Numerical Simulation Of The Effect Of Microbubbles On High Intensity Focused Ultrasound Acoustic Pressure Field

Because of its non-invasive and repeatable therapeutic characteristics, high intensity focused ultrasound(HIFU) treatment technique has been used clinically to treat solid tumors of soft tissue, including breast cancer, prostate cancer, uterine fibroid, etc. When using HIFU to treat deep tumors such as liver tumor and brain tumor, due to the distance of ultrasonic propagation is long and needs to pass through the bone tissue with strong attenuation, making ultrasound energy is greatly attenuated such that focal energy is not sufficient to coagulate tumor tissue. Besides the volume of focal region formed by a single HIFU exposure is small, resulting in the long time required for the treatment of a larger volume of tumors. Ultrasound microbubble contrast agent with synergistic effect of HIFU treatment has attracted the attention of many researchers.ObjectiveDue to the lack of energy reaching the target area, the treatment time is too long or cannot kill tumor tissue during the HIFU treatment of deep tumors such as liver tumor and brain tumor. Microbubble contrast agents can enhance the therapeutic effect of HIFU by using the cavitation effect of ultrasonic wave. In this paper, by using numerical simulation to study the effect of initial gas volume fraction, initial bubble radius, exciting acoustic pressure and exciting frequency on the acoustic pressure field of HIFU, which provide an effective theoretical data and related basis for the application of ultrasound microbubble contrast agent enhanced HIFU in clinical treatment.MethodsIn this research, the gas-liquid mixture and tissue-mimicking numerical simulation model were established. By using the acoustic propagation equation in gas-liquid mixture and Keller bubble motion equation to simulate the acoustic pressure field and bubble radius distribution of HIFU propagation in bubble-containing water based on finite difference time domain method and Runge-Kutta method in axisymmetric cylindrical coordinate system. The effect of the initial gas volume fraction, initial bubble radius, exciting acoustic pressure andexciting frequency on the acoustic pressure field of HIFU was analyzed and discussed.ResultsAs the initial gas volume fraction increasing, the amplitude of instantaneous bubble radius without obvious difference and the reflected wave was observed at the junction of gas-liquid mixture with pure water and the amplitude of instantaneous pressure of the reflected ultrasonic wave increases gradually in gas-liquid mixture;the peak of the bubble radius decreases gradually in tissue-mimicking; the acoustic pressure at focus decreases gradually and the focus moves to the transducer side in gas-liquid mixture and tissue-mimicking;When the initial bubble radius is greater than the bubble resonance radius, as the initial bubble radius increasing, the amplitude of instantaneous bubble radius decreases gradually and the amplitude of instantaneous pressure of the reflected ultrasonic wave at the junction of gas-liquid mixture with pure water decreases gradually in gas-liquid mixture; the peak of the bubble radius increases first and then decreases in tissue-mimicking, the amplitude of bubble radius decreases gradually at tissue-mimicking boundary; the focal pressure increases gradually and the focus moves away from the transducer side in gas-liquid mixture and tissue-mimicking;As the exciting acoustic pressure increasing, the amplitude of instantaneous bubble radius increases gradually and the amplitude of instantaneous pressure of the reflected ultrasonic wave at the junction of gas-liquid mixture with pure water increases gradually and the ratio of acoustic pressure at focus to exciting acoustic pressure decreases first and then increases in gas-liquid mixture; the ratio of acoustic pressure at focus to exciting acoustic pressure decreases gradually and when the exciting acoustic pressure is low, the change of bubble radius is almost coincide with the variation law of acoustic pressure, but the exciting acoustic pressure increases to a certain extent, bubble oscillation becomes more intense and the scope of fluctuation expands in tissue-mimicking; the acoustic pressure at focus increases gradually and the focus position is constant in gas-liquid mixture and tissue-mimicking;When the exciting frequency is greater than the bubble resonance frequency, as the exciting frequency increasing, the amplitude of instantaneous bubble radiusdecreases gradually and the amplitude of instantaneous pressure of the reflected ultrasonic wave at the junction of gas-liquid mixture with pure water decreases gradually in gas-liquid mixture; the peak of the bubble radius in tissue-mimicking and the amplitude of the bubble radius at tissue-mimicking boundary decrease gradually;the focal pressure increases gradually and the focus moves away from the transducer side in gas-liquid mixture and tissue-mimicking.ConclusionsMicrobubbles influence the ultrasonic propagation, increasing the attenuation of ultrasonic intensity; The greater the initial gas volume fraction, the greater the ultrasonic attenuation, the focus position has a clear shift; When the initial microbubble radius and the exciting frequency are greater than the microbubble resonance radius and resonance frequency respectively, the greater the initial microbubble radius and the exciting frequency, the weaker the microbubble oscillation, the smaller the ultrasonic attenuation, the focus position has a clear shift;The greater the exciting acoustic pressure, the more intense the microbubble oscillation, the greater the ultrasonic attenuation, the focus position does not change.