Comparison of ultrasound scattering behavior of Optison® and a liquid perfluorocarbon nanoparticle contrast agent

Previously we reported that useful ultrasound contrast enhancement could be obtained using a liquid emulsion contrast agent developed in our laboratory. Although the physical basis for this enhancement was not completely characterized, the effect was reproducible in vivo and in vitro. We now 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 and acoustic pressure, and would show evidence of scattering agent destruction as the duration of ultrasound exposure increased. Both low power (0.65 MPa) unipolar pulses and high-power (3 MPa) unipolar pulses were used to measure the attenuation coefficient of Optison and that of the emulsion. The coefficient was also measured using a high-power unipolar pulse preceded by 5 cycles of a 1 MHz sine-wave of the same amplitude, emitted at a high repetition frequency (5 kHz). Both specimens were held in a pressurized chamber 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) as the major component. Particle size was measured at 276 nm. We varied ambient hydrostatic pressure (-50 to 200 mm Hg in 50 mm steps), duration of exposure to the acoustic field (2, 20, 40, 80 s), and peak positive and negative acoustic pressure (0.65 to 3.0 MPa).