Acoustic scattering properties of multilayer structured magnetic microbubbles

Normal ultrasound contrast agents (UCAs) loaded with magnetic nanoparticles are called magnetic microbubbles (MMBs), which can be used for multimodal imaging, thrombolytic therapy, and targeted drug delivery. MMBs are often studied by in situ measurement techniques, however scattering model is the basis of inversion techniques. Therefore, we developed a scattering model of multilayer structured MMBs with magnetic fluid inner layer and phospholipid outer layer, in which outer layer's viscoelasticity and the effect of nanoparticles on inner layer's density are considered, while scattered sound fields in each region were obtained by solving normal series. The MMBs model was compared with other bubbles, and numerically analyze its acoustic scattering characteristics, including the effects of radius, magnetic nanoparticle volume fraction, inner layer thickness and outer layer characteristics parameters. The results show that when the volume fraction α of magnetic nanoparticles in the inner layer does not exceed 0.1,magnetic nanoparticles have a two-sided effect on resonant scattering of MMBs, depending mainly on its radius and have critical radius value, if radius of MMBs exceeds this value, the particles will enhance scattering and vice versa; for a given microbubble radius, by increasing α (not more than 0.1)resonant scattering of MMBs will be stronger; decreasing the thickness of the inner and outer film layers or the Larmé constant, the scattering will also be stronger. This study provides theoretical guidance for the optimal structural design of MMBs and its in situ monitoring and therapeutic applications.