The effect of dilution medium on the measurement of in-vitro properties of ultrasound contrast agents

Although clinical detection of ultrasound contrast agents requires injection of the agent into the human circulatory system, most in-vitro measurements have been made with agents diluted in aqueous media such as saline or Isoton(R). The differences between blood and saline have the potential to alter observed physical properties. We present a series of experiments to identify differences in in-vitro behavior of a contrast agent diluted in different media. In one set of experiments, Definity(TM), a suspension of lipid-encapsulated microbubbles of perfluoropropane, was exposed to ultrasound from a clinical ultrasound system. The agent was diluted in either blood or USP saline in a small test cell immersed in a water bath at either room temperature or 37 degrees C. The resulting B-mode images were digitized and the mean videointensity in a region within the cell as a function of time determined. The average videointensity and the mean integrated videointensity were used to compare the two conditions. In all cases, the duration of the videointensity was significantly greater for Definity(TM) in blood than in saline. The time for the signal to decay to one half the initial videointensity was 400 seconds for blood vs. 90 seconds for saline. The mean integral of the normalized videointensity over time was in all cases greater for Definity(TM) in blood than in saline. We concluded that Definity(TM) is significantly more stable to ultrasound exposure when it is diluted in blood than when diluted in saline. Further, the rate of disappearance of contrast agent in blood as well as in saline is related to the acoustic output of the ultrasound scanhead and to increases in the ambient pressure. The two factors were additive.