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-acrylon itryl copolymer), and Albunex (compared to 5% human albumin), at conce ntrations comparable to those used for ultrasound imaging, enhance H2O 2 production (through the superoxide-dependent pathway) in air-saturat ed aqueous solutions exposed to 47 kHz ultrasound above the cavitation threshold. These agents also act as scavengers of H-. atoms and (OH)- O-. radicals, thus lowering H2O2 formation (by recombination of (OH)-O -. radicals) in argon-saturated solutions. EPR spin trapping also reve als that secondary radicals derived from the contrast agents are produ ced by reactions with H-. and (OH)-O-. which are formed by pyrolysis o f water inside cavitation bubbles. In addition, the contrast agents th emselves undergo pyrolysis reactions in the cavitation bubbles as demo nstrated by formation of methyl radicals. Possible deleterious consequ ences of the formation of sonochemical intermediates may have to be as sessed, particularly since some of the echo contrast agents have been shown to lower the cavitation threshold of diagnostic ultrasound. Unli ke the microspheres formed from organic molecules, inorganic microsphe res, Eccospheres, because of their stability and inert nature with res pect to participation in free radical processes, appear to be suitable tools for enhancing the yields of aqueous sonochemical reactions. Pub lished by Elsevier Science.