Measuring the effects of ultrasound on contrast agents

In vitro acoustic studies generally assume that known and reproducible quantities of contrast media are being evaluated, without need for agent retesting after dilution and handling. This assumption is incorrect, because duration of ultrasound contrast is substantially reduced by dilution of encapsulated, gas-filled agents into gas-poor solutions. Contrast agent persistence is enhanced and diffusive gas loss is slowed, but not eliminated, by filling microspheres with core gases having lower aqueous solubility. Since diffusive gas loss promotes safe elimination of gas bubbles in vivo, it is a desirable property that may prove unavoidable for commercial contrast agents. For non-encapsulated contrast agents containing emulsified perfluorocarbon liquids, exposure to threshold levels of ultrasound energy increased the dose of available contrast, by causing superheated liquid droplets to boil and form new microbubbles. This sudden activation threshold was not observed in encapsulated microsphere formulations, where the same perfluorocarbon was converted to vapor phase prior to insonation. These findings show that effective contrast agent doses can rapidly increase or decrease under conditions commonly used for in vitro acoustic testing. To avoid potential misinterpretation of experimental results, the effects of ultrasound independent factors on contrast agent stability need to be carefully considered.