Abstract: An ultrasonic imaging agent is produced by a continuous sonication processing of an aqueous solution of heat-denaturable biocompatible protein. The solution is carefully preheated to a temperature of incipient protein denaturation without forming insolubilized protein. A gaseous fluid, preferably air, is added to the solution. In the sonication, the air-containing solution is foamed, increasing the formation and concentration of microbubbles, and the solution is further heated to insolubilize a portion of the protein, thereby encapsulating the microbubbles and forming particulate microspheres.

Abstract: A microbubble-type ultrasonic imaging agent is provided comprising a parenterally-administerable aqueous medium containing a dispersion of microspheres predominantly of diameters less than 10 microns, wherein the microspheres consist of gas microbubbles encapsulated with water-insolubilized biocompatible material. The imaging agent is characterized by having a concentration of greater than 100.times.10.sup.6 microspheres per milliliter, and a stability such that this concentration is maintained for over 4 weeks at a temperature of 20.degree. to 25.degree. C.