Comparative analysis of renal flow using contrast power Doppler and gray-scale ultrasound

Our previous studies have shown that renal perfusion can be visualized by imaging the transit of a contrast agent through the parenchyma of the organ using gray scale (GS) and power Doppler (PD) ultrasound. However, the relative merits and the sensitivities of the two imaging methods are not known. This study compares the effectiveness of the two modes in visualizing kidney perfusion at the clinical dose of contrast agents. Gray scale and power Doppler images of the dog kidneys (N = 5) were recorded using a clinical ultrasound scanner at 4-7 MHz. A fixed longitudinal plane of the kidney was imaged by mounting the transducer on the animal with a specially designed holder. A dose of 0.1 ml/kg of Echogen (Sonus Pharmaceuticals, Costa Mesa, CA) was injected intravenously and GS and PD images were recorded simultaneously on two separate time-encoded video tapes during the passage of the contrast agent through the kidneys. The enhancement of GS and PD images was assessed qualitatively by three radiologists. The quantitative assessment was made by measuring the regional and global enhancements of digitized B-scan and PD images. Regional measurements were made by comparing brightness of the post contrast images with that of a pre-contrast reference image pixel by pixel. Student t-test (p less than or equal to 0.01) was used to determine the statistical significance of the change. The regions representing statistically significant differences were encoded on the image in color with brightness proportional to the magnitude of change. The regions with no significant change were represented in gray scale. This generated a series of new images, referred to as StatMap, with color representing regions of perfusion. Changes in power Doppler images were visually detectable with high confidence in all five dogs by all three radiologists. There was no perceptible changes in B-scans. Computer analysis of PD images yielded characteristic indicator dilution curves in all five dogs with an initial rise time of 2-5 sec and a peak at 7-20 sec. The enhancement in PD lasted for 97-400 seconds. The peak to pre-injection Doppler power ratio was 2.41 +/- 0.85. There were no detectable changes in gray scale images except in one dog which exhibited a small change (similar to 2% of the base line value). The StatMap images of PD exhibited perfusion over the entire kidney, whereas the GS images showed perfusion to be sparsely distributed. The qualitative and quantitative analyses show PD to be more sensitive to contrast enhancement than B-scan imaging, making the former more suitable for visualizing renal perfusion.