A microbubble preparation formed of a plurality of microbubbles comprising a first gas and second gas surrounded by a membrane such as a surfactant, wherein the first gas and the second gas are present in a molar ration of from about 1:100 to abut 1000:1, and wherein the first gas has a vapor pressure...http://www.google.com/patents/US6280704?utm_source=gb-gplus-sharePatent US6280704 - Ultrasonic imaging system utilizing a long-persistence contrast agent
