New theoretical models for lipid-shelled ultrasound contrast agents

Abstract

Analysis of existing theoretical models for lipid-coated contrast agents is presented. A general theoretical approach to the development of zero-thickness encapsulation models, allowing the testing of different rheological laws for encapsulation, is proposed. Based on available experimental data, analysis of the rheological behavior of encapsulating lipid shells is made. The problems of the existing models, such as the dependence of shell parameters on the initial bubble radius and the "compression-only" behavior, are discussed. Physical explanations for the observed effects and new theoretical models for their simulation are offered. In particular, it is shown that the inclusion of nonlinear shell viscosity allows one to model the "compression-only" behavior. It is also very important to select an appropriate rheological law describing the dependence of the shell viscosity on the shear rate. A correct choice reduces considerably the spread of the experimentally estimated values of the shell parameters and makes them look more ordered. Whereas with an inappropriate rheological law, the experimental data may look rather chaotic.