Noninvasive ultrasound imaging of inflammation using microbubbles targetedto activated leukocytes

Background-Lipid microbubbles used for perfusion imaging with ultrasound ar e retained within inflamed tissue because of complement-mediated attachment to leukocytes within venules. We hypothesized that incorporation of phosph atidylserine (PS) into the microbubble shell may enhance these interactions by amplifying complement activation and thereby allow ultrasound imaging o f inflammation. Methods and Results-In 6 mice, intravital microscopy of tissue necrosis fac tor-alpha -treated cremaster muscle was performed to assess the microvascul ar behavior of fluorescein-labeled lipid microbubbles with and without PS i n the shell. Ten minutes after intravenous injection, microbubble attachmen t to leukocytes within inflamed venules was greater for PS-containing than for standard lipid microbubbles (20+/-4 versus 10+/-3 per 20 optical fields , P<0.05). The ultrasound signal from retained microbubbles was assessed in the kidneys of 6 mice undergoing renal ischemia-reperfusion injury and in 6 control kidneys. The signal from retained microbubbles in control kidneys was low (<2.5 video intensity units) for both agents. After ischemia-reper fusion, the signal from retained microbubbles was 2-fold higher for PS-cont aining than for standard lipid microbubbles (18+/-6 versus 8+/-2 video inte nsity units, P<0.05). An excellent relation was found between the ultrasoun d signal from retained microbubbles and the degree of renal inflammation, a ssessed by tissue myeloperoxidase activity. Conclusions-We conclude that noninvasive assessment of inflammation is poss ible by ultrasound imaging of microbubbles targeted to activated leukocytes by the presence of PS in the lipid shell.