Abstract: Microparticles useful for enhancing the ultrasound image of a tissue or organ consist of liquid and/or gas core material which is encapsulated by a biocompatible, tanned protein shell. These stabilized microparticles are useful as ultrasonic imaging agents, and are additionally useful in the further production of functionalized microparticles for in vivo imaging. In particular, targeting molecules such as antibodies or other ligands can be attached to the strengthened exterior surface of the stabilized microparticles to impart target-specificity to the microparticles. The targeting molecules may also provide hydrophilicity to the exterior surface, thus increasing the recirculation time of the microparticles. The targeting molecules may be attached directly to the exterior surface of the microparticles, or they may be attached via a bifunctional spacer arm, which may itself be hydrophilic.