CONTRAST AGENTS CONTAINING TWO TYPES OF GAS FILLED MICROBUBBLES

Ultrasound contrast agents comprising two or more types of gas-containing microparticles which differ in their susceptibility to ultrasonic pressure. Microparticle types having relatively low susceptibility to ultrasonic pressure provide a response to high acoustic energy in the near field where microparticle types with relatively high susceptibility tend to be inactivated through destruction. More highly susceptible microparticle types are effective in the lower acoustic energy environment of the far field, where lower susceptibility microparticle types will tend to exhibit little interaction.