Real-time measurement of renal blood flow in healthy subjects using contrast-enhanced ultrasound

Abstract

Current methods for measuring renal blood flow (RBF) are time consuming and not widely available. Contrast-enhanced ultrasound (CEU) is a safe and noninvasive imaging technique suitable for assessment of tissue blood flow, which has been used clinically to assess myocardial blood flow. We tested the utility of CEU in monitoring changes in RBF in healthy volunteers. We utilized CEU to monitor the expected increase in RBF following a high protein meal in healthy adults. Renal cortical perfusion was assessed by CEU using low mechanical index (MI) power modulation Angio during continuous infusions of Definity. Following destruction of tissue microbubbles using ultrasound at a MI of 1.0, the rate of tissue replenishment with microbubbles and the plateau acoustic intensity (AI) were used to estimate the RBF velocity and cortical blood volume, respectively. Healthy adults (n = 19, mean age 26.6 yr) were enrolled. The A.beta parameter of CEU, representing mean RBF increased by 42.8%from a baseline of 17.05 +/- 6.23 to 23.60 +/- 6.76 dB/s 2 h after the ingestion of the high-protein meal (P = 0.002). Similarly, there was a 37.3%increase in the beta parameter, representing the geometric mean of blood velocity after the high protein meal (P < 0.001). The change in cortical blood volume was not significant (P = 0.89). Infusion time of Definity was 6.3 +/- 2.0 min. The ultrasound contrast agent was tolerated well with no serious adverse events. CEU is a fast, noninvasive, and practical imaging technique that may be useful for monitoring renal blood velocity, volume, and flow.

Fig. 1.

Contrast-enhanced ultrasound (CEU) images of the kidneys 0.1 (A), 1.0 (B), and 2.1 s (C) after replenishment of tissue with contrast agent (microbubbles). Note that renal hilum, interlobar, and arcuate arteries fill with contrast agent very rapidly (A) followed in a second or two by renal cortex (B). Due to low velocity and flow of blood to the medulla, complete opacification of medullary pyramids may take several seconds, making distinction between cortex and medulla easy (B and C). A white dashed line is drawn around the kidney to better demonstrate its boundaries.

Fig. 2.

Graphic depiction of changes in acoustic intensity vs. time after destruction of microbubbles in the tissue with high-energy ultrasound wave.

Fig. 3.

Sequential ultrasound images of the kidney before and after high-energy impulse. A: at steady state and before destruction of microbubbles. B: destruction of microbubbles in the tissue using high-energy ultrasound wave. C–I: 0.1–16 s after high-energy pulse showing gradual replenishment of kidney tissue with microbubbles. Note that the filling of large arteries occurs almost immediately after the impulse (0.13 s after high-energy pulse; C), followed by the cortex (1–2.25 s after high-energy pulse; D–G,) and much later by the medulla (10 and 16 s after high-energy pulse, respectively; H and I).

Fig. 4.

Replenishment of microbubbles in the renal cortex after destruction of microbubbles with high-energy pulse. The top section depicts the kidney 6 s after replenishment with contrast agent. The arrow points to a single region of interest (ROI) within the renal cortex. Graphic demonstration of tissue replenishment with microbubbles over time for the selected ROI using the QLAB software is shown at the bottom.

Fig. 5.

Box plots demonstrating changes from baseline in different CEU parameters after ingestion of a high-protein meal.