Gas or air filled microbubble suspensions in aqueous phases usable as
imaging contrast agents in ultrasonic echography. They contain
surfactants and, optionally, hydrophilic stabilizers. The surfactants can
be in the form of liposomes. The suspensions are obtained by exposing the
surfactants to air or a gas before or after admixing with an aqueous
phase. One can impart outstanding resistance against collapse under
pressure to these gas-filled microbubbles used as contrast agents in
ultrasonic echography by using as fillers gases whose solubility in
water, expressed in liter of gas by liter of water under standard
conditions, divided by the square root of the molecular weight does not
exceed 0.003. Contrast agents with particular mixtures of gases are also
disclosed that have advantageous properties.