Potential mechanism for vessel invagination caused by bubble oscillations

In medical ultrasound, acoustically excited bubbles are relevant to both imaging and therapeutic applications and have been implicated in causing vascular damage. A current paradigm for understanding interactions between bubbles and vessels considers the distention of small vessels and the impingement of bubble jets on vessel walls to be the most likely damage mechanisms. However, recent high-speed photographs suggest a type of interaction that is characterized by a prominent invagination of the vessel wall (i.e., an inward deflection toward the lumen) that appears to exceed any accompanying distention. To elucidate mechanisms for such behavior, a confined flow geometry between an oscillating bubble and a nearby boundary is modeled and compared to fully spherical flow. From a Bernoulli-type equation for an incompressible and inviscid liquid, the pressure attributable to a bubble at a nearby boundary is found to become biased toward negative values as the flow becomes more confined and less spherical. Such negative values are consistent with invagination. Using radial bubble dynamics inferred from a high-speed photographic sequence of a bubble in a vessel, the aforementioned model was used to simulate the pressure radiated by the bubble at the vessel wall. At the 1 MHz acoustic frequency, the simulated negative pressure is 2.5 times the positive pressure; in turn, the observed vessel displacement inward was about 6 times the corresponding outward displacement.