[Contrast media in ultrasonography]

Microbubbles of air or other gases are ideally suited as echo-enhancers because of the high difference in acoustic impedance of gas/fluid interfaces. The microbubbles are 1-7 microns in diameter and capable of surviving pulmonary passage to produce enhancement of especially the Doppler signal after intravenous injection. The main clinical role of ultrasound (US)-contrast is to enhance the Doppler signals (up to 20 dB) in inconclusive Doppler examinations. Experience with US-contrast agents comes mainly from use of the galactose based agent Levovist (Schering AG, Germany). The US-contrast agents have further properties: a) Air bubbles in the sound fields will reverberate and return sound with harmonic frequencies. This phenomenon can be used in "harmonic imaging". B) Short intense sound impulses will destroy the bubbles resulting in a mosaic of colours (stimulated acoustic emission). This phenomenon may be used in tumour diagnostics in the liver.