Assessment of renal blood flow with contrast ultrasonography

Sonicated albumin microspheres, a digitalizing ultrasound system, and a mathematical model for flow were used to determine whether blood flow in the canine kidney could be assessed with contrast ultrasound. Albunex ultrasound contrast microspheres were injected into the aorta while ultrasound images of the kidney and aorta were recorded simultaneously. Ultrasound data were obtained during contrast injections at 93 different renal blood flow rates in nine dogs. Contrast dose was calibrated to ultrasound system response for both aortic and renal images. A linear relationship between microbubble concentration used and pixel intensity was established (r = 0.89 for aortic images and r = 0.91 for renal images). Renal blood flow was manipulated from baseline by means of a hydraulic renal artery occluder and by intravenous dopamine or fenoldopam infusion. Blood flow calculated with contrast ultrasonography was compared with direct measurement obtained with an electromagnetic flow probe at each flow rate. Direct measurement correlated with rates calculated with contrast ultrasonography (r = 0.84, 95% confidence limits from 0.75 to 0.90). Overall, calculations tended to overestimate absolute flow measurements, and overestimation of flow tended to be greater during pharmacologically manipulated flow rates. We conclude the changes and trends in renal blood flow can be serially assessed in vivo with contrast ultrasonography, but technical limitations of present commercial ultrasounds systems preclude absolute quantification at this time.