Contrast ultrasonography of the kidney: a new method for evaluation of renal perfusion in vivo

Association of Contrast-Enhanced Ultrasound-Derived Kidney Cortical Microvascular Perfusion with Kidney Function

BACKGROUND

Individuals with chronic kidney disease (CKD) have decreased kidney cortical microvascular perfusion, which may lead to worsening kidney function over time, but methods to quantify kidney cortical microvascular perfusion are not feasible to incorporate into clinical practice. Contrast-enhanced ultrasound (CEUS) may quantify kidney cortical microvascular perfusion, which requires further investigation in individuals across the spectrum of kidney function.

METHODS

We performed CEUS on a native kidney of 83 individuals across the spectrum of kidney function and calculated quantitative CEUS-derived kidney cortical microvascular perfusion biomarkers. Participants had a continuous infusion of the microbubble contrast agent (Definity) with a flash-replenishment sequence during their CEUS scan. Lower values of the microbubble velocity (β) and perfusion index (β×A) may represent lower kidney cortical microvascular perfusion. Multivariable linear regression models tested the associations of the microbubble velocity (β) and perfusion index (β×A) with estimated glomerular filtration rate (eGFR).

RESULTS

Thirty-eight individuals with CKD (mean age±SD 65.2±12.6 years, median [IQR] eGFR 31.5 [18.9-41.5] ml/min per 1.73 m²), 37 individuals with end stage kidney disease (ESKD; age 54.8±12.3 years), and eight healthy volunteers (age 44.1±15.0 years, eGFR 117 [106-120] ml/min per 1.73 m²) underwent CEUS without side effects. Individuals with ESKD had the lowest microbubble velocity (β) and perfusion index (β×A) compared with individuals with CKD and healthy volunteers. The microbubble velocity (β) and perfusion index (β×A) had moderate positive correlations with eGFR (β: r_s=0.44, P<0.001; β×A: r_s=0.50, P<0.001). After multivariable adjustment, microbubble velocity (β) and perfusion index (β×A) remained significantly associated with eGFR (change in natural log transformed eGFR per 1 unit increase in natural log transformed biomarker: β, 0.38 [95%, CI 0.17 to 0.59]; β×A, 0.79 [95% CI, 0.45 to 1.13]).

CONCLUSIONS

CEUS-derived kidney cortical microvascular perfusion biomarkers are associated with eGFR. Future studies are needed to determine if CEUS-derived kidney cortical microvascular perfusion biomarkers have prognostic value.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

When is contrast-enhanced sonography preferable over conventional ultrasound combined with Doppler imaging in renal transplantation?

Conventional ultrasound in combination with colour Doppler imaging is still the standard diagnostic procedure for patients after renal transplantation. However, while conventional ultrasound in combination with Doppler imaging can diagnose renal artery stenosis and vein thrombosis, it is not possible to display subtle microvascular tissue perfusion, which is crucial for the evaluation of acute and chronic allograft dysfunctions. In contrast, real-time contrast-enhanced sonography (CES) uses gas-filled microbubbles not only to visualize but also to quantify renal blood flow and perfusion even in the small renal arterioles and capillaries. It is an easy to perform and non-invasive imaging technique that augments diagnostic capabilities in patients after renal transplantation. Specifically in the postoperative setting, CES has been shown to be superior to conventional ultrasound in combination with Doppler imaging in uncovering even subtle microvascular disturbances in the allograft perfusion. In addition, quantitative perfusion parameters derived from CES show predictive capability regarding long-term kidney function.

Impact of complete revascularization with percutaneous coronary intervention on survival in patients with at least one chronic total occlusion

AIMS

This study sought to determine the impact on survival of successful drug-eluting stent-supported percutaneous coronary intervention (PCI) for chronic total occlusion (CTO).

METHODS AND RESULTS

Comparison of long-term cardiac survival of consecutive patients who underwent PCI for at least one CTO and who were stratified into successful and failure procedures. From 2003 to 2006, 486 patients underwent PCI for 527 CTO. CTO-PCI was successful in 344 patients (71%) and 361 lesions (68%). Multivessel PCI was performed in 62% in the CTO-PCI failure group and in 71% in the CTO-PCI success group (P = 0.062). Cardiac survival rate was higher in the CTO-PCI success group compared with CTO-PCI failure group (91.6 +/- 2.0 vs. 87.4 +/- 2.9%; P = 0.025), in patients with multivessel disease and CTO-PCI success compared with CTO-PCI failure (91.4 +/- 2.2 vs. 86.6 +/- 3.1%; P = 0.021), and in patients with complete revascularization when compared to patients with incomplete revascularization (94.0 +/- 1.7 vs. 83.8 +/- 3.6%; P < 0.001).

CONCLUSION

Successful CTO-PCI confers a long-term survival benefit. Improvement in survival is driven by the differences in the outcome of patients with multivessel disease and who were completely revascularized.

The powerful microbubble: from bench to bedside, from intravascular indicator to therapeutic delivery system, and beyond

This review discusses the development, current applications, and therapeutic potential of ultrasound contrast agents. Microbubbles containing gases act as true, intravascular indicators, permitting a noninvasive, quantitative analysis of the spatial and temporal heterogeneity of blood flow and volumes within the microvasculature. These shelled microbubbles are near-perfect reflectors of acoustic ultrasound energy and, when injected intravenously into the bloodstream, reflect ultrasound waves within the capillaries without disrupting the local environment. Accordingly, microbubble ultrasound contrast agents are clinically useful in enhancing ultrasound images and improving the accuracy of diagnoses. More recently, ultrasound contrast agents have been used to directly visualize the vasa vasorum and neovascularization of atherosclerotic carotid artery plaques, thus suggesting a new paradigm for diagnosis and treatment of atherosclerosis. Future applications of these microscopic agents include the deliver of site-specific therapy to targeted organs in the body. Medical therapies may use these microbubbles as carriers for newer therapeutic options.

Contrast-enhanced sonography of the renal transplant using triggered pulse-inversion imaging: preliminary results

The purpose of our study was to quantify renal transplant parenchymal sonographic enhancement using pulse-inversion imaging (PII) and intermittent emission after contrast administration by means of bolus and infusion techniques, and to evaluate renal perfusion functional indices. A total of 34 patients, presenting with minor abnormalities (n = 14) and cortical perfusion changes due to parenchymal disorders (n = 12) or renal artery stenosis (n = 8) were included. Cardiac-triggered contrast-enhanced PII ultrasound (US) was performed after administration of SHU 508 A (Schering AG, Berlin, Germany), using a high mechanical index, a frame rate of one image every four cardiac cycles for bolus study, and a decreasing frame rate for infusion study. Compared to baseline values, peak enhancement ratio ranged from 5.6 to 14.7 using a bolus administration, and reached a value of 2.1 to 4.0 using infusion technique. Qualitative analysis showed heterogeneous enhancement in most allografts presenting with acute parenchymal disease (p = 0.03). In bolus studies, time to peak, wash-in and wash-out slopes increased in renal transplants with parenchymal disease and renal artery stenosis (p = 0.0001). Infusion administration exhibited no plateau in signal level, and no significant difference in enhancement ratio was found between groups of patients. Triggered PII after contrast agent administration provides morphologic and quantitative information about cortical vascularity in renal transplants.

Quantification of renal blood flow with contrast-enhanced ultrasound

OBJECTIVES

The goal of this study was to determine the ability of contrast-enhanced ultrasound (CEU) to quantify renal tissue perfusion.

BACKGROUND

The kinetics of tracers used to assess renal perfusion are often complicated by countercurrent exchange, tubular transport or glomerular filtration. We hypothesized that, because gas-filled microbubbles are pure intravascular tracers with a rheology similar to that of red blood cells, CEU could be used to quantify renal tissue perfusion.

METHODS

During a continuous venous infusion of microbubbles (SonoVue), regional renal perfusion was quantified in nine dogs using CEU by destroying microbubbles and measuring their tissue replenishment with intermittent harmonic imaging. Both renal blood volume fraction and microbubble velocity were derived from pulsing-interval versus video-intensity plots. The product of the two was used to calculate renal nutrient blood flow. Renal arterial blood flow was independently measured with ultrasonic flow probes placed directly on the renal artery and was increased using dopamine and decreased by placement of a renal artery stenosis.

RESULTS

An excellent correlation was found between cortical nutrient blood flow using microbubbles and ultrasonic flow probe-derived renal blood flow (r = 0.82, p < 0.001) over a wide range (2.5 fold) of flows.

CONCLUSIONS

Ultrasound examination during microbubble infusion can be used to quantify total organ as well as regional nutrient blood flow to the kidney.

Contrast-enhanced power Doppler harmonic imaging--influence on visualization of renal vasculature

To compare contrast-enhanced power Doppler (PD) harmonic imaging (CHI) with contrast-enhanced power Doppler fundamental imaging (CPD) in the depiction of renal cortical vessels, 20 healthy volunteers were subjected to PD imaging and HI assessment of the kidney after bolus injection of Levovist(R) (SH U 508A). System settings were standardized and the pulse-repetition frequencies (PRF) systematically toggled from 750 to 500 and 250. Videotapes were independently reviewed by three readers with regard to the presence of artefacts, the degree of Doppler signal enhancement, demarcation of vessels and the extent of visualization. The assessments were graded separately for each PRF in accordance with a multistage scoring system. In comparison to contrast-enhanced PD, artefacts were significantly lower with CHI for all PRF (p = 0.0001). Vessels were better visualized (p = 0.002) and less blurred (p = 0.006) with CHI than with CPD. There was no significant difference in the extent of Doppler signal increase between CPD and the contrast-enhanced harmonic mode. Combination of the contrast-enhanced harmonic method and PD allows the PRF to be lowered and, by balancing the greater susceptibility of PD to interference from clutter, increases the likelihood of detection of flow in small vessels.

Assessment of Renal Perfusion in a Canine Model Using FS069, A New Transpulmonary Echocontrast Agent

We have previously demonstrated the safety and efficacy of FS069, a new transpulmonary echocontrast agent, for myocardial opacification. To our knowledge, no information exists regarding the use of this agent for transcutaneous assessment of renal perfusion. We studied 14 mongrel dogs using intravenously administered FS069. Renal ultrasound imaging was performed with a Hewlett-Packard Sonos 1500 using a 3.5-MHz transducer. Renal blood flow (ReBF) was altered using renal artery occlusion in four dogs and dipyridamole (0.56 mg/kg IV) in ten dogs. Renal perfusion was quantitatively assessed before and after each intervention using background subtracted peak intensity. ReBF was assessed with radiolabeled microspheres in ten dogs. Renal opacification was observed in all 14 dogs at baseline. The intravenous contrast dose required to produce optimal renal opacification ranged from 0.3-0.7 cc. After renal artery occlusion, peak intensity was reduced from 5.4 +/- 5.8 to 0.93 +/- 1.1 units (r = 0.99, P < 0.008). As expected, blood pressure and ReBF dropped in all ten dogs after dipyridamole administration. Interestingly, peak intensity increased in all but one dog. An inverse correlation (r = -0.75, P = 0.02) was observed between ReBF and peak intensity (percent change from baseline). The inverse relation between ReBF and peak intensity observed suggests vasoconstriction of the afferent arterioles in response to dipyridamole and a reduced clearance of the contrast. These findings are in agreement with previous data demonstrating decreased renal thallium clearance postdipyridamole administration. Our data document the feasibility to assess renal perfusion under various flow states after intravenous injection of FS069.

Intraoperative perfusion echocardiography

The ability to assess perfusion intraoperatively should enable end-organ evaluation of the effects of therapeutic choices and provide a basis for understanding the mechanisms of disease. Several experimental techniques for assessment of tissue perfusion are being evaluated; contrast echocardiography appears to be adaptable to the perioperative setting because of its portability and relatively modest cost. With further improvements in commercial ultrasound imaging devices and ultrasonic contrast agents, intraoperative contrast echocardiography may prove to be a technique for quantitation of tissue perfusion. Contrast echocardiography is currently being used intraoperatively to assess cardioplegia distribution, coronary bypass graft patency, and coronary artery collateral vessel distribution. In addition, relative change in renal blood flow can be assessed during renal transplant surgery. With continued advancement of ultrasound technology providing linear (or known) acoustic signal response and wider dynamic range for detection of small and large concentrations of contrast agents, tissue blood flow may soon be evaluated with even greater precision.

Ultrasound contrast agents: a review

During the past 25 years, many attempts have been made to establish effective ultrasound contrast agents for both cardiac and noncardiac applications. The ideal ultrasound contrast agent would be: (a) nontoxic; (b) injectable intravenously; (c) capable of passing through the pulmonary, cardiac and capillary circulations; and (d) stable for recirculation. A variety of potential ultrasound contrast agents have been or are now under development. Present and future ultrasound contrast agents should provide for increased diagnostic capabilities in a variety of normal and abnormal vessels and organs throughout the body. These agents will enhance tumor vascularity, delineate areas of ischemia, as well as improve visualization of vascular stenosis. Future developments with modification of ultrasound equipment should increase the capabilities of these agents to improve imaging as well as Doppler sensitivity.

Flow quantitation by radio frequency analysis of contrast echocardiography

Contrast echocardiography has the potential for measuring cardiac output and regional blood flow. However, accurate quantitation is limited both by the use of non-standard contrast agents and by the electronic signal distortion inherent to the echocardiographic instruments. Thus, the aim of this study is to quantify flow by combining a stable contrast agent and a modified echo equipment, able to sample the radio frequency (RF) signal from a region of interest (ROI) in the echo image. The contrast agent SHU-454 (0.8 ml) was bolus injected into an in vitro calf vein, at 23 flow rates (ranging from 376 to 3620 ml/min) but constant volume and pressure. The ROI was placed in the centre of the vein, the RF signal was processed in real time and transferred to a personal computer to generate time-intensity curves. In the absence of recirculation, contrast washout slope and mean transit time (MTT) of curves (1.11-8.52 seconds) yielded excellent correlations with flow: r = 0.93 and 0.95, respectively. To compare the accuracy of RF analysis with that of conventional image processing as to flow quantitation, conventional images were collected in the same flow model by two different scanners: a) the mechanical sector scanner used for RF analysis, and b) a conventional electronic sector scanner. These images were digitized off-line, mean videodensity inside an identical ROI was measured and time-intensity curves were built. MTT by RF was shorter than by videodensitometric analysis of the images generated by the same scanner (p < 0.001). In contrast, MTT by RF was longer than by the conventional scanner (p < 0.001). Significant differences in MTT were also found with changes in the gain setting controls of the conventional scanner. To study the stability of the contrast effect, 6 contrast injections (20 ml) were performed at a constant flow rate during recirculation: the spontaneous decay in RF signal intensity (t1/2 = 64 +/- 8 seconds) was too long to affect MTT significantly. In conclusion, the combination of a stable contrast agent and a modified echocardiographic instrument provides accurate quantitation of flow in an in vitro model; RF analysis is more accurate than conventional processing as to flow quantitation by contrast echocardiography.