Effect of gas-containing microspheres and echo contrast agents on free radical formation by ultrasound

Stabilized microbubbles (microspheres) are widely used to enhance the contrast of ultrasound imaging. Our data provide direct evidence that the contrast agents, Levovist, PVC-AN (polyvinylidene chloride-acrylonitryl copolymer), and Albunex (compared to 5% human albumin), at concentrations comparable to those used for ultrasound imaging, enhance H2O2 production (through the superoxide-dependent pathway) in air-saturated aqueous solutions exposed to 47 kHz ultrasound above the cavitation threshold. These agents also act as scavengers of √H atoms and √OH radicals, thus lowering H2O2 formation (by recombination of √OH radicals) in argon-saturated solutions. EPR spin trapping also reveals that secondary radicals derived from the contrast agents are produced by reactions with √H and √OH which are formed by pyrolysis of water inside cavitation bubbles. In addition, the contrast agents themselves undergo pyrolysis reactions in the cavitation bubbles as demonstrated by formation of methyl radicals. Possible deleterious consequences of the formation of sonochemical intermediates may have to be assessed, particularly since some of the echo contrast agents have been shown to lower the cavitation threshold of diagnostic ultrasound. Unlike the microspheres formed from organic molecules, inorganic microspheres, Eccospheres, because of their stability and inert nature with respect to participation in free radical processes, appear to be suitable tools for enhancing the yields of aqueous sonochemical reactions.