Contrast-enhanced Ultrasonography in Small Liver Tumors (< 3 cm)

Ultrasonography is a safe, convenient, low cost and noninvasive diagnostic modality for liver tumors. Power Doppler sonography may demonstrate fine tumor vessels in small lesions and hypovascular lesions. However, it has limitations including motion artifacts, less sensitivity to slow vascular flow, poor demonstration of deep-seated lesions (> 7 cm in depth), and high sensitivity to tissue motion (heart beat or aortic pulsation). Owing to improvements in contrast agents and new technologies such as harmonic and pulse inversion imaging, contrast-enhanced ultrasound (CEUS) has improved the detection rate compared with Doppler ultrasound in studies of liver lesions. The enhanced vascular patterns have been proved to correlate well with the findings from dynamic computed tomography or magnetic resonance imaging. CEUS provides the ability to detect small focal liver lesions and even metastatic liver tumors of less than 1 cm in diameter. This review attempts to determine ways to allow the diagnosis of small hepatocellular carcinomas (HCCs), especially in cirrhotic patients, using CEUS. Because HCCs are small, the feeding arteries are fine and the arterial blood flow to the tumor is slow, CEUS used in the diagnosis of nodules of 1–2 cm in cirrhotic patients is not satisfactory. The portal and late phases in pulse inversion imaging may provide more information to detect small lesions in the cirrhotic liver and improve the diagnostic sensitivity and specificity. Contrast-enhanced flash echo with subtraction mode is another way of detecting this type of small tumor. In the arterial phase, some tumors are hard to identify, owing to the isoechoic status of the tumors with respect to the surrounding liver parenchyma. However, these small lesions may be shown by flash echo subtraction imaging. Concurrent delayed phase imaging is useful in the diagnosis of small hypovascular HCCs. In conclusion, CEUS improves the diagnostic accuracy of focal liver lesions, even in tumors as small as 1–2 cm. This safe, convenient, low cost and noninvasive diagnostic modality should be promoted in routine clinical practice.