Contrast media in liver sonography: correlation with enhanced dynamic magnetic resonance imaging

Both color Doppler sonography and magnetic resonance are imaging techniques which do not use ionizing radiations, but despite this common feature there remain many differences between them. Thus, color Doppler sonography is a cost-effective technique using mechanical waves and providing real-time images while magnetic resonance imaging is much more expensive, uses magnetic fields and radiofrequency energy and provides static images. The former method is very sensitive in detecting focal liver lesions > 1 cm, but its specificity in characterization is not as good, not even with the color Doppler technique. The main differences between color Doppler sonography, with and/or without echocontrast agents, and contrast-enhanced (Gadolinium chelates) dynamic magnetic resonance imaging in focal liver lesions can be summarized as follows: (1) magnetic resonance imaging depicts tumor vascularization only after paramagnetic contrast media injection. Enhanced images completely depend upon the contrast agent and cannot be achieved without it. In contrast, color Doppler signal is not modified by the contrast agent, it just becomes stronger. (2) Contrast-enhanced magnetic resonance signal (as well as contrast-enhanced computed tomography signal) provides more pieces of information than color Doppler signal about the flow characteristics of liver nodules--i.e. it shows not only blood flow (hyper-/hypovascular nodule), but also the interstitial spread of the agent and its wash-out. For example, hepatocellular carcinoma and focal nodular hyperplasia have similar perfusion while agent spread and wash-out decrease very quickly in the former and more slowly in the latter, except for the low decrease of the central scar. (3) Color Doppler technology improvements, higher sensitivity to slow flows and better signal/noise ratio reduce the applications of contrast-enhanced sonography in focal liver lesions because the agents modify only sensitivity and not the imaging in slow flow studies. (4) The higher cost of contrast studies is justified only in selected cases, namely treatment follow-up in the lesions with rich pretreatment vascularization. Finally, the higher cost of contrast magnetic resonance studies is justified to increase sensitivity and especially to allow lesion characterization.