Previously we reported that useful ultrasound contrast enhancement could be obtained using a liquid emulsion contrast agent developed in our laboratory . The physical basis for this enhancement was unknown at the time of our report, although the effect was reproducible, in vivo and in vitro. We hypothesize that microbubble formation does not play a significant role in the acoustic behavior of these contrast agents, otherwise attenuation and backscatter would be significantly affected by changes in hydrostatic pressure, acoustic pressure, and show evidence of scattering agent destruction as the duration of ultrasound exposure increased. Measurements of attenuation coefficient and backscatter of Optison and the emulsion were made in a water bath maintained at temperatures ranging from 37° to 50°C. The emulsion nanoparticles were produced by methods standard in our laboratory using perfluorooctyl bromide (PFOB: boiling point 142°C) or perfluorooctane (PFO: boiling point 99°C) as the major component. Particle size was measured at 435 and 375 ±30nm respectively. We varied ambient hydrostatic pressure (-50 to 200 mm Hg in 50 mm steps), duration of exposure to insonifying acoustic field (2, 20, 40, 80 s), peak positive and negative acoustic pressure (0.038 to 3.0 MPa).