INSERM U930 CNRS ERL3106, Université François Rabelais, CHU Bretonneau, 2 Boulevard Tonnellé, 37044 Tours Cedex 9, France.
Phys Med Biol. 2011 Nov 7;56(21):6951-67. doi: 10.1088/0031-9155/56/21/012.
Interest in the problem under consideration in this study is motivated by targeted ultrasound imaging where one has to deal with microbubble contrast agents pulsating near blood vessel walls. A modified Rayleigh–Plesset equation is derived that describes the oscillation of a contrast agent microbubble near an elastic wall of finite thickness. It is assumed that the medium behind the wall is a fluid but it is shown that the equation obtained is easily transformable to the case that the medium behind the wall is an elastic solid. In contrast to the model of a rigid wall, which predicts decreasing natural frequency of a bubble near the wall, the elastic wall model reveals that the bubble natural frequency can both decrease and increase, and in cases of interest for medical applications, the bubble natural frequency usually increases. It is found that the influence of an elastic wall on the acoustic response of a bubble is determined by the ratio between a cumulative parameter, which integrally characterizes the mechanical properties of the wall and has the dimension of density, and the density of the liquid surrounding the bubble. It is shown that the acoustic influence of the arterial wall on the bubble is weak and apparently cannot be used to recognize the moment when the bubble approaches the wall. However, in experiments where the behavior of bubbles near various plastic walls is observed, changes in the bubble response, such as increasing natural frequency and decreasing oscillation amplitude, are detectable.
本研究关注的问题源于靶向超声成像，在该成像中，需要处理靠近血管壁脉动的微泡造影剂。推导出了一个改良的瑞利-普莱塞特方程，用于描述造影微泡在有限厚度弹性壁附近的振动。假设壁后的介质是流体，但可以证明，所得到的方程很容易转换为壁后介质是弹性固体的情况。与预测靠近壁的气泡自然频率降低的刚性壁模型不同，弹性壁模型表明，气泡自然频率既可以降低也可以增加，并且在对医学应用感兴趣的情况下，气泡自然频率通常会增加。研究发现，弹性壁对气泡声响应的影响取决于累积参数与气泡周围液体密度之比，该累积参数综合表征了壁的力学性能，具有密度的维度。研究表明，动脉壁对气泡的声学影响较弱，显然不能用于识别气泡接近壁的时刻。然而，在观察各种塑料壁附近气泡行为的实验中，可以检测到气泡响应的变化，例如自然频率增加和振动幅度减小。
