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An ultrasound system has been developed to generate contrast microbubbles in vivo. Possible uses include diagnosis of reflux in the urinary tract. In vivo and in vitro experiments were designed to elucidate the microbubble generation process using 1.8 MHz acoustic bursts at 125 ms. In in vivo experiments on rabbits, the peak rarefactional pressure threshold for contrast generation, as visualized with a diagnostic ultrasound system, decreased with increasing pCO/sub 2/. For the in vitro aqueous studies the threshold decreased almost a factor of two for increasing particle concentration (0.2 /spl mu/m dia. polystyrene) from 10/sup 8/ to 10/sup 10/ particles/cc. The thresholds were at least twice as high for more saturated fluids, and CO/sub 2/ samples had considerably lower thresholds than respective under-saturations in air. At a fixed pressure amplitude, echogenicity tended to increase with both increasing particle and gas content; this was more pronounced for samples containing CO/sub 2/. Even in a restricted-nuclei environment such as the human urinary bladder, generation of vaporous cavitation should be possible; however subsequently, abundant gas is needed to grow vaporous bubbles to persistent and imageable sizes, to assist in the diagnosis of urinary reflux.