Renal vascular Doppler imaging: clinical benefits of power mode

RADIOLOGICAL IMAGING IN RENAL TRANSPLANTATION

SUMMARY – Radiological diagnostic methods have a significant role in the preoperative and postoperative care of patients after kidney transplantation. Improvement and innovations in technology, but also the growing experience of the radiologists who deal with kidney transplant patients as part of the transplant team lead to earlier detection of complications in the postoperative period, which are the leading cause of transplant failure. In this article, we describe, through diagnostic imaging examples, detailed evaluation of all possible complications that can occur after kidney transplantation, with evaluation of different possible diagnostic methods that can be used in the preoperative assessment and postoperative follow up and care of the transplanted patient. The goal of this article is to demonstrate and summarize in detail the possible complications of renal transplantation and how to best diagnostically approach them, with special reference to ultrasound which is the main imaging method for this group of conditions.

Abdominal thromboses of splanchnic, renal and ovarian veins

Thromboses of abdominal veins outside the iliac-caval axis are rare but clinically relevant. Early deaths after splanchnic vein thrombosis occur in 5-30% of cases. Sequelae can be liver failure or bowel infarction after splanchnic vein thrombosis, renal insufficiency after renal vein thrombosis, ovarian infarction after ovarian vein thrombosis. Local cancer or infections are rare in Budd-Chiari syndrome, and common for other sites. Inherited thrombophilia is detected in 30-50% of patients. Myeloproliferative neoplasms are the main cause of splanchnic vein thrombosis: 20-50% of patients have an overt myeloproliferative neoplasm and/or carry the molecular marker JAK2 V617F. Renal vein thrombosis is closely related to nephrotic syndrome; finally, ovarian vein thrombosis can complicate puerperium. Heparin is used for acute treatment, sometimes in conjunction with systemic or local thrombolysis. Vitamin K-antagonists are recommended for 3-6 months, and long-term in patients with Budd-Chiari syndrome, unprovoked splanchnic vein thrombosis, or renal vein thrombosis with a permanent prothrombotic state such as nephrotic syndrome.

Imaging of renovascular disease

A variety of different imaging techniques have been used for the diagnosis of renal vascular diseases. The wide range of renal vascular diseases include congenital renal artery and vein variations, aneurysms, arteriovenous malformations (AVMs), renal artery stenosis, renal vein thrombosis, vasculitis, and traumatic injuries, such as dissection and vascular pedicle injury. In this article, we discuss the role of invasive and noninvasive imaging in each of these abnormalities and their typical features. Because renal artery stenosis is an important vascular abnormality encountered in clinical practice, imaging of this entity will be emphasized.

[Atherosclerotic renal artery disease diagnosis update]

Atherosclerotic renal artery disease represents a cause of which little is known but not a cause to be neglected for hypertension and renal insufficiency. Even though its occurrence remains badly defined, atherosclerotic renal artery disease is constantly on the rise due to the aging population, the never prevailing hypertension and diabetes mellitus. This review aims to give a clinical profile of patients presenting with atherosclerotic renal artery disease and to discuss, in the light of study results, which diagnostic evaluation should be used considering the sequence and the benefit and risk of each in order to initiate a personalized treatment. Patients affected by atherosclerotic renal artery disease are likely to have more complications and more extensive target-organ damage than patients without renal artery stenosis. The evolution of the atherosclerotic renal artery disease is in general slow and progressive. Nevertheless, certain clinical cases manifest themselves with the onset of acute renal failure bought upon by the administration of blockers of the rennin-angiotensin-aldosterone system, or by some other causes responsible for a sudden drop in renal plasma flow (e.g., thrombosis of the renal artery). The relationship between atherosclerotic renal artery disease and atherosclerosis is complex, and mediators implicated in the pathophysiology of renovascular disease may also contribute to the progression of cardiovascular damage. An early assumption of the atherosclerotic renal artery stenosis is warranted to determine the adapted treatment (i.e., medical treatment, revascularisation...) just as the assumption and the correction of the more general cardiovascular risk factors.

Renal vascular perfusion index in a canine model

Decreased renal perfusion plays an important role in the progression toward renal failure. In this study, a novel measure was proposed to quantify renal perfusion using canine model. Serial renal vascular images at different vascular areas including the whole vascular tree, interlobar, arcuate and interlobular vessels were captured. Image processing software was designed to analyze the changes of power Doppler intensity of colored pixels within regions-of-interest (ROI). For a given ROI, the power Doppler vascular index (PDVI) was found to fluctuate with the cardiac cycle. It was also noted that the power Doppler signals generated by arterial vessels have different fluctuating waveforms and different phase compared with the signal derived from venous vessels. A power Doppler correlation-map was developed to differentiate the arteries and veins in the ROI. Using the serial power Doppler images and the derived flow direction information, the interlobular perfusion can be strongly quantified. The renal vascular perfusion index (RVPI) defined as the ratio of PDVI(max) versus PDVI(min) was significantly higher in the interlobular vessel areas than three other areas for seven healthy dogs. The RVPI resembles the systolic/diastolic (S/D) ratio that commonly reflects arterial hemodynamics. RVPI and power Doppler correlation-map reveal more "dynamic" sense of vascular perfusion and provide a novel approach for the examination of renal function in clinical practice.

Power Doppler imaging in acute renal vein occlusion and recanalization: a canine model

OBJECTIVE

To evaluate the dynamic changes of the power Doppler (PD) in acute renal vein occlusion and recanalization in a canine model.

MATERIALS AND METHODS

We performed a PD of the kidney during graded renal vein occlusion and recanalization induced by balloon inflation and deflation in nine dogs. The PD images were transferred to a personal computer, and the PD signals were quantified.

RESULTS

We observed the temporal change of the PD signal during renal vein occlusion and recanalization, with a decrease in the PD signal during occlusion and an increase during recanalization. The mean PD signal decreased gradually as the renal vein was occluded, and conversely increased gradually with sequential relief of occlusion. The sequential change of the mean value of the PD signal was statistically significant.

CONCLUSION

The PD can detect a change in renal blood flow during acute renal vein occlusion and recanalization in a canine model. The PD may be used as a helpful tool for the early detection of acute renal vein thrombosis and the monitoring of renal perfusion.

[Doppler US of the urinary system and renal vessels in normal and pathologic conditions]

The interest of Doppler ultrasound (US) in urinary tract investigation has dramatically increased during the past 10 years. It can provide useful diagnostic informations in pedicular and intrarenal vascular disorders, some medical nephropathies, in the assessment of renal tumors and urinary tract disorders including urolithiasis, acute obstruction and ureteral reflux. Recent technical improvements including the development of US contrast agents (microbubbles) have contributed to increase the diagnostic accuracy of the technique.

Signal losses with real-time three-dimensional power Doppler imaging

Power Doppler imaging (PDI) has been shown to be influenced by the wall filter when assessing arterial stenoses. Real-time 3-D Doppler imaging may likely become a widespread practice in the near future, but how the wall filter could affect PDI during the cardiac cycle has not been investigated. The objective of the study was to demonstrate that the wall filter may produce unexpected major signal losses in real-time 3-D PDI. To test our hypothesis, we first validated binary images obtained from analytical simulations with in vitro PDI acquisitions performed in a tube under pulsatile flow conditions. We then simulated PDI images in the presence of a severe stenosis, considering physiological conditions by finite element modeling. Power Doppler imaging simulations revealed important signal losses within the lumen area at different instants of the flow cycle, and there was a very good concordance between measured and predicted PDI binary images in the tube. Our results show that the wall filter may induce severe PDI signal losses that could negatively influence the assessment of vascular stenosis. Clinicians should therefore be aware of this cause of signal loss to properly interpret power Doppler angiographic images.

Use of power Doppler ultrasound to monitor renal perfusion during burn shock

BACKGROUND

Renal cortical blood flow can be quantified by means of power Doppler ultrasound (PDUS) image analysis. We hypothesized that renal cortical perfusion, estimated by PDUS image intensity (PDUSII), would decrease during burn shock and improve during resuscitation in a porcine model.

METHODS

Eight anesthetized swine sustained a 75% scald injury. Resuscitation began 6h postburn. Renal cortical blood flow was measured directly using fluorescent microspheres (CORFLO), and was estimated noninvasively by PDUSII. PDUSII, CORFLO, and cardiopulmonary data were recorded every 2h.

RESULTS

PDUSII decreased significantly from preburn to postburn hour 6, and increased with resuscitation by hour 8. CORFLO correlated well with PDUS image intensity (n=48, r(2)=0.696) but poorly with urine output (n=48, r(2)=0.252).

CONCLUSION

PDUS in this study was superior to the urine output in assessing renal cortical microvascular blood flow during shock and resuscitation, and may be useful in the care of injured patients.

Renal allograft compartment syndrome: an underappreciated postoperative complication

PURPOSE

Renal allograft compartment syndrome (RACS) is early graft dysfunction secondary to retroperitoneal hypertension and resultant ischemia. Our purpose was to identify the incidence, therapies and outcomes of patients with RACS.

METHODS

All patients who underwent a renal transplant between 2000 and 2005 were reviewed. Patients with signs of acute allograft dysfunction were identified. RACS was diagnosed via visual allograft hypoperfusion and/or with preoperative Doppler ultrasound.

RESULTS

Among 458 patients, 11 (2%) were diagnosed with RACS. Characteristics between patient groups were similar. Five (45%) patients displayed adequate initial allograft function after transplantation. Doppler ultrasound was diagnostic. Six (55%) patients displayed poor initial allograft function and were classified as early presenters of RACS. Allograft function improved dramatically upon decompression.

CONCLUSIONS

Clinicians must remain aware of RACS as a potential diagnosis when patients display rapid deterioration in kidney performance after good initial allograft function. Doppler ultrasound is useful in diagnosing late presenters.

[Diagnostic imaging of peripheral renal vascular disorders]

Peripheral vascular disorders of the kidney involve the intrarenal branches of the renal vascular tree. It include occlusive (infarction and cortical necrosis) and non-occlusive vascular lesions (acquired arteriovenous fistulas, arteriovenous malformation, false aneurysms and microaneurysms). Initial diagnosis relies on color Doppler US and CT angiography. Angiography plays a therapeutic role. MR imaging provides useful diagnostic information on perfusion disorders especially in patients with renal insufficiency.

Evaluation of Renal Cortical Perfusion by Noninvasive Power Doppler Ultrasound During Vascular Occlusion and Reperfusion

BACKGROUND

Urine output, a frequently used resuscitation end point, is presumed to represent renal cortical perfusion. However, no noninvasive method for direct measurement of renal perfusion exists. Power Doppler ultrasound (PDUS) is a method that reportedly is sensitive to low-velocity and microvascular blood flow and can depict it. This study aimed to develop a quantitative technique for PDUS image analysis, and to evaluate the ability of PDUS to quantify cortical perfusion during renal ischemia induced by vascular occlusion.

METHODS

A method was developed to determine the mean gray-scale intensity of PDUS images from within the renal cortex (PDUS image intensity). This index was hypothesized to represent renal cortical microvascular blood flow. Renal cortical blood flow was determined using fluorescent microspheres in five swine. Renal artery flow was measured with an ultrasonic flow probe. Power Doppler ultrasound was performed at baseline; at 75%, 50%, and 25% of baseline renal artery flow; and during reperfusion.

RESULTS

Subjectively, PDUS images showed decreases in image intensity corresponding to renal artery occlusion and increases after reperfusion. Cortical blood flow correlated well with renal artery flow (n = 25; r2 = 0.868) and with PDUS image intensity (n = 25; r2 = 0.844).

CONCLUSION

Noninvasive power Doppler ultrasound image intensity correlated well with invasively measured renal cortical blood flow, and may be useful during resuscitation of injured and critically ill patients.

A comparison of Power Doppler with conventional sonographic imaging for the evaluation of renal artery stenosis

Background

Power Doppler (PD) has improved diagnostic capabilities of vascular sonography, mainly because it is independent from the angle of insonation. We evaluated this technique in a prospective comparison with conventional imaging, consisting in Duplex and Color Doppler, for the evaluation of Renal Artery (RA) stenosis.

Methods

Sensitivity, specificity and predictive values of PD and conventional imaging were assessed in a blinded fashion on eighteen patients, 9 with angiographic evidence of unilateral RA stenosis (hypertensive patients) and 9 with angiographically normal arteries (control group). PD images were interpreted with an angiography-like criteria.

Results

In the control group both techniques allowed correct visualization of 16 out of the 18 normal arteries (93% specificity). Only in five hypertensive patients RA stenosis was correctly identified with conventional technique (56% sensitivity and 86% negative predictive value); PD was successful in all hypertensive patients (100% sensitivity and negative predictive value), since the operators could obtain in each case of RA stenosis a sharp color signal of the whole vessel with a clear "minus" at the point of narrowing of the lumen. All results were statistically significant (p < 0.01).

Conclusions

This study demonstrates that PD is superior to conventional imaging, in terms of sensitivity and specificity, for the diagnosis of RA stenosis, because it allows a clear visualization of the whole stenotic vascular lumen. Especially if it is used in concert with the other sonographic techniques, PD can enable a more accurate imaging of renovascular disease with results that seem comparable to selective angiography.

Complementary roles of color-flow duplex imaging and intravascular ultrasound in the diagnosis of renal artery fibromuscular dysplasia: should renal arteriography serve as the "gold standard"?

OBJECTIVES

The purpose of this study was to compare color-flow duplex imaging (CFDI), intravascular ultrasound (IVUS), and renal arteriography in diagnosing renal artery (RA) fibromuscular dysplasia (FMD) and correlating with the hemodynamic response to balloon angioplasty (BA) in patients with drug-resistant hypertension.

BACKGROUND

Renal arteriography is generally regarded as the gold standard for diagnosing RA FMD. The observation that CFDI and IVUS depicted endoluminal abnormalities suggestive of RA FMD in some patients with normal renal arteriograms prompted comparison of these modalities in a consecutive series of patients.

METHODS

Twenty hypertensive patients with CFDI suggestive of RA FMD (mid-to-distal flow derangement and velocity augmentation) underwent renal arteriography, IVUS, and BA, with both immediate and long-term blood pressure (BP) response assessment.

RESULTS

All patients were women, aged 31 to 86 years (mean 62 years). On IVUS, various endoluminal defects (eccentric ridges; fluttering membranes; spiraling folds) were depicted at locations predicted by CFDI and were uniformly identified at sites where arteriography depicted classic evidence of FMD (8 patients). However, similar defects were detected by IVUS when angiography was borderline (7 patients) or normal (5 patients). Balloon angioplasty eliminated (16 patients) or reduced (4 patients) the IVUS findings and lowered systolic BP in all (mean reduction 53 mm Hg, p < 0.0001). This reduction was maintained during follow-up of 4 to 22 (mean 13) months (mean reduction 44 mm Hg, p < 0.0001), independent of baseline angiographic appearance.

CONCLUSIONS

Both CFDI and IVUS depict the blood flow and endoluminal abnormalities of RA FMD. Balloon angioplasty eliminates or improves IVUS findings and produces substantial, sustained BP reduction, an effect that is independent of baseline arteriographic appearance, calling into question the legitimacy of arteriography as the diagnostic gold standard.

[Vasculo-renal disease]

The kidney as an apo-exocrine organ has two important functions: the control of homeostasis and arterial blood pressure. Any pathological disorders witch alters the renal blood flow results in two consequences: renovascular hypertension and renal insufficiency. Renal revascularization looks for with priority the improvement of renal function; the good response and control of hypertension shall come after. The goals of this paper is to actualize the sophisticated diagnostic methods (Angiography, Eco-Doppler, AngioCT, AngioMR, Angioscopy; and the use of contrast potential-mediums), functionality tests (RN and MR with Captopril) and management (Percutaneous transluminal angioplasty, simple or covered Stents, Embolizations, Occlusive balloons, Fibrinolysis and Trombus Aspiration; direct Arterial Surgery and renal Autotransplantation) of different renovascular diseases. (Blunt renal injuries, Stenosis, Aneurysms. A-V fistuls, Acute Occlusions or arterial and venous, Thrombosis. Nutcraker Syndrome). The possibility of the presence of an inverse venous Fraley's Syndrome is presented and discussed.

CONCLUSIONS

A controversy of the last technical methods are established and discussed and a Guideline is recommended for each renal vascular disease.

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.

[Power Doppler sonography and acute pyelonephritis in children: comparison with Tc-DMSA scintigraphy]

Acute pyelonephritis is a common infection in children. The clinical and biological diagnosis is still sometimes difficult. For most authors, Technecium 99m dimercaptosuccinic acid scintigraphy is considered as the gold standard tool for diagnosis but it is invasive and expensive. The aim of our study was to compare the sensitivity and the specificity of B-mode sonography and power doppler to DMSA-Tc scintigraphy in acute pyelonephritis.

PATIENTS AND METHODS

Forty-nine children were enrolled in this study with suspicion of pyelonephritis. All infants underwent doppler sonography and scintigraphy within 48 hours after their hospitalization. Doppler sonography criteria were increased kidney size, thickness of sinus wall, vascular defect, and various echogenicity of the kidneys (focal or diffuse hyperechogenicity or focal hypoechogenicity).

RESULTS

Among 28 children with a positive scintigraphy, 15 had a positive doppler sonography (sensitivity 54%) and 13 had a negative doppler sonography. Among 21 children with a negative scintigraphy, 20 had a negative doppler sonography (specificity 95%) and one had a positive doppler sonography.

CONCLUSION

In clinically suspected acute pyelonephritis, doppler sonography has a high specificity. A positive doppler sonography should avoid the use of scintigraphy.

Renal infarct: contrast-enhanced power Doppler sonographic findings

Power Doppler sonography (PDUS) is a promising technique for the diagnosis of renal infarcts. PDUS's efficacy may be enhanced by using sonographic contrast agents. We evaluated 3 cases of renal infarction using PDUS and the sonographic contrast agent Levovist. The findings were compared with those of other imaging modalities, such as scintigraphy, CT, and angiography. In case 1, PDUS showed a patent interlobar artery only in the lower part of the right kidney and no other perfusion of the right renal parenchyma. Contrast-enhanced PDUS showed patchy areas of preserved perfusion in the lower and middle-upper anterior portions of the kidney. In case 2, PDUS showed diffuse and patchy perfusion defects in the anterolateral portion of the right kidney. On contrast-enhanced PDUS, no signal enhancement was seen in these areas, but the perfusion defects were better delineated. In case 3, PDUS showed no perfusion in the upper pole of the kidney; the nonperfused area extended to the posterior upper portion of the kidney but could not be distinguished from normal tissue. After injection of the contrast agent, there was no enhancement of the posterior extension of the upper pole infarct, but Doppler signals from the surrounding normal parenchyma were enhanced, so the area was more clearly demarcated. The administration of the contrast agent facilitated the visualization of the infarcts in all 3 cases.

Renal allograft vasculopathy: ultrasound findings in a non-human primate model of chronic rejection

The purpose was to determine whether decreased cortical flow detected with power Doppler (PD) ultrasound in renal allografts in cynomolgus monkeys marks the presence or onset of chronic renal allograft vasculopathy. The 2D grey scale and PD ultrasound findings of 24 consecutively implanted non-life-supporting renal allografts in cynomolgus monkeys that underwent either 24 h (n=15) or 48 h (n=9) cold ischaemia times were recorded and compared with the results of histology performed every 2 weeks post-operatively. 13 allografts developed vasculopathies, 10 of which had PD scores equal to 1 (severe reduction of cortical flow). A PD score of 1 occurred in only one instance in the group of allografts without vasculopathies and this was due to necrosis. Allografts without vasculopathies otherwise had either PD scores of 3 (normal flow; n=2) or 2 (reduced flow; n=4). Allografts subjected to 48 h cold ischaemia times were smaller than those with 24 h cold ischaemia times (significant at weeks 5-11, p<0.05), but a reduction in graft size associated with vasculopathies occurred infrequently. In conclusion, the finding of reduced renal cortical flow detected by PD ultrasound during serial examination of non-life-supporting renal allografts is highly supportive of a diagnosis of graft vasculopathy due to arteriolar intimal proliferation, and illustrates an excellent method of monitoring changes in cortical perfusion in allografts in animal models. The combination of findings of reduced or absent cortical flow together with severe graft enlargement is highly suggestive of the presence of not only vasculopathies but also tissue damage and degeneration.

Power Doppler US: evaluation of the morphology of stenoses with a flow phantom

PURPOSE

To determine the importance of technical settings at power Doppler ultrasonography (US) for the evaluation of stenoses.

MATERIALS AND METHODS

A computer-controlled phantom was used to induce a reproducible flow across a calibrated 71% stenosis in an 8.4-mm-diameter tube. Two transducers, 2-4 and 5-10 MHz, working at depths of 3.0 and 11.5 cm, respectively, with different beam angles (40 degrees, 60 degrees, and 90 degrees ), were used to simulate evaluation of pulsatile flow across normal and stenotic vessels in various conditions. For each condition, gain, pulse repetition frequency, and wall filter were progressively turned from low to high values. Two observers measured in a blinded fashion the apparent lumen of the stenotic and normal vessels on longitudinal and transverse images with the use of power Doppler US.

RESULTS

When the high-frequency transducer was used, gain significantly affected both stenotic and feeding vessel measurement, whereas pulse repetition frequency and filter only affected feeding vessel evaluation. When the low-frequency transducer was used, all factors, including flow velocity and beam angle, played a significant role (P <.001). In most conditions, overestimation of the lumen and underestimation of the lumen of the feeding vessel led to severe underestimation of the degree of stenosis.

CONCLUSION

Power Doppler US cannot be used to measure stenoses accurately. Underestimation of the degree of the stenosis was significantly higher with the low-frequency probe than with the high-frequency probe.

Evaluation of ureterocele with Doppler sonography

Ureterocele is a common disorder characterized by cystic dilatation of the terminal submucosal portion of 1 or both ureters. Conventional imaging modalities for the diagnosis of ureterocele include intravenous urography and voiding cystourethrography; gray-scale sonography, augmented by color Doppler and spectral analysis studies, can be useful in diagnosing and managing this condition as well. We describe the diagnosis of ureterocele using transvaginal gray-scale and color Doppler sonography with spectral analysis in 2 women. Color Doppler sonography demonstrated urine flow into the bladder, and spectral analysis was used to measure the flow velocity. Together, these modalities can provide additional information about urinary flow dynamics, aid in implementing treatment for patients with ureterocele, and eliminate the need for invasive diagnostic procedures.