Early experience in the use of Levovist ultrasound contrast in the evaluation of liver masses

The Use of Acoustic Radiation Force Decorrelation-Weighted Pulse Inversion for Enhanced Ultrasound Contrast Imaging

OBJECTIVES

The use of ultrasound imaging for cancer diagnosis and screening can be enhanced with the use of molecularly targeted microbubbles. Nonlinear imaging strategies such as pulse inversion (PI) and "contrast pulse sequences" (CPS) can be used to differentiate microbubble signal, but often fail to suppress highly echogenic tissue interfaces. This failure results in false-positive detection and potential misdiagnosis. In this study, a novel acoustic radiation force (ARF)-based approach was developed for superior microbubble signal detection. The feasibility of this technique, termed ARF decorrelation-weighted PI (ADW-PI), was demonstrated in vivo using a subcutaneous mouse tumor model.

MATERIALS AND METHODS

Tumors were implanted in the hindlimb of C57BL/6 mice by subcutaneous injection of MC38 cells. Lipid-shelled microbubbles were conjugated to anti-VEGFR2 antibody and administered via bolus injection. An image sequence using ARF pulses to generate microbubble motion was combined with PI imaging on a Verasonics Vantage programmable scanner. ADW-PI images were generated by combining PI images with interframe signal decorrelation data. For comparison, CPS images of the same mouse tumor were acquired using a Siemens Sequoia clinical scanner.

RESULTS

Microbubble-bound regions in the tumor interior exhibited significantly higher signal decorrelation than static tissue (n = 9, P < 0.001). The application of ARF significantly increased microbubble signal decorrelation (n = 9, P < 0.01). Using these decorrelation measurements, ADW-PI imaging demonstrated significantly improved microbubble contrast-to-tissue ratio when compared with corresponding CPS or PI images (n = 9, P < 0.001). Contrast-to-tissue ratio improved with ADW-PI by approximately 3 dB compared with PI images and 2 dB compared with CPS images.

CONCLUSIONS

Acoustic radiation force can be used to generate adherent microbubble signal decorrelation without microbubble bursting. When combined with PI, measurements of the resulting microbubble signal decorrelation can be used to reconstruct images that exhibit superior suppression of highly echogenic tissue interfaces when compared with PI or CPS alone.

Contrast-Enhanced Hepatic Sonography

Hepatic applications of sonography include evaluation of the gallbladder and bile ducts, assessment of patients with suspected portal hypertension, and evaluation of focal and diffuse parenchymal liver disease. Ultrasound imaging (US) is generally a reliable and accurate method to diagnose a variety of hepatic abnormalities; however, in some cases there is a need to improve sonography's sensitivity and specificity, particularly for the detection and characterization of hepatic masses. Recent clinical experience has shown that the addition of a US contrast agent can enhance sonography's diagnostic capabilities including improving assessments of hepatic blood flow and the detection and characterization of hepatic neoplasms. One intravenously administered US contrast agent is now available in the United States, but its approved indication is limited to echocardiographic applications. Additional agents that have more diagnostic applications are likely to become available. Contrast-enhanced sonography may play a significant role in the diagnosis and management of patients with suspected hepatic abnormalities. This article describes two generic types of US contrast agents—vascular agents and tissue-specific agents—and includes a review of recent published reports on clinical applications of contrast-enhanced hepatic sonography.

Contrast media-enhanced power Doppler sonography for evaluation of hemangiomas and malignant tumors in the liver

AIM

To evaluate the characterization of liver hemangiomas and malignant tumors using power Doppler sonography before and after intravenous injection of a sonocontrast agent.

METHODS

Forty-five patients with 57 liver tumors (22 hemangiomas, 24 metastases, 10 hepatocellular carcinomas (HCC) and one cholangiocellular carcinoma) were examined prospectively. The distribution (peripheral, central, mosaic) and extent (none, minimal, moderate and strong) of intratumoral flow pattern in each sonographic examination was subjectively classified.

RESULTS

The administration of the sonocontrast agent by bolus injection caused enhancement to gradually increase up to 2 min and lasted for 4-5 min. After injection of contrast agent, flow signals appeared or increased in 34 tumors. No signal enhancement was observed in 18 hemangiomas, four metastases and one HCC. The sensitivity and specificity of intratumoral vascularity for the detection of malignant liver tumors was 37.1 and 90.9% for unenhanced power Doppler sonography, and 85.7 and 81.8% for contrast-enhanced power Doppler sonography, respectively.

CONCLUSION

Contrast-enhanced power Doppler sonography is superior to unenhanced power Doppler sonography in the demonstration of malignant tumor vascularity, and is helpful in differentiating between hemangiomas and malignant liver tumors. A specific flow pattern within the tumor is not established in primary and metastatic malignant tumors with contrast-enhanced power Doppler sonography.