Ultrasound imaging of focal liver lesions with a second-generation contrast agent

For several years, the major diagnostic objective in using US contrast agents has been to detect flow in the circulation at a lower level than that otherwise possible in Doppler techniques. However, it turned out that enhanced color and power Doppler could provide fine details on small and deep vessels, but were not sensitive enough to detect contrast agents in the microcirculation. As a result, the usefulness of contrast agents in the field of liver imaging was limited to the evaluation of the vascular architecture of focal lesions. Recently, the development of new US techniques that produce images based on nonlinear acoustic effects of US interaction with microbubble contrast agents has opened new prospects for US imaging of the liver. These contrast-specific imaging techniques, in fact, can display microbubble enhancement in gray-scale, thus maximizing contrast and spatial resolution and enabling the analysis of the microcirculation. These techniques offer high sensitivity either to microbubble movement or to microbubble collapse independent of the level of the applied acoustic peak pressure. At low acoustic peak pressure levels grossly reflected by a low mechanical index (MI) the microbubble destruction can be reasonably neglected and the microbubble movement effect due to blood circulation is predominant. In contrast, when increasing the acoustic peak pressure levels (high MI), the destruction phenomena become the most important: the signal, in fact, is produced by microbubble collapse, and is related to microbubble concentration and not to blood flow velocity (1–3). We performed a pilot clinical study aimed at investigating the usefulness of both destructive (high MI) and non-destructive (low MI) gray-scale US techniques, in combination with a second-generation contrast agent, for the detection and characterization of focal liver lesions.