Value of image subtraction in 3D gadolinium-enhanced MR angiography of the renal arteries

The objective of this study was to evaluate quantitatively and qualitatively the effect of image subtraction on the image quality of three-dimensional (3D) gadolinium-enhanced MR angiograms of the renal arteries. Breath-hold 3D gadolinium MR angiography (MRA) as well as conventional contrast angiography of the renal arteries was performed on 20 patients with suspected renovascular hypertension. MR angiograms were acquired before and during dynamic infusion of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). Contrast-enhanced images were compared with images that had undergone voxel-by-voxel signal intensity subtraction of contrast-enhanced data from precontrast data. One false positive finding for significant renal artery stenosis was recorded with MRA using conventional angiography as the gold standard. Image subtraction did not alter the diagnosis at MRA in any case. The mean contrast-to-noise ratio (CNR) was significantly higher (P < .05) on the subtraction MR angiograms compared to the nonsubtracted MR angiograms. There was no significant difference in the signal-to-noise ratio (SNR). Qualitative analysis revealed a significant improvement in image quality after image subtraction with respect to visualization of the distal renal arteries. In conclusion, image subtraction improves the quality of renal MRA in terms of both CNR and visualization of the distal renal arteries.

Renal artery stenosis and accessory renal arteries: accuracy of detection and visualization with gadolinium-enhanced breath-hold MR angiography

PURPOSE

To determine the accuracy of gadolinium-enhanced breath-hold magnetic resonance (MR) angiography in the diagnosis of renal artery stenosis and visualization of accessory renal arteries.

MATERIALS AND METHODS

Forty-four patients suspected of having renal artery stenosis and 10 potential kidney donors, all of whom were scheduled to undergo elective intraarterial digital subtraction angiography (DSA), were studied. Three-dimensional gradient-echo gadolinium-enhanced MR angiography was performed at 1.5 T with the following parameters: repetition time, 13.5 msec; echo time, 3.5 msec; flip angle, 60 degrees; 195 x 512 matrix; 400-mm field of view; and 6-cm imaging volume consisting of 15 4-mm-thick partitions reconstructed every 2 mm. Gadopentetate dimeglumine (30 mL) was injected with a power injector. MR angiograms were assessed before the standard of reference, intraarterial DSA, was performed.

RESULTS

Four MR angiograms were not evaluable because of poor image quality. MR angiography enabled visualization of all but one of the 121 arteries. In four small accessory arteries, a stenosis could not be excluded owing to inadequate spatial resolution. MR angiography enabled the correct diagnosis in 30 of the 31 arteries with a grade 2 (50%-99%) stenosis and in seven of the 10 occluded arteries. Sensitivity and specificity for correct identification of a grade 2 stenosis were 97% and 92%, respectively.

CONCLUSION

Gadolinium-enhanced MR angiography is an accurate, minimally invasive method for detecting renal artery stenosis and is reliable for visualizing accessory renal arteries.

[Renal artery stenosis in pediatric age. Clinical and instrumental observations of two patients]

Two male patients, four and six years old, affected with monolateral renal artery stenosis are reported. One of them presented recurrent facial nerve palsy, while the other was affected by renal calculosis. The diagnostic suspicion was confirmed by arteriography. They were submitted respectively to aortorenal bypass and nephrectomy, with blood pressure normalization.

Hypertension and renal failure

Images

The cost-effectiveness of the diagnosis of renal artery stenosis

The objective of this study was to assess the cost effectiveness of eight strategies to diagnose renovascular hypertension (RVHT) followed by treatment with percutaneous transluminal angioplasty (PTRA) with or without stent placement. The eight diagnostic strategies were compared with a reference strategy, i.e. antihypertensive medication. The diagnostic imaging techniques under consideration were captopril renography, spiral computed tomography angiography (CTA), magnetic resonance angiography (MRA) and conventional angiography. Cost-effectiveness analysis was carried out from the perspective of the health care system, based on data from the literature. A model was developed to predict the reduction in 10-year morbidity and 10-year mortality owing to myocardial infarction, stroke and chronic renal failure achieved after PTRA compared with the reference strategy. Life-years gained over a 10-year follow-up period and the incremental cost-effectiveness ratio per life-year saved were the outcome measures. The strategy CTA followed by angiography was more effective, but more costly, than captopril renography followed by angiography, with an incremental cost-effectiveness ratio per life-year gained of Dfl 64700. Combining captopril renography with CTA was even more effective, but the incremental cost-effectiveness ratio per life-year gained was Dfl 236400. Strategies including MRA were not cost-effective. The results suggest that diagnostic strategies that include CTA are more effective than captopril renography in detecting renal artery stenosis (> 50%) and cost saving due to prevented myocardial infarction, stroke or chronic renal failure. MRA is even more effective, but in order to achieve an acceptable cost-effectiveness ratio, the costs would need to be reduced. The cost-effectiveness of the diagnostic strategies is sensitive to the pre-test probability of RVHT. So, careful clinical evaluation, in order to achieve a pre-test probability of at least 20%, is an essential component of the complete workup strategy in patients suspected to have RVHT.

Variations in blood flow waveforms in stenotic renal arteries by 2D phase-contrast cine MRI

Waveform variations in blood flow measurements through stenotic renal arteries have been reported already with echo Doppler studies. We studied these variations with MRI in 14 patients (mean age, 60 years) with suspected renal arterial stenosis (24 patent arteries, four occluded). Flow measurements were successful in 15 arteries and unsuccessful in nine, due to practical limitations. Seven healthy younger volunteers (mean age, 28 years) and five healthy older volunteers (mean age, 58 years) were recruited for comparison purposes. In patients, the severity of stenoses was also assessed by digital subtraction angiography and intraarterially measured transstenotic pressure drops. We found flow patterns to be statistically significantly (P < 0.01) age-related. Younger healthy subjects showed shorter wave duration, higher diastolic flow, and total blood flow per minute. Also, with increasing stenosis severity, the systolic wave became more damped and the systolic wave duration became statistically significantly (P = .03) longer.

University of Miami Division of Clinical Pharmacology Therapeutic Rounds: ischemic renal disease

Ischemic renal disease (IRD) is defined as a significant reduction in glomerular filtration rate and/or loss of renal parenchyma caused by hemodynamically significant renal artery stenosis. IRD is a common and often overlooked clinical entity that presents in the setting of extrarenal arteriosclerotic vascular disease in older individuals with azotemia. IRD is an important cause of chronic renal failure and end-stage renal disease (ESRD), and many patients with a presumed diagnosis of hypertensive nephrosclerosis may actually have undiagnosed ischemic nephropathy as the cause of their ESRD. The primary reason for establishing the diagnosis of IRD is the hope that correction of a renal artery stenosis will lead to improvement of renal function or a delay in progression to ESRD. There are six typical clinical settings in which the clinician could suspect IRD: acute renal failure caused by the treatment of hypertension, especially with angiotensin-converting enzyme inhibitors; progressive azotemia in a patient with known renovascular hypertension; acute pulmonary edema superimposed on poorly controlled hypertension and renal failure; progressive azotemia in an elderly patient with refractory or severe hypertension; progressive azotemia in an elderly patient with evidence of atherosclerotic disease; and unexplained progressive azotemia in an elderly patient. It is important for the clinician to identify IRD, because IRD represents a potentially reversible cause of chronic renal failure in a hypertensive patient.

Renal MR angiography: a comprehensive approach

Renal artery MR angiography has now emerged as a safe, accurate approach to renal arteriography. A comprehensive examination, including both three-dimensional (3D) dynamic gadolinium-enhanced and 3D phase contrast MRA techniques, allows evaluation of both the aorta-renal and splanchnic arterial anatomy as well as the hemodynamic significance of any stenoses identified. The 3D gadolinium-enhanced MRA technique produces a contrast arteriogram but without risks of iodinated contrast or ionizing radiation. The 3D phase contrast technique is a flow-based technique, which may show dephasing in the presence of hemodynamically significant stenoses. A comprehensive examination should also include T1- and T2-weighted imaging for the assessment of potential neoplastic masses and the ubiquitous renal cysts. Through trial and error over the course of over a thousand examinations, this comprehensive approach to the MR evaluation of renal vascular pathology has emerged.

Renal MR angiography

Three-dimensional gadolinium MR angiography (3D-Gd-MRA) accurately visualizes the renal arteries with almost no degradation from inplane saturation or motion artifacts. The diagnostic accuracy for detecting and grading of renal artery stenosis, as well as the assessment of other vascular pathology, approaches that of conventional x-ray angiography. For the renovascular system, this technique requires precise contrast media bolus timing since multiple successively enhancing structures are present. Details on performing renal MR angiography, strategies for image analysis, and examples of common renal vascular pathology are reviewed. The 3D-Gd-MRA protocol can be easily combined with other MR imaging techniques to provide a comprehensive assessment of the hemodynamic and functional significance of renal artery stenosis.

Renovascular disease in older adults

Renovascular disease (RVD) as a cause of end-stage renal disease (ESRD) has been under-recognized until recently. As the population ages, the number of individuals with atherosclerotic RVD will increase, as will the number of patients with renovascular hypertension (RVH) and ESRD. The anatomic presence of atherosclerotic RVD as a threat to renal function has become a more pressing concern than that of hypertension secondary to RVD. This article reviews the causes, assessment, and treatment of RVD in older adults.

Objective stenosis quantification from post-stenotic signal loss in phase-contrast magnetic resonance angiographic datasets of flow phantoms and renal arteries

In this study a semi-automated and observer-independent algorithm for quantifying post-stenotic signal loss (PSL) in three-dimensional phase-contrast (PC) magnetic resonance angiography (MRA) of patients with renal artery stenosis is presented. This algorithm was developed on MRA datasets of stenotic phantoms, included in a flow circuit with stationary flows. The length and the severity of the PSL (incorporating both the length and the degree of PSL) in the MRA datasets were proposed for quantifying the stenoses. The algorithm was tested in renal arteries; ten patients with renal artery stenosis and seven healthy volunteers were investigated. Digital subtraction angiography was performed in the patients and served as the gold standard. Stenosis severity showed better correlation with the severity of the PSL than with the length, both for in vitro and in vivo measurements. Spearman correlation coefficients (rs) showed statistically significant correlations between the severity of the PSL and parameters determined by digital subtraction angiography, i.e., percent diameter stenosis (rs = 0.90). The length of the PSL showed no correlation with the diameter stenosis (rs = 0.37). In conclusion, this study presents a semi-automated and observer-independent way of quantifying signal loss, and the severity of the PSL is proposed for quantifying stenoses, rather than the length of PSL.

Traumatic renal artery occlusion: a review of the literature

To better define what constitutes appropriate treatment for traumatic renal artery occlusion, we analyzed our experience along with 147 other case reports from the literature. We recently reported our 15-year experience with 12 patients (13 injuries) who presented to our trauma centers with renal artery occlusion secondary to blunt trauma. This experience prompted a review of the literature. From this review, we identified an additional 19 cases of bilateral and 128 cases of unilateral renal artery occlusion that met our inclusion criteria. Of the 20 patients with bilateral renal artery occlusion, surgical revascularization was attempted in 16 and successful in 9 (56%). Of the 139 patients with unilateral renal artery occlusion, surgical revascularization was attempted in 34 and successful in 9 (26%). Evidence of decreased renal function was noted in 67% of those who had a successful revascularization for unilateral injury at a mean 1.8-year follow-up, whereas 12% experienced hypertension at a mean 3.1-year follow-up. Hypertension developed in 34 (32%) of the 105 patients who did not have revascularization attempted and was present by a mean 97 days postinjury. Surgical revascularization for unilateral renal artery occlusion seldom results in a successful outcome. Revascularization is indicated in patients with bilateral renal artery occlusion and in those with injury to a solitary kidney. Patients who are observed must be followed closely for development of hypertension.

Standard dose Gd-DTPA dynamic MR of renal arteries

Renal MR contrast enhancement depends on the timing of image acquisition. Limited human trials have demonstrated efficacy of renal artery stents on salvage of renal function. This study assessed the ability of dynamic gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) administration to demonstrate renal artery stenosis and renal stent patency compared to conventional angiography as the gold standard. Twenty subjects referred for renal angiography underwent 22 dynamic MR studies, including 7 with renal artery stenting (Palmaz P204 or P201, Johnson & Johnson, Sydney, Australia). All were examined with conventional angiography and after dynamic Gd-DTPA infusion. Coronal MR images of the kidneys were acquired using a GE Signa 1.5-T magnet (General Electric Medical Systems, Milwaukee, WI) (fast spoiled gradient echo [FSPGR]; TE=4.2 msec, TR=68-150 msec, flip angle=75 degrees) 0 to 600 seconds after iv bolus injection of 15 ml of Gd-DTPA during sequential breath-hold acquisitions, 13 to 32 seconds each. All 51 renal arteries (13 stenosed, 38 normal) were detected with dynamic MRI. Severity of renal artery stenosis was classified correctly with an accuracy of 98% (95% confidence interval [CI]: 85-100), yielding 98% specificity and 100% sensitivity. All nine renal stents were visualized with 100% accurate patency documentation. FSPGR MRI with bolus Gd-DTPA administration can provide adequate time and spatial resolution to demonstrate renal artery stenosis.

Can the IVIM model be used for renal perfusion imaging?

OBJECTIVE

Renal perfusion imaging may provide information about the hemodynamic significance of a renal artery stenosis and could improve noninvasive characterization when combined with angiography. It was proposed previously that diffusion sequences could provide useful perfusion indices based on the intravoxel incoherent motion (IVIM) model. Owing to motion artifacts, diffusion imaging has been restricted to relatively immobile organs like the brain. With the availability of single-shot echo-planar imaging (EPI) our purpose was to evaluate the IVIM model in renal perfusion.

METHODS AND MATERIAL

Eight volunteers underwent diffusion-sensitive magnetic resonance (MR) imaging of the kidneys using a spin echo (SE) EPI sequence. The diffusion coefficients determined by a linear regression analysis and fits to the IVIM function were calculated.

RESULTS AND CONCLUSION

Our preliminary experience does not support the possibility of obtaining perfusion information using the IVIM model in the kidneys.

Renal artery stenosis in kidney transplants

Transplant renal artery stenosis (TRAS) is an increasingly recognized complication of renal transplantation, with a reported incidence of between 1% and 23%. The clinical features include refractory hypertension, new-onset hypertension, allograft dysfunction, and the presence of a bruit over the graft. In this report, we describe the investigation and treatment of one such patient and review the current diagnostic approaches and therapy in this setting.

Atherosclerotic stenosis of the renal arteries. Indications for intervention

Advances in the ability to diagnose atherosclerotic stenosis of the renal arteries have increased awareness of this disorder over the past decade, and several medical specialties have become interested in its pathophysiology, diagnosis, and therapy. Improved surgical and anesthetic techniques and the development of endovascular technology have paved the way for more aggressive management of atherosclerotic renal artery stenosis. However, there is a lack of prospective data supporting this aggressive approach, and there is little agreement among physician groups concerning the appropriateness of revascularization in many patient scenarios. We will examine the available data and make recommendations about which patient groups might benefit most from renal artery revascularization (surgical or endovascular).

Renal artery stenosis: evaluation with breath-hold, three-dimensional, dynamic, gadolinium-enhanced versus three-dimensional, phase-contrast MR angiography

PURPOSE

To compare breath-hold, three-dimensional, gadolinium-enhanced magnetic resonance (MR) angiography with three-dimensional, phase-contrast MR angiography in the evaluation of renal artery stenosis.

MATERIALS AND METHODS

Fifty-five consecutive adult patients with clinical suspicion of renovascular disease were prospectively examined with three-dimensional, phase-contrast MR angiography and breath-hold, three-dimensional MR angiography with injection of a standard dose of gadopentetate dimeglumine to evaluate the number of renal arteries and the presence and degree of stenosis. The standard of reference was intraarterial digital subtraction angiography.

RESULTS

Gadolinium-enhanced MR angiography depicted all 105 main renal arteries, whereas phase-contrast MR angiography depicted 104. The number of accessory renal arteries depicted was significantly higher with gadolinium-enhanced (17 of 18) than with phase-contrast (11 of 18) studies (P = .04). Both techniques depicted 27 of the 29 stenoses (sensitivity, 93%; P > .05). Sensitivities, specificities, and accuracies in the diagnosis of hemodynamically significant stenosis (> 50% narrowing) were, respectively, 94%, 96%, and 96% for phase-contrast and 100%, 97%, and 98% for gadolinium-enhanced MR angiography (P > .05).

CONCLUSION

Gadolinium-enhanced MR angiography is superior to phase-contrast MR angiography in accessory renal artery depiction. No statistically significant difference in the assessment of stenosis has been found between the two techniques.

Colour-coded duplex sonography in the diagnostic assessment of vascular complications after kidney transplantation in children

BACKGROUND

Vascular complications are a major cause of dysfunction or transplant loss in children. Arterial or venous occlusion, transplant renal artery stenosis (TRAS) and some arteriovenous (AV) fistula require rapid detection and prompt intervention. The present study was performed to determine the accuracy of colour Doppler sonography (CDS) in the early and late phase after renal transplantation and to correlate the results with angiographic and intraoperative findings.

OBJECTIVE

CDS is the preferred imaging modality with a high diagnostic accuracy for follow-up of renal transplantation in children. The indication for angiography should be established on the basis of the CDS diagnosis.

MATERIALS AND METHODS

In 87 children (mean age 10.9 years, range 2-17), 423 CDS examinations were performed after renal transplantation. Angiography was performed in 17 cases; surgery was necessary in 16 patients.

RESULTS

CDS correctly identified 8/8 arterial or venous occlusions and 7/7 TRAS. The only false positive diagnosis of TRAS was due to misinterpretation of an iliac artery stenosis. Six AV fistulae were diagnosed by CDS. The overall positive predictive value of CDS was 94 % in this study.

CONCLUSION

CDS is a noninvasive, non-radiation producing imaging modality with a high diagnostic accuracy. It is the method of choice in the assessment of vascular complications after renal transplantation in children.

Relation between arteriosclerosis in the coronary and renal arteries

This study analyzes the severity of coronary artery disease in terms of the severity of renal artery disease in 609 patients undergoing coronary and renal angiography. The presence of renal artery disease of any severity is strongly suggestive of advanced coronary artery disease.