The invention relates to ultrasound contrast agents comprising vesicles comprising a protein capable of formation of gas-containing vesicles, wherein the vesicles contain gas which comprises sulphur hexafluoride or a low molecular weight fluorinated hydrocarbon. These contrast agents exhibit stability in vivo upon administration so as to permit ultrasound visualization while allowing rapid subsequent elimination from the system.
Ultrasound contrast agents comprising microbubbles of gas or a gas precusor encapsulated in a protein shell, e.g. of human serum albumin, the protein being crosslinked with biodegradable crosslinking groups, exhibit stability in vivo upon administration so as to permit ultrasound visualisation while allowing rapid subsequent elimination from the system.