Simulation Study on the Effect of Multi-layer Biological Tissue on Focus Shift in High-Intensity Focused Ultrasound Therapy

Introduction: During the treatment of soft tissue tumors with high-intensity focused ultrasound (HIFU), the focus may shift away from the desired point due to tissue heterogeneity. By studying the effect of biological tissue on focus shift, it can provide a theoretical basis for the safety and reliability of HIFU therapy. Material and Methods: The finite difference time domain (FDTD) method was used to construct the simulation model of HIFU irradiated multi-layer biological tissue. Based on the Westervelt nonlinear acoustic propagation equation, the focus position change caused by the thickness of biological tissue and ultrasonic transducer during HIFU irradiation were simulated and calculated. The effects of ultrasonic transducer’s electric power, irradiation frequency and tissue thickness on the focus position shift were analyzed and discussed. Results: With the increase of electric power of HIFU transducer, the sound pressure at the focal point rose and the focal point approached the transducer side. With the increase of irradiation frequency of transducer, the sound pressure at the focus increased and the focus shifted away from transducer. With the increase of the thickness of biological tissue, the amplitude of sound pressure at the focal point decreased gradually. If the sound velocity of biological tissue was greater than that of water, the focus was close to the transducer side. If the sound velocity of biological tissue was less than the sound velocity of water, the focus moved to the side away from the transducer. For biological tissue with sound velocity greater than (or less than) water, the greater the sound velocity, the greater the relative shift distance difference of focal position. Conclusion: As the electric power and frequency of ultrasonic transducer increased, the focus of HIFU moved toward and away from the transducer, respectively. For multi-layer biological tissue, the focus shift direction depended on the sound velocity relationship between biological tissue and water. © (2023), (Mashhad University of Medical Sciences). All rights reserved.