多层膜磁性微泡的非线性声振动特性

超顺磁性氧化铁纳米粒子与造影剂微泡结合形成磁性微泡,用于产生多模态造影剂,以增强医学超声和磁共振成像.将装载有纳米磁性颗粒的微泡包膜层看作由磁流体膜与磷脂膜组合而成的双层膜结构,同时考虑磁性纳米颗粒体积分数a对膜密度及黏度的影响,从气泡动力学基本理论出发,构建多层膜结构磁性微泡非线性动力学方程.数值分析了驱动声压和频率等声场参数、颗粒体积分数、膜层厚度以及表面张力等膜壳参数对微泡声动力学行为的影响.结果表明,当磁性颗粒体积分数较小且a≤0.1时,磁性微泡声响应特性与普通包膜微泡相似,微泡的声频响应与其初始尺寸和驱动压有关;当驱动声场频率f为磁性微泡共振频率f0的2倍(f=2f0)时,微泡振动失稳临界声压最低;磁性颗粒的存在抑制了泡的膨胀和收缩但抑制效果非常有限;磁性微泡外膜层材料的表面张力参数K及膜层厚度d也会影响微泡的振动,当表面张力参数及膜厚取值分别为0.2—0.4 N/m及50—150 nm时,可观察到气泡存在不稳定振动响应区.

Nonlinear acoustic characteristics of multilayer magnetic microbubbles

The combination of superparamagnetic iron oxide nanoparticles(SPIOs)with ultrasonic contrast agent(UCA)microbubble is called magnetic microbubble(MMB)and has been used to produce multimodal contrast agents to enhance medical ultrasound and magnetic resonance imaging.The nanoparticles are either covalently linked to the shell or physically entrapped into the shell.Considering the effect of the volume fraction of SPIOs on the shell density and viscosity,a nonlinear dynamic equation of magnetic microbubbles(MMBs)with multilayer membrane structure is constructed based on the basic theory of bubble dynamics.The influences of the driving sound pressure and frequency,particle volume fraction,shell thickness and surface tension on the acoustic-dynamics behavior of microbubbles are numerically analyzed.The results show that when the volume fraction of magnetic particles is small and a≤0.1,the acoustic properties of magnetic microbubbles are similar to those of ordinary UCA microbubbles.The acoustic response of the microbubble depends on its initial size and driving pressure.The critical sound pressure of microbubble vibration instability is lowest when the driving sound field frequency is twice the magnetic microbubble resonance frequency f0(f=2f0).The presence of magnetic particles inhibits the bubbles from expanding and contracting,but the inhibition effect is very limited.The surface tension parameter K of the outer film material and thickness of the shell also affect the vibration of the microbubble.When K and film thickness are 0.2–0.4 N/m and 50–150 nm respectively,it is observed that the bubble has an unstable vibration response region.