Ultrasound contrast agents: resonances of coated bubbles

The Brandaris 128 high speed camera was designed and built in order to analyze the dynamics of single oscillating contrast agent microbubbles under influence of ultrasound. In this thesis we report on our findings. First, in Ch. 2, a new model was introduced which takes into account the physical properties of a lipidmonolayer coating on a gasmicrobubble. Three parameters describe the properties of the shell: a buckling radius, the compressibility of the shell, and a break-up shell tension. The model presents an original non-linear behavior at large amplitude oscillations, termed compression–only, induced by the buckling of the lipid monolayer. This prediction is validated by experimental recordings with the Brandaris camera, operated at several million frames per second. The effect of aging, or the resultant of repeated acoustic pressure pulses on bubbles, is predicted by the model. Further, the model corrects a flaw in the shell elasticity term previously used in the dynamical equation for coated bubbles. The break-up ismodeled by a critical shell tension above which gas is directly exposed to water.