Doinikov A., Teterev A.

Encapsulated microbubbles, known as contrast agents, are used in ultrasound medical diagnostics to increase the backscattered echo from blood and to provide thus better flow visualization, as well as in therapy as a vehicle for targeted drug delivery. Ultrasound contrast agents typically consist of a gas core surrounded by a shell of albumin or lipid. All current theoretical models of contrast agents assume that the encapsulating coating behaves as an elastic (or viscoelastic) solid. This assumption appears to be adequate for albumin-shelled microbubbles. However, experimental data available in the literature for lipid-shelled bubbles indicate that an elastic solid cannot be a good approximation for lipid monolayer shells. Lipid coating exhibits properties of a viscoelastic fluid rather than a viscoelastic solid. The present paper substantiates this observation by thoroughly analyzing the experimental data and proposes a new theoretical model for a lipid-shelled contrast agent microbubble. The model describes the encapsulating coating as the Maxwell fluid and incorporates the translational motion of the bubble. Theoretical predictions of the model are checked against experimentally measured translational displacements of lipid-shelled microbubbles.