Postema Michiel, Schmitz Georg
Institute for Medical Engineering, Department of Electrical Engineering and Information Technology, Ruhr-Universität Bochum, Building IC, 6/146, D-44780 Bochum, Germany.
Ultrason Sonochem. 2007 Apr;14(4):438-44. doi: 10.1016/j.ultsonch.2006.09.013. Epub 2007 Jan 10.
Ultrasound contrast agents consist of microscopically small bubbles encapsulated by an elastic shell. These microbubbles oscillate upon ultrasound insonification, and demonstrate highly nonlinear behavior, ameliorating their detectability. (Potential) medical applications involving the ultrasonic disruption of contrast agent microbubble shells include release-burst imaging, localized drug delivery, and noninvasive blood pressure measurement. To develop and enhance these techniques, predicting the cracking behavior of ultrasound-insonified encapsulated microbubbles has been of importance. In this paper, we explore microbubble behavior in an ultrasound field, with special attention to the influence of the bubble shell. A bubble in a sound field can be considered a forced damped harmonic oscillator. For encapsulated microbubbles, the presence of a shell has to be taken into account. In models, an extra damping parameter and a shell stiffness parameter have been included, assuming that Hooke's Law holds for the bubble shell. At high acoustic amplitudes, disruptive phenomena have been observed, such as microbubble fragmentation and ultrasonic cracking. We analyzed the occurrence of ultrasound contrast agent fragmentation, by simulating the oscillating behavior of encapsulated microbubbles with various sizes in a harmonic acoustic field. Fragmentation occurs exclusively during the collapse phase and occurs if the kinetic energy of the collapsing microbubble is greater than the instantaneous bubble surface energy, provided that surface instabilities have grown big enough to allow for break-up. From our simulations it follows that the Blake critical radius is not a good approximation for a fragmentation threshold. We demonstrated how the phase angle differences between a damped radially oscillating bubble and an incident sound field depend on shell parameters.
超声造影剂由包裹在弹性壳内的微小气泡组成。这些微气泡在超声照射下会振荡，并表现出高度非线性行为，从而提高其可检测性。涉及超声破坏造影剂微气泡壳的（潜在）医学应用包括释放爆发成像、局部药物递送和无创血压测量。为了开发和改进这些技术，预测超声照射下包裹微气泡的破裂行为一直很重要。在本文中，我们研究了超声场中的微气泡行为，特别关注气泡壳的影响。声场中的气泡可被视为一个受迫阻尼简谐振荡器。对于包裹微气泡，必须考虑壳的存在。在模型中，假设胡克定律适用于气泡壳，已纳入一个额外的阻尼参数和一个壳刚度参数。在高声幅下，已观察到破裂现象，如微气泡破碎和超声开裂。我们通过模拟不同尺寸的包裹微气泡在谐波声场中的振荡行为，分析了超声造影剂破碎的发生情况。破碎仅在坍缩阶段发生，并且如果坍缩微气泡的动能大于瞬时气泡表面能，且表面不稳定性已发展到足以允许破裂时，就会发生破碎。从我们的模拟结果可知，布莱克临界半径不是破碎阈值的良好近似。我们展示了阻尼径向振荡气泡与入射声场之间的相位角差如何取决于壳参数。
