NONLINEAR CONTRAST ENHANCEMENT OF FOCAL LIVER LESIONS

A - CHARACTERIZATION OF FOCAL LIVER LESIONS

Due to the dual blood supply of the liver tissue by the hepatic artery (25-30%) and the portal vein (70-75%), three vascular phases can be defined in all the contrast imaging modalities and even in CEUS. The arterial phase starts from 10-20 sec after contrast administration into a peripheral vein and persists for ~ 10-15 sec; followed by the portal vein phase which usually lasts 2 min after i.v. contrast injection. The last phase (parenchymal or sinusoidal) persists until clearance of the contrast medium from the hepatic parenchyma.  It has two different lengths: 4-6 min for SonoVue and 15-20 min for Levovist, which suggests several mechanisms including sinusoid pooling and RES / Kupffer’s cells uptake. This differs from the extracellular equilibrium phase observed in contrast agent CT and MR imaging (EFSUMB Study Group. Guidelines for the Use of Contrast Agents in Ultrasound 2004). In our opinion the evaluation criteria of the US and CEUS examinations should be specified; meaning that the terms Hypo-, Iso- and Hyper- echogenic should be used in the description of the conventional US examinations; basket patterns, peripheral or central, regular or irregular vessels for color Doppler examinations.
Terms to be used for CEUS should be :

1. Type of enhancement: Rim, Marginal, Nodular; Centripetal, Centrifugal filling.
2. Degree of enhancement or enhancing: Hypo-iso- and Hyper-enhancement or enhancing - Lower, Equal or Higher than surrounding parenchymal.
3. Time of washout of contrast in the different phases - portal and parenchymal.
4. Lack of enhancement in the different phases - arterial portal and    parenchymal.
5. Sustained enhancement in the different phases - arterial portal and parenchymal.

Hypo-, Iso- and Hyper- echoic terms might be useful to describe static phenomena underlying the conventional US imaging but are ambiguous for explaining dynamic phenomena underlying the CEUS imaging. It might be appropriate to uniform terms related to the perfusion behavior of the lesions which shows the evolution of lesion vascular volume over time.