VARIABLES CONTROLLING CONTRAST GENERATION IN A URINARY-BLADDER MODEL

An ultrasound system has been developed to generate microbubbles in vi vo for use as ultrasound contrast agent. Possible applications include diagnosis of reflux in the urinary tract. In experiments designed to elucidate the contrast microbubble generation process, acoustic bursts (at 1.8 MHz, 125 ms) were propagated through a latex rubber balloon, modeled after a rabbit urinary bladder, containing fluids of various a ir and carbon dioxide saturations and concentrations of cavitation nuc lei (0.198-mu m-diam polystyrene particles). The peak rarefactional pr essure threshold for contrast microbubble generation, as visualized wi th a diagnostic ultrasound system, decreased approximately a factor of 2 for increasing particle concentration from 10(8) to 10(10) particle s/cc, with the lowest threshold of 5.24 MPa. For samples with gas satu rations below 50% and 10(10) particles/cc, the average thresholds were at least twice as high as those of more saturated fluids (with mean t hreshold for saturated fluids of 6.45 MPa), and samples containing CO2 had considerably lower thresholds than respective under-saturations i n air. At a fixed pressure amplitude, echogenicity tended to increase with both increasing particle concentration and gas saturation; this w as more favorable for samples containing COL?. Even in a restricted-nu clei environment such as the urinary bladder, generation of vaporous c avitation should be possible; however, subsequently, abundant gas is n eeded to grow vaporous bubbles to persistent and imageable sizes, to a ssist in the diagnosis of urinary reflux. (C) 1998 Acoustical Society of America.