Advances in ultrasound systems for hepatic lesions in Japan.

Abstract

Since the late 1990s, the first-generation agent Levovist (SH U 508A; Schering AG, Berlin, Germany), which consists of air microbubbles, has been used as an ultrasound contrast agent to enhance liver lesions in Japan. In addition to its utility during the vascular phases (arterial and portal) of contrast-enhanced ultrasonography (US), this contrast agent exhibits a parenchyma-specific phase and can accumulate within the liver for up to 20 min [1]. However, the parenchyma-specific contrast of Levovist was reportedly effective only when the imaging was performed at a high acoustic power using a high mechanical index (MI), and the effect was transient. Therefore, this phenomenon cannot be used in real-time examinations during the parenchyma-specific phase, and the visualization of the whole liver is limited to a single scan [2]. During the past 8 years, however, diagnostic images using ultrasound systems have significantly advanced with the development of ultrasound contrast agents, contrastenhanced three-dimensional (3D) US, fusion imaging, and high-resolution contrast-enhanced US with a high frame rate. A second-generation ultrasound contrast agent known as Sonazoid (Daiichi Sankyo, Tokyo, Japan), which is a lipidstabilized suspension of perfluorobutane gas microbubbles, was approved for clinical use in patients with liver lesions and for phase-inversion harmonic gray-scale US in Japan in January 2007. Similar to Levovist, contrast-enhanced US with Sonazoid exhibits two phases of contrast enhancement: vascular and post-vascular phase. During the postvascular phase of Sonazoid-enhanced US, we scanned the whole liver using this modality at a low MI, resulting in the minimal destruction of the Sonazoid microbubbles; this method allowed the detection of small malignant lesions as perfusion defects as well as the detection of small hypervascular hepatocellular carcinoma (HCC) lesions that cannot be detected using conventional US [3]. During the post-vascular phase of Sonazoid-enhanced US, nearly all moderately and poorly differentiated HCCs appear as hypoechoic lesions and are detected as perfusion defects, whereas the majority of well-differentiated HCCs and nearly all early HCCs exhibit an isoechoic pattern [4]. We must therefore remain aware that the majority of well-differentiated HCCs and nearly all early HCCs cannot be detected as perfusion defects during the post-vascular phase of Sonazoid-enhanced US [5]. Therefore, the arterial phase findings are important for evaluating the vascularity of each hepatic lesion. Three-dimensional (3D) US allows the volume of interest (VOI) to be viewed in three orthogonal planes, supplying more spatial information and facilitating an easier anatomic assessment. Since 2009, with the assistance of a high MI contrast mode and high MI contrast conditions (which reduce microbubbles in microvessels but not in relatively large vessels, such as tumor vessels and portal veins), arterial phase Sonazoid-enhanced 3D US has been used to facilitate detailed observations of tumor vessels [6]. In the post-vascular phase, perfusion defect images obtained using a high MI contrast mode of Sonazoid-enhanced 3D US are clearer than those obtained using a low MI contrast mode, since the high MI contrast mode showed a significant difference in intensity between the lesion and the surrounding liver parenchyma. The process of Sonazoid-enhanced 3D US includes two steps: data acquisition and image reconstruction. With equipment development, a & Kazushi Numata kz-numa@urahp.yokohama-cu.ac.jp