Background-Routine methods capable of assessing tissue inflammation noninvasively are currently not available. We hypothesized that tissue retention of microbubbles targeted to the endothelial cell adhesion molecule P-selectin would provide a means to assess inflammation with ultrasound imaging. Methods and Results-Phospholipid microbubbles targeted to P-selectin (MB p ) were created by conjugating monoclonal antibodies against murine P-selectin to the lipid shell. The microvascular behaviors of MB p and control microbubbles without antibody (MB) or with isotype control antibody (MB iso ) were assessed by intravital microscopy of cremasteric venules of control and tumor necrosis factor (TNF)-␣-stimulated wild-type mice. Retention of all microbubbles increased (PϽ0.05) with TNF-␣ treatment because of increased attachment to activated leukocytes. Extensive attachment of MB p directly to the venular endothelium or to adherent platelet-leukocyte aggregates was observed in TNF-␣-stimulated mice, resulting in 4-fold greater (PϽ0.01) retention of MB p than either MB iso or MB. Enhanced retention of MB p was completely abolished in TNF-␣-stimulated P-selectin-deficient mice. The ultrasound signal from microbubbles retained in inflamed tissue was assessed by contrast-enhanced renal ultrasound imaging of the kidneys of mice undergoing ischemia-reperfusion injury. In wild-type mice, this signal was significantly higher (PϽ0.05) for MB p (12Ϯ2 U) than either MB iso (6Ϯ3 U) or MB (5Ϯ3 U). In P-selectin-deficient mice, the signal for MB p was equivalent to that from control microbubbles. Conclusions-Microvascular retention of microbubbles targeted to P-selectin produces strong signal enhancement on ultrasound imaging of inflamed tissue. These results suggest that site-targeted microbubbles may be used to assess inflammation, tissue injury, and other endothelial responses noninvasively with ultrasound.

-We conclude that noninvasive assessment of inflammation is possible by ultrasound imaging of microbubbles targeted to activated leukocytes by the presence of PS in the lipid shell.