is in agreement with available literature and allows to validate the model effectiveness. The FE model offers new insights to predict secondary heating effects of ultrasound propagation in the skull region and to improve treatment planning.