Mechanism of hepatic parenchyma-specific contrast of microbubble-based contrast agent for ultrasonography: microscopic studies in rat liver.

OBJECTIVE: The objective of this study was to elucidate the mechanism of hepatic parenchyma-specific contrast of Sonazoid (microbubble contrast agent) using microscopic techniques. MATERIALS AND METHODS: Sonazoid was intravenously injected into rats to investigate the microbubble dynamics and distribution within hepatic microcirculation in exteriorized liver using intravital microscopy and to observe dose dependency of ultrasound hepatic contrast effect. In vitro and in vivo uptake of microbubbles by Kupffer cells was examined using confocal laser scanning microscopy. RESULTS: Intravital observation demonstrated freely flowing microbubbles in the sinusoid and some microbubbles co-localized with Kupffer cells. The microbubbles internalized in Kupffer cells were identified with reflected light by confocal laser scanning microscopy. The percentage of Kupffer cells taking up microbubbles was about 1% at clinical dose at which the homogeneous hepatic contrast was observed. CONCLUSIONS: The hepatic parenchyma-specific contrast by Sonazoid is due to distribution of the microbubbles in Kupffer cells.

Characterization of tumor imaging with microbubble-based ultrasound contrast agent, sonazoid, in rabbit liver.

In order to investigate improvement of hepatic tumor detectability by Sonazoid with phase inversion imaging, the contrast effects on the liver of metastatic carcinoma-model rabbits were evaluated by videodensitometry and visual assessment. Correlation between the contrast enhancement of Sonazoid and histopathology was examined using the same animals. Electron microscopy was performed on hepatic tissue from another healthy rabbits to identify the distribution of Sonazoid microbubbles. As a result, all tumors were smaller than 12 mm in diameter, and after intravenous injection of Sonazoid, they were surrounded with a ring of enhanced signal during the early phase (up to 30 s), followed by a clear contrast defect during the delayed phase (after 10 min). Histopathologic observation revealed that the ring-enhancement was caused by neovasculature in the tumor, and the contrast defects corresponded to living and dead parts of tumors, which lack Kupffer cells. Videodensitometric differences between tumor and healthy tissue markedly increased at delayed phase, and visual detectability of tumors was improved when Sonazoid was used. Ultrastructural analysis showed microbubble-like structures in Kupffer cells, which indicated that Sonazoid microbubbles were taken up with these cells. In conclusion, Sonazoid, used with phase inversion imaging, greatly increases the detectability of small hepatic tumors by highlighting neovascularity at early phase and providing clear contrast defects due to absence of Kupffer cells, which take up Sonazoid microbubbles, at delayed phase.

Gray-scale liver enhancement with Sonazoid (NC100100), a novel ultrasound contrast agent; detection of hepatic tumors in a rabbit model.

The liver contrast effects of Sonazoid in two ultrasonographic imaging modes, gray-scale conventional and harmonic, were examined as a time-related study in normal rabbits, and evaluated quantitatively and visually with tumor-model rabbits to estimate the diagnostic potential. Peak enhancement of vessels and parenchyma was observed 1 min after injection in both modes, although signal enhancement in the parenchyma lasted for 120 min compared with rapid decay (5-10 min) in vessels. When Sonazoid was intravenously injected into metastatic carcinoma-model (VX-2) rabbits, all hepatic tumors showed ring enhancement in the early phase followed by clear contrast defects in the delayed phase, because signal enhancement remained only in normal parenchyma. Visual analysis scores for the diagnosis of tumors were improved by Sonazoid injection, and the videodensitometric differences between tumor and normal tissues were significantly greater after injection. Although the harmonic mode tended to show better contrast effects, the conventional mode provided significant contrast enhancement in this hepatic tumor-model. Sonazoid might be useful for the detection of undifferentiated tumors in the liver by making it possible to visualize neovascularity in the early phase and clear contrast defects in the delayed phase, not only in the harmonic but also in the conventional mode.