Cavitation of microbubble contrast agents with ultrasound produces shear-mediated vasodilation and an increase in tissue perfusion. We investigated the influence of the size of the cavitation volume by comparing flow augmentation produced by two-dimensional (2D) versus three-dimensional (3D) therapeutic ultrasound. We also hypothesized that cavitation could augment flow beyond the ultrasound field through release of vasodilators that are carried downstream.

In 11 rhesus macaques, cavitation of intravenously administered lipid-shelled microbubbles was performed in the proximal forearm flexor muscles unilaterally for 10 min. Ultrasound cavitation (1.3 MHz, 1.5 MPa peak negative pressure) was performed with 2D or 3D transmission with beam elevations of 5 and 25 mm, respectively, and pulsing intervals (PIs) sufficient to allow complete postdestruction refill (5 and 12 sec for 2D and 3D, respectively). Contrast ultrasound perfusion imaging was performed before and after cavitation, using multiplane assessment within and beyond the cavitation field in 1.5-cm increments. Cavitation in the hindlimb of mice using 2D ultrasound at a PI of 1 or 5 sec was performed to examine microvascular flow changes from cavitation in only arteries versus the microcirculation.

In primates, the degree of muscle flow augmentation in the center of the cavitation field was similar for 2D and 3D conditions (five- to sixfold increase for both, P < .01 vs baseline). The spatial extent of flow augmentation was only modestly greater for 3D cavitation because of an increase in perfusion with 2D transmission that was detected outside of the cavitation field. In mice, cavitation in the microvascular compartment (PI 5 sec) produced the greatest degree of flow augmentation, yet cavitation in the arterial compartment (PI 1 sec) still produced a three- to fourfold increase in flow (P < .001 vs control). The mechanism for flow augmentation beyond the cavitation zone was investigated by in vitro studies that demonstrated cavitation-related release of vasodilators, including adenosine triphosphate and nitric oxide, from erythrocytes and endothelial cells.

Compared with 2D transmission, 3D cavitation of microbubbles generates a similar degree of muscle flow augmentation, possibly because of a trade-off between volume of cavitation and PI, and only modestly increases the spatial extent of flow augmentation because of the ability of cavitation to produce conducted effects beyond the ultrasound field.