Mechanism of Parenchymal Enhancement of the Liver with a Microbubble-based US Contrast Medium: An Intravital Microscopy Study in Rats

PURPOSE: To investigate the mechanism of prolonged contrast material enhancement of the liver observed with the lipid-shell ultrasonographic (US) contrast agent AF0150, with use of intravital microscopy. MATERIALS AND METHODS: Eight Sprague-Dawley rats were used. Six received fluorescent microspheres to label the Kupffer cells; two were used as controls. The edge of the middle lobe of the liver was transilluminated with white light. Fluorescent microspheres were observed under fluorescence light. After injection of AF0150, behavior of microbubbles was observed for 6 minutes while viewing a single high-power field. Multiple other fields were then assessed for stationary bubbles and their relation to Kupffer cells. The number of bubbles in motion, aggregated, stationary, and associated with labeled cells were counted. RESULTS: Of 590 bubbles, 34 (5.8%) became stationary and 556 (94.2%) kept moving. Of the 34 stationary microbubbles, 21 dislodged within 30 seconds. Microbubbles were homogeneously distributed throughout the lobule, in contrast to the dominant periportal distribution of the labeled Kupffer cells. Among 83 stationary bubbles observed from all fields of view, only 14 (17%) were associated with fluorescent-labeled cells. CONCLUSION: The late parenchymal liver enhancement effect of AF0150 is likely not related to Kupffer-cell uptake, but rather to a mechanical slowdown within the sinusoids.