Biol. Pharm. Bull. 28(6) 972—977 (2005)

focal lesions, which is based on their distinctive echogenicities; the gray-scale morphologic features. To improve diagnostic ability of detection and characterization of tumors, contrast imaging has been used in diagnostic imaging, e.g. X-ray computed tomography (CT) or magnetic resonance imaging (MRI), but has not yet been popular in US since contrast agents for US are not at clinical satisfaction level. Intravenous US contrast agents available for clinical use or the agents under development are basically microbubbles whose size is comparable to or less than red blood cells. First-generation agent consisted of air-microbubbles, Levovist (SH U 508A; Schering, Berlin, Germany), is commercially available in many countries, but short imaging duration and weak contrast effect due to instability of the microbubbles could provide only limited diagnostic information. In order to improve the contrast effects, second-generation agents that use less soluble gases for microbubbles are being developed and now clinically available in limited western countries. Viewpoint from US imaging machine, to detect microbubbles more sensitively, harmonic imaging technology has been developed to visualize the harmonic echo from the microbubbles: backscatter signal (echo) from the microbubbles contains more harmonic component than that from tissues. One of the second-generation agents, Sonazoid (previously NC100100, Amersham Health, Oslo, Norway) has been reported to enhance perfusion of the myocardium and other organs. In the previous study, we have characterized contrast enhancement of Sonazoid on the liver of metastatic carcinoma-model (VX-2) in rabbits, and demonstrated promising contrast effect on the detection and delineation of liver tumors. This is due to uniqueness of Sonazoid whose microbubbles are likely to be taken up by Kupffer cells (liver macrophages) in the healthy liver and enhances contrast of the liver parenchyma during the delayed phase, which usually occurs within 10 min after the injection. In contrast, tumor that lacks Kupffer cells was not enhanced resulting in clear negative contrast of the tumor. In the previous study, comparison of the tumor contrast with histopathology of the tumors could not be examined in detail, since resolution of US images that had been acquired using the harmonic imaging technique was not enough to compare with microscopic images. To improve resolution and signal-to-noise ratio of this harmonic technique, phase inversion imaging (PII) technique was then developed and now become available in the commercial US machine. PII can detect harmonic echoes from the microbubble contrast agent with greater sensitivity and without the loss of spatial resolution often seen in the harmonic imaging. Localization of the microbubbles in the liver Kupffer cells has been demonstrated in rat by using transmission electron microscopy (TEM). However, the liver contrast effect in rats, such as dose and contrast duration, have not been fully characterized since rat liver was too small to image using clinical US machine. It is favorable to determine the microbubble distribution in the same species whose liver contrast with Sonazoid has already been characterized. In this study, we used normal and the rabbits with VX-2 tumor in the liver 1) to demonstrate hepatic tumor detectability of Sonazoid with PII by visual assessment and videodensitometric analysis, 2) to compare the contrast effect and histopathology of the tumor by taking advantage of high resolution of PII, 3) to identify the microbubbles distribution in the rabbit liver at the delayed phase by TEM.