[Duplex ultrasound with echo contrast media]

Intravenous echo contrast agents transversing the lungs lead to improved detection of blood flow in arteries and veins due to enhancement of the Doppler signal by 20-25 dB. This echo enhancement improves the visualisation of vessel sections that are otherwise difficult to examine due to obesity, to deep vessel location, scarring or low flow state. The available contrast agents that pass through the lungs and are used in controlled studies, possess similar physico-chemical properties: mean bubble diameter 2-4 microns, viscosity and pH value close to that of the blood. The contrast agents enable the plotting of densitometric wash-in and wash-out curves from the heart and the vessels to aid in the investigation of tissue perfusion. Echo contrast agents induce some technical side effects that must be known to avoid annoying diagnostic pitfalls: spectral bubble noise, colour blooming, acoustic shadowing and changes in Doppler frequency shifts. Their origin and measures to avoid them are discussed. The contrast agents that pass through the lungs open up new fields of application in the investigation of blood flow.