Detection of hepatic VX2 tumors in rabbits: comparison of conventional US and phase-inversion harmonic US during the liver-specific late phase of contrast enhancement

Abstract

Objective: To compare phase-inversion sonography during the liver-specific phase of contrast enhancement using a microbubble contrast agent with conventional B-mode sonography for the detection of VX2 liver tumors.

Materials and methods: Twenty-three rabbits, 18 of which had VX2 liver tumor implants, received a bolus injection of 0.6 g of Levovist (200 mg/ml). During the liver-specific phase of this agent, they were evaluated using both conventional sonography and contrast-enhanced phase-inversion harmonic imaging (CEPIHI). Following sacrifice of the animals, pathologic analysis was performed and the reference standard thus obtained. The conspicuity, size and number of the tumors before and after contrast administration, as determined by a sonographer, were compared between the two modes and with the pathologic findings.

Results: CE-PIHI demonstrated marked hepatic parenchymal enhancement in all rabbits. For VX2 tumors detected at both conventional US and CE- PIHI, conspicuity was improved by contrast-enhanced PIHI. On examination of gross specimens, 52 VX2 tumors were identified. Conventional US correctly detected 18 of the 52 (34.6%), while PIHI detected 35 (67.3%) (p < 0.05). In particular, conventional US detected only three (8.3%) of the 36 tumors less than 10 mm in diameter, but CE-PIHI detected 19 such tumors (52.8%) (p < 0.05).

Conclusion: Compared to conventional sonography, PIHI performed during the liver-specific phase after intravenous injection of Levovist is markedly better at detecting VX2 liver tumors.

Fig. 1

A. Unenhanced baseline transverse conventional US image obtained in a rabbit shows a 10 mm, slightly hyperechoic hepatic VX2 tumor (arrows). B. Transverse phase-inversion harmonic US image obtained in the same animal five minutes after contrast injection shows homogeneous enhancement of normal liver parenchyma. Note the hepatic VX2 tumor, seen as a well-defined hypoechoic enhancement defect (arrows). C. Transverse section of a gross specimen reveals an oval-shaped, whitish VX2 liver tumor (arrows).

Fig. 2

A. Unenhanced subcostal transverse sonogram of rabbit liver depicts a heterogeneous isoechoic lesion (arrows). B. Contrast-enhanced transverse phase-inversion US image obtained slightly inferior to the unenhanced image (A) reveals three nonenhancing foci measuring 2-12 mm in diameter. Two additional lesions (arrows) are identified, and their conspicuity is much greater than at unenhanced conventional imaging (A). C. Transverse section of the liver reveals five tumors at the same location. Compared to B, two additional lesions (arrows) less than 5 mm in diameter are apparent.