Schematic representation of the ARFI technique. On a conventional gray-scale US image (oblique scan including the right kidney and the lowest portion of the right lobe of the liver), acoustic push pulses (curved lines) are generated together with the main US beam. From the push pulses originate shear waves (dashed horizontal lines) propagating perpendicular to the main US beam, which are sampled by tracking beams (arrows) parallel to the main beam. The actual distance covered by the shear waves before their attenuation is limited: the data concerning tissue response can only be achieved within a small ROI (rectangular box along the straight vertical line)