Vascular damage by ultrasound-activated microbubble induced vessel invagination

Vascular bioeffects produced by ultrasound contrast agent microbubbles are primarily manifested as damage to microvessels. The objective of this work is to directly observe the transient dynamics of bubble-vessel interactions and correlate the observed interactions with associated vascular damage. Microbubbles were perfused into microvessels in ex vivo rat mesenteries and then excited by a single 2 us long ultrasound pulse at 1 MHz. Meanwhile, 14 high-speed photomicrographic images were acquired using 50 ns shutter speeds. The targeted region was then examined by histology and transmission electron microscopy (TEM). Image registration was used to identify the specific vessels that the corresponding high-speed images were captured. The recorded high-speed images revealed that bubble-vessel interactions caused vessel wall distention (motion outward against the surrounding tissue) and invagination (motion inward toward the lumen). Invagination exceeding distention was observed in 60 out of 70 cases. Significant vessel invagination was correlated with vascular damage that was characterized by a separation of the endothelium from the surrounding tissue as revealed by both the histology and TEM analyses. The separation of the endothelium from the surrounding tissue is consistent with damage caused by tensile stresses at the vessel walls that lead to vessel invagination. This suggests that invagination may be an important mechanism by which microbubbles cause vascular damage.