Nonlinear dynamics of lipid-shelled ultrasound microbubble contrast agents

Encapsulated gas microbubbles, known as contrast agents, are widely used in ultrasound medical applications. The present study is devoted to modelling of the spatio-temporal dynamics of lipid-shelled contrast agents. A theoretical model is proposed that describes the radial and translational motion of a lipid-shelled microbubble in an ultrasound field. The model approximates the behaviour of the lipid shell by the linear 3-constant Oldroyd constitutive equation, incorporates the translational motion of the bubble, and accounts for acoustic radiation losses due to the compressibility of the surrounding liquid. The values of the shell parameters appearing in the model are evaluated by fitting simulated radius-time curves to experimental ones. The results are then used for the simulation of the translational motion of contrast agent bubbles of various radii and the evaluation of the relationship between equilibrium radii of lipid-shelled agents and their resonance frequencies in the regime of nonlinear oscillation.