High-spatial-resolution contrast-enhanced MR angiography of the renal arteries: a prospective comparison with digital subtraction angiography

Contrast Agent-Free 3D Renal Ultrafast Doppler Imaging Reveals Vascular Dysfunction in Acute and Diabetic Kidney Diseases

To combat the irreversible decline in renal function associated with kidney disease, it is essential to establish non-invasive biomarkers for assessing renal microcirculation. However, the limited resolution and/or vascular sensitivity of existing diagnostic imaging techniques hinders the visualization of complex cortical vessels. Here, a 3D renal ultrafast Doppler (UFD) imaging system that uses a high ultrasound frequency (18 MHz) and ultrahigh frame rate (1 KHz per slice) to scan the entire volume of a rat's kidney in vivo is demonstrated. The system, which can visualize the full 3D renal vascular branching pyramid at a resolution of 167 µm without any contrast agent, is used to chronically and noninvasively monitor kidneys with acute kidney injury (AKI, 3 days) and diabetic kidney disease (DKD, 8 weeks). Multiparametric UFD analyses (e.g., vessel volume occupancy (VVO), fractional moving blood volume (FMBV), vessel number density (VND), and vessel tortuosity (VT)) describe rapid vascular rarefaction from AKI and long-term vascular degeneration from DKD, while the renal pathogeneses are validated by in vitro blood serum testing and stained histopathology. This work demonstrates the potential of 3D renal UFD to offer valuable insights into assessing kidney perfusion levels for future research in diabetes and kidney transplantation.

Non-contrast renal MRA using multi-shot gradient echo EPI at 3-T MRI

OBJECTIVES

The purpose of this study was to investigate the feasibility of non-contrast renal MRA using multi-shot gradient echo planar imaging (MSG-EPI) with a 3-T MRI system.

METHODS

Seventeen healthy volunteers underwent non-contrast renal MRA using MSG-EPI and balanced steady-state free precession (b-SSFP) sequences on a 3-T MRI system. Two radiologists independently recorded the images' contrast, noise, sharpness, artifacts, and overall quality on 4-point scales. The signal-to-noise ratio (SNR) for the renal artery, the contrast ratio (CR) between the renal artery and erector spinae, and acquisition time were compared between the two sequences.

RESULTS

The SNR and CR were significantly higher with MSG-EPI than with the b-SSFP sequence (17.80 ± 3.67 vs. 10.84 ± 2.86 and 0.77 ± 0.05 and 0.66 ± 0.09, respectively; p < 0.05), and the acquisition time was significantly lower (164.5 ± 34.0 vs. 261.5 ± 39.3 s, respectively; p < 0.05). There were significant differences in image contrast, noise, sharpness, artifacts, and overall image quality between the two sequences (p < 0.01).

CONCLUSIONS

The MSG-EPI sequence is a promising technique that can shorten the scan time and improve the image quality of non-contrast renal MRA with a 3-T MRI system.

KEY POINTS

• The multi-shot gradient echo planar imaging with an inversion pulse is a brand-new fast scan technique for an unenhanced renal MRA. • The image quality of multi-shot gradient echo planar imaging is better than that of b-SSFP for an unenhanced renal MRA.

Abdominal Magnetic Resonance Angiography

MR imaging hardware and software improvements have led to new applications for contrast-enhanced and noncontrast-enhanced magnetic resonance angiography in the abdomen and pelvis. Higher magnetic field strength MR imaging scanners have greater signal-to-noise ratio and contrast-to-noise ratio, which is used to improve spatial resolution or temporal resolution for these techniques. New noncontrast-enhanced sequences offer high-resolution magnetic resonance angiography without contrast and provide additional hemodynamic information. Magnetic resonance angiography is particularly well suited to imaging patients with chronic mesenteric ischemia, renal vascular disease, pelvic congestion syndrome, and vascular malformations.

The capability of inflow inversion recovery magnetic resonance compared to contrast-enhanced magnetic resonance in renal artery angiography

PURPOSE

To assess the capability of inflow inversion recovery (IFIR) magnetic resonance angiography (MRA), compared with contrast-enhanced MRA (CE-MRA) as reference standard, in evaluating renal artery stenosis (RAS).

METHODS

Seventy-two subjects were examined by IFIR MRA with respiratory-gated, prior to CE-MRA with a 1.5-T scanner. Two readers evaluated the quality of IFIR MRA images and renal artery depiction on artery-by-artery basis. The agreement of two methods to assess RAS was analyzed using the Kappa test. The relationship between image quality of IFIR MRA and respiratory rate was analyzed by ANOVA test.

RESULTS

The visibility of renal artery branch vessels was significantly higher using IFIR MRA than CE-MRA (p < 0.05). A good agreement of two methods in evaluating stenosis grade, and a near-perfect inter-observer agreement for IFIR MRA (Kappa value 0.98) and CE-MRA (Kappa value 0.93), were demonstrated. As RAS ≥50%, the sensitivity and specificity of IFIR MRA were 92 and 98% in reader 1, 93 and 98% in reader 2, respectively. The image quality was significantly better in patients with stable respiration (p < 0.01).

CONCLUSIONS

IFIR MRA in patients with stable respiration has higher visibility of renal artery branch vessels than CE-MRA, and a good agreement with CE-MRA in evaluating stenosis grade. It could be used to evaluate RAS for screening, and monitoring treatment.

Unenhanced respiratory-navigated NATIVE® TrueFISP magnetic resonance angiography in the evaluation of renal arteries: Comparison with contrast-enhanced magnetic resonance angiography

PURPOSE

To compare unenhanced three-dimensional (3D) NATIVE^® true fast imaging with steady-state precession (TrueFISP) magnetic resonance (MR) angiography with the more conventional MR angiography technique obtained after intravenous administration of a gadolinium chelate in the evaluation of renal arteries and their branches in patients with suspected renal artery stenosis.

MATERIALS AND METHODS

A total of 39 patients (25 men, 14 women) with a mean age of 51.4±17.5years (SD) (range: 10-82years) were included in the study. All patients with suspected renal artery stenosis underwent unenhanced 3D NATIVE^® TrueFISP MR angiography and contrast-enhanced MR angiography. The two MR angiography methods were compared by two independent readers for image quality using a four-point scale, diagnostic performance and grading of renal artery stenosis on a total of 78 renal arteries.

RESULTS

For both readers image quality of unenhanced 3D NATIVE^® TrueFISP MR angiography (3.12 to 3.63) was greater than that of contrast-enhanced MR angiography (1.94 to 2.71) for renal artery ostium-trunk and the left renal artery segmental branches. The sensitivity of 3D NATIVE^® TrueFISP MR angiography for the diagnosis of renal artery stenosis was 100% for both readers for the right renal artery and 66% and 80% for the left renal artery for reader 1 and reader 2, respectively. Agreement between 3D NATIVE^® TrueFISP MR angiography and CE-MR angiography was 95% (74/78) for reader 1 and 92% (72/78) for reader 2.

CONCLUSION

Unenhanced NATIVE^® TrueFISP magnetic resonance angiography can play an additional role in the evaluation of renal arteries in patients with hypertension, especially in subjects at risk of nephrogenic systemic fibrosis.

Accuracy of unenhanced magnetic resonance angiography for the assessment of renal artery stenosis

Purpose

To evaluate the accuracy of unenhanced magnetic resonance angiography (U-MRA) using balanced steady-state free precession (SSFP) sequences with inversion recovery (IR) pulses for the evaluation of renal artery stenosis.

Materials and methods

U-MRA was performed in 24 patients with suspected main renal artery stenosis. Two radiologists evaluated the quality of the imaging studies and the ability of U-MRA to identify hemodynamically significant main renal artery stenosis (RAS) defined as a stenosis ≥50% when compared to gold standard tests: contrast-enhanced magnetic resonance angiography (CE-MRA) (18 patients) or digital subtraction arteriography (DSA) (6 patients).

Results

A total of 44 main renal arteries were evaluated. Of them, 32 renal arteries could be assessed with U-MRA. When CE-MRA or DSA was used as the reference standard, nine renal arteries had hemodynamically significant RAS. U-MRA correctly identified eight out of nine arteries as having ≥50% RAS, and correctly identified 22 out of 23 arteries as not having significant RAS, with a sensitivity of 88.8%, a specificity of 95.65%, positive and negative predictive value of 88.8% and 95.65%, respectively, and an accuracy of 93.75%. Renal artery fibromuscular dysplasia (FMD) was observed in the two misclassified arteries.

Conclusion

U-MRA is a reliable diagnostic method to depict normal and stenotic main renal arteries. U-MRA can be used as an alternative to contrast-enhanced magnetic resonance angiography or computer tomography angiography in patients with renal insufficiency unless FMD is suspected.

Atherosclerotic renal artery stenosis and revascularization

Atherosclerotic renovascular disease is the most common cause of secondary hypertension. The patients with renovascular disease are at increased risk for adverse cardiac outcomes. Recent trials comparing medical therapy alone to medical therapy with stenting are flawed, but lay to rest any existing debate that unselected revascularization is unwarranted; however, revascularization may be appropriate in high-risk populations. Defining an appropriate population for revascularization is an area of ongoing study. Furthermore, delivery of optimal medical therapy in this population is inadequate. This review describes recent developments in renal artery revascularization.

Renal artery stenosis: epidemiology and treatment

Renal artery stenosis (RAS) is a frequently encountered problem in clinical practice. The disease encompasses a broad spectrum of pathophysiologies and is associated with three major clinical syndromes: ischemic nephropathy, hypertension, and destabilizing cardiac syndromes. The two most common etiologies are fibromuscular dysplasia and atherosclerotic renal artery disease with atherosclerotic disease accounting for the vast majority of cases. Atherosclerotic renovascular disease has considerable overlap with atherosclerotic disease elsewhere and is associated with a poor prognosis. A wide range of diagnostic modalities and treatment approaches for RAS are available to clinicians, and with the advent of endovascular interventions, selecting the best course for a given patient has only grown more challenging. Several clinical trials have demonstrated some benefit with revascularization but not to the extent that many had hoped for or expected. Furthermore, much of the existing data is only marginally useful given significant flaws in study design and inherent bias. There remains a need for further identification of subgroups and appropriate indications in hopes of maximizing outcomes and avoiding unnecessary procedures in patients who would not benefit from treatment. In recent decades, the study of RAS has expanded and evolved rapidly. In this review, we will attempt to summarize the amassed body of literature with a focus on the epidemiology of RAS including prevalence, overlap with other atherosclerotic disease, and prognosis. We will also outline existing diagnostic and treatment approaches available to clinicians as well as summarize the findings of several major clinical trials. Finally, we will offer our perspective on future directions in the field.

Unenhanced respiratory-gated magnetic resonance angiography (MRA) of renal artery in hypertensive patients using true fast imaging with steady-state precession technique compared with contrast-enhanced MRA

OBJECTIVE

This study was aimed to evaluate the accuracy of "True Fast Imaging with Steady-State Precession" (TrueFISP) MR angiography (MRA) for diagnosis of renal arterial stenosis (RAS) in hypertensive patients.

METHODS

Twenty-two patients underwent both TrueFISP MRA and contrast-enhanced MRA (CE-MRA) on a 1.5-T MR imager. Volume of main renal arteries, length of maximal visible renal arteries, number of visualized branches, stenotic grade, and subjective quality were compared. Paired 2-tailed Student t test and Wilcoxon signed rank test were applied to evaluate the significance of these variables.

RESULTS

Volume of main renal arteries, length of maximal visible renal arteries, and number of branches indicated no significant difference between the 2 techniques (P > 0.05). Stenotic degree of 10 RAS was greater on CE-MRA than on TrueFISP MRA. Qualitative scores from TrueFISP MRA were higher than those from CE-MRA (P < 0.05).

CONCLUSIONS

TrueFISP MRA is a reliable and accurate method for evaluating RAS.

B-flow imaging for the measurement of residual lumen diameter of renal artery stenosis

BACKGROUND

The aim of this study was to assess the accuracy and efficacy of B-flow imaging (BFI) in the diagnosis and measurement of renal artery stenosis (RAS) compared with color duplex ultrasonography (CDU) and digital subtraction angiography (DSA).

METHODS

Fifty-one consecutive patients with RAS diagnosed and measured with DSA were subsequently and independently examined with BFI for the measurement of residual lumen diameter, and with CDU for the measurement of peak systolic velocity and renal-aortic velocity ratio. The diagnostic performances of BFI and CDU in determining 60-99% RAS were compared by receiver operating characteristic curve analysis. The agreement between DSA and BFI stenosis measurements was evaluated with Bland-Altman method.

RESULTS

The area under curve was 0.983 for BFI and 0.959 for CDU, without a significant difference in diagnostic performances (p = 0.26). BFI yielded an 88% sensitivity and 94% specificity in the diagnosis of RAS with a 3% underestimation of RAS compared to DSA (95% confidence interval 1.4%, 4.6%).

CONCLUSIONS

BFI is an accurate method that minimally underestimates RAS. It might provide an additional benefit to CDU in patients with RAS.

Contrast-enhanced MR angiography of the abdomen with highly accelerated acquisition techniques

PURPOSE

To demonstrate that highly accelerated (net acceleration factor [R(net)] ≥ 10) acquisition techniques can be used to generate three-dimensional (3D) subsecond timing images, as well as diagnostic-quality high-spatial-resolution contrast material-enhanced (CE) renal magnetic resonance (MR) angiograms with a single split dose of contrast material.

MATERIALS AND METHODS

All studies were approved by the institutional review board and were HIPAA compliant; written consent was obtained from all participants. Twenty-two studies were performed in 10 female volunteers (average age, 47 years; range, 27-62 years) and six patients with renovascular disease (three women; average age, 48 years; range, 37-68 years; three men; average age, 60 years; range, 50-67 years; composite average age, 54 years; range, 38-68 years). The two-part protocol consisted of a low-dose (2 mL contrast material) 3D timing image with approximate 1-second frame time, followed by a high-spatial-resolution (1.0-1.6-mm isotropic voxels) breath-hold 3D renal MR angiogram (18 mL) over the full abdominal field of view. Both acquisitions used two-dimensional (2D) sensitivity encoding acceleration factor (R) of eight and 2D homodyne (HD) acceleration (R(HD)) of 1.4-1.8 for R(net) = R · R(HD) of 10 or higher. Statistical analysis included determination of mean values and standard deviations of image quality scores performed by two experienced reviewers with use of eight evaluation criteria.

RESULTS

The 2-mL 3D time-resolved image successfully portrayed progressive arterial filling in all 22 studies and provided an anatomic overview of the vasculature. Successful timing was also demonstrated in that the renal MR angiogram showed adequate or excellent portrayal of the main renal arteries in 21 of 22 studies.

CONCLUSION

Two-dimensional acceleration techniques with R(net) of 10 or higher can be used in CE MR angiography to acquire (a) a 3D image series with 1-second frame time, allowing accurate bolus timing, and (b) a high-spatial-resolution renal angiogram.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.11110242/-/DC1.

Highly accelerated contrast-enhanced MR angiography: improved reconstruction accuracy and reduced noise amplification with complex subtraction

Contrast-enhanced magnetic resonance angiography is routinely performed using parallel imaging to best capture the first pass of contrast material through the target vasculature, followed by digital subtraction to suppress the appearance of unwanted signal from background tissue. Both processes, however, amplify noise and can produce uninterpretable images when large acceleration factors are used. Using a phantom study of contrast-enhanced magnetic resonance angiography, we show that complex subtraction processing prior to partially parallel reconstruction improves reconstruction accuracy relative to magnitude subtraction processing for reduction factors as large as 12. Time-resolved contrast-enhanced magnetic resonance angiographic data obtained with complex subtraction in volunteers supported the results of the phantom study and when compared with magnitude subtraction processing demonstrated reduced geometry factors as well as improved image quality at large reduction factors.

Low-dose, time-resolved, contrast-enhanced 3D MR angiography in the assessment of the abdominal aorta and its major branches at 3 Tesla

RATIONALE AND OBJECTIVES

The aims of this study were to evaluate the effectiveness of low-dose, contrast-enhanced (CE), time-resolved, three-dimensional magnetic resonance angiography (MRA) in the assessment of the abdominal aorta and its major branches at 3 T and to compare the results with those of high-spatial resolution CE MRA.

MATERIALS AND METHODS

Twenty-two consecutive patients (eight men, 14 women; mean age, 43.9 +/- 17.9 years) underwent CE time-resolved three-dimensional MRA and high-spatial resolution three-dimensional MRA. Studies were performed using a 3-T magnetic resonance system; gadolinium-based contrast medium was administered at a dose of 3 to 5 mL for time-resolved MRA, followed by 0.1 mmol/kg gadopentetate dimeglumine for single-phase CE MRA. For analysis purposes, the abdominal arterial system was divided into 11 arterial segments, and image quality as well as the presence and degree of vascular pathology were evaluated by two independent magnetic resonance radiologists.

RESULTS

A total of 242 arterial segments were visualized with good image quality. Time-resolved MRA was able to visualize the majority of arterial segments with good definition in the diagnostic range. Vascular pathologies (stenosis, occlusion) or abnormal vascular anatomy was detected in 19 arterial segments, with good interobserver agreement (kappa = 0.78). All image findings were detected with time-resolved CE MRA by both observers and were confirmed by correlative imaging.

CONCLUSION

Low-dose, time-resolved MRA at 3 T yields rapid and important anatomic and functional information in the evaluation of the abdominal vasculature. Because of its limited spatial resolution, time-resolved MRA is inferior to CE MRA in demonstrating fine vascular details.

Direct comparison of sensitivity encoding (SENSE) accelerated and conventional 3D contrast enhanced magnetic resonance angiography (CE-MRA) of renal arteries: effect of increasing spatial resolution

PURPOSE

To assess the effect of attaining higher spatial resolution in contrast-enhanced magnetic resonance angiography (MRA) of renal arteries using parallel imaging, sensitivity encoding (SENSE), by comparing the SENSE contrast-enhanced (CE) MRA against a conventional CE-MRA protocol with identical scan times, injection protocol, and other acquisition parameters.

MATERIALS AND METHODS

Numerical simulations and a direct comparison of SENSE-accelerated versus conventional acquisitions were performed. A total of 41 patients (18 male) were imaged using both protocols for a direct comparison. Both protocols used fluoroscopic triggering, centric encoding, breath-holding, equivalent injection protocol, and lasted approximately 30 seconds.

RESULTS

Simulated point-spread functions were narrower for the SENSE protocol compared to the conventional protocol. In the patient study, although the SENSE protocol produced images with lower signal-to-noise ratio (SNR), image quality was better for all segments of the renal arteries. In addition, ringing of kidney parenchyma and renal artery blurring were significantly reduced in the SENSE protocol. Finally, reader confidence improved with the SENSE protocol.

CONCLUSION

Despite a reduction in SNR, the higher-resolution SENSE CE-MRA provided improved image quality, reduced artifacts, and increased reader confidence compared to the conventional protocol.

Non-contrast-enhanced MR imaging of renal artery stenosis at 1.5 tesla

Balanced steady-state free precession (Bal-SSFP) techniques produce excellent anatomic images of renal arteries without the use of contrast agents and are relatively flow-insensitive. Electrocardiography (ECG)-triggered and non-ECG-triggered sequences have been shown to be quite sensitive for detection of regional arterial stenosis (RAS), and the already high specificity is likely to increase with further refinement of the techniques. Bal-SSFP sequences can be used as a screening tool or as an alternative to contrast-enhanced (CE) magnetic resonance angiography (MRA) when contrast agents are contraindicated. In addition to morphologic imaging of RAS, non-CE techniques can be used in functional assessment of hemodynamic significance. The complimentary tools can be used alone or in combination with CE-MRA for MR imaging of renal vascular hypertension.

Diagnosis and management of atherosclerotic renal artery stenosis: improving patient selection and outcomes

Renal artery stenosis (RAS) is common among patients with atherosclerosis, and is found in 20-30% of individuals who undergo diagnostic cardiac catheterization. Renal artery duplex ultrasonography is the diagnostic procedure of choice for screening outpatients for RAS. Percutaneous renal artery stent placement is the preferred method of revascularization for hemodynamically significant RAS, and is favored over balloon angioplasty alone. Stent placement carries a class I recommendation for atherosclerotic RAS according to ACC and AHA guidelines. Discordance exists between the very high (>95%) procedural success rate and the moderate (60-70%) clinical response rate after renal stent placement, which is likely to be a result of poor selection of patients, inadequate angiographic assessment of lesion severity, and the presence of renal parencyhmal disease. Physiologic lesion assessment using translesional pressure gradients, and measurements of biomarkers (e.g. brain natriuretic peptide), or both, could enhance the selection of patients and improve clinical response rates. Long-term patency rates for renal stenting are excellent, with 5-year secondary patency rates greater than 90%. This Review will outline the clinical problem of atherosclerotic RAS and its diagnosis, and will critically assess treatment options and strategies to improve patients' outcomes.

Renal contrast-enhanced MR angiography: timing errors and accurate depiction of renal artery origins

PURPOSE

To investigate bolus timing artifacts that impair depiction of renal arteries at contrast material-enhanced magnetic resonance (MR) angiography and to determine the effect of contrast agent infusion rates on artifact generation.

MATERIALS AND METHODS

Renal contrast-enhanced MR angiography was simulated for a variety of infusion schemes, assuming both correct and incorrect timing between data acquisition and contrast agent injection. In addition, the ethics committee approved the retrospective evaluation of clinical breath-hold renal contrast-enhanced MR angiographic studies obtained with automated detection of contrast agent arrival. Twenty-two studies were evaluated for their ability to depict the origin of renal arteries in patent vessels and for any signs of timing errors.

RESULTS

Simulations showed that a completely artifactual stenosis or an artifactual overestimation of an existing stenosis at the renal artery origin can be caused by timing errors of the order of 5 seconds in examinations performed with contrast agent infusion rates compatible with or higher than those of hand injections. Lower infusion rates make the studies more likely to accurately depict the origin of the renal arteries. In approximately one-third of all clinical examinations, different contrast agent uptake rates were detected on the left and right sides of the body, and thus allowed us to confirm that it is often impossible to optimize depiction of both renal arteries. In three renal arteries, a signal void was found at the origin in a patent vessel, and delayed contrast agent arrival was confirmed.

CONCLUSION

Computer simulations and clinical examinations showed that timing errors impair the accurate depiction of renal artery origins.

Renal artery stenosis evaluation: diagnostic performance of gadobenate dimeglumine-enhanced MR angiography--comparison with DSA

PURPOSE

To prospectively determine diagnostic performance and safety of contrast material-enhanced (CE) magnetic resonance (MR) angiography with 0.1 mmol per kilogram of body weight gadobenate dimeglumine for depiction of significant steno-occlusive disease (> or =51% stenosis) of renal arteries, with digital subtraction angiography (DSA) as reference standard.

MATERIALS AND METHODS

This multicenter study was approved by local institutional review boards; all patients provided written informed consent. Patient enrollment and examination at centers in the United States complied with HIPAA. Two hundred ninety-three patients (154 men, 139 women; mean age, 61.0 years) with severe hypertension (82.2%), progressive renal failure (11.3%), and suspected renal artery stenosis (6.5%) underwent CE MR angiography with three-dimensional spoiled gradient-echo sequences after administration of 0.1 mmol/kg gadobenate dimeglumine at 2 mL/sec. Anteroposterior and oblique DSA was performed in 268 (91.5%) patients. Three independent blinded reviewers evaluated CE MR angiographic images. Sensitivity, specificity, and accuracy of CE MR angiography for detection of significant steno-occlusive disease (> or =51% vessel lumen narrowing) were determined at segment (main renal artery) and patient levels. Positive and negative predictive values and positive and negative likelihood ratios were determined. Interobserver agreement was analyzed with generalized kappa statistics. A safety evaluation (clinical examination, electrocardiogram, blood and urine analysis, monitoring for adverse events) was performed.

RESULTS

Of 268 patients, 178 who were evaluated with MR angiography and DSA had significant steno-occlusive disease of renal arteries at DSA. Sensitivity, specificity, and accuracy of CE MR angiography for detection of 51% or greater stenosis or occlusion were 60.1%-84.1%, 89.4%-94.7%, and 80.4%-86.9%, respectively, at segment level. Similar values were obtained for predictive values and for patient-level analyses. Few CE MR angiographic examinations (1.9%-2.8%) were technically inadequate. Interobserver agreement for detection of significant steno-occlusive disease was good (79.9% agreement; kappa = 0.69). No safety concerns were noted.

CONCLUSION

CE MR angiography performed with 0.1 mmol/kg gadobenate dimeglumine, compared with DSA, is safe and provides good sensitivity, specificity, and accuracy for detection of significant renal artery steno-occlusive disease.

Isotropic high-spatial-resolution contrast-enhanced 3.0-T MR angiography in patients suspected of having renal artery stenosis

The purpose of this study was to prospectively evaluate the diagnostic performance of contrast material-enhanced magnetic resonance (MR) angiography performed at 3 T for assessment of renal artery stenosis (RAS) by using parallel acquisition techniques with high acceleration factors and with digital subtraction angiography (DSA) as the reference standard. The study was institutional review board approved, and written informed consent was obtained from all patients. Twenty-nine patients (18 men, 11 women; mean age, 57.1 years +/- 14.3 [standard deviation]) suspected of having RAS underwent MR angiography. Images were evaluated qualitatively and quantitatively. The interobserver variability, sensitivity, specificity, and positive and negative predictive values of 3-T MR angiography, as compared with DSA (performed in 15 patients), were calculated. All examinations yielded good or excellent image quality. The sensitivity and specificity of MR angiography in grading significant (>75%) stenosis were 94% and 96%, respectively. Owing to its high sensitivity, contrast-enhanced 3-T MR angiography can be used reliably to exclude RAS and can serve as a useful screening method in the diagnostic work-up of patients with arterial hypertension.

Renal Artery Assessment with Nonenhanced Steady-State Free Precession versus Contrast-enhanced MR Angiography

PURPOSE

To prospectively assess the diagnostic accuracy of nonenhanced three-dimensional (3D) steady-state free precession (SSFP) magnetic resonance (MR) angiography for detection of renal artery stenosis (RAS), with breath-hold contrast material-enhanced MR angiography performed as the reference standard.

MATERIALS AND METHODS

The study was local ethics committee approved; all patients gave written informed consent. Fifty-three patients (30 male, 23 female; mean age, 58 years) with arterial hypertension and suspected of having RAS were examined with 1.5-T 3D SSFP renal MR angiography. Stenosis grade, maximal visible vessel length, and subjective image quality were compared. Sensitivity, specificity, accuracy, and negative predictive value (NPV) were calculated on artery-by-artery and patient-by-patient bases. The significance of the results was assessed with the paired two-sided t test for continuous variables and with the marginal homogeneity test for categorical variables. Cohen kappa statistics were used to estimate interobserver agreement.

RESULTS

One hundred eight renal arteries with 20 significant (>or=50%) stenoses were detected with contrast-enhanced MR angiography. At artery-by-artery analysis, sensitivity, specificity, accuracy, and NPV of nonenhanced SSFP MR angiography for RAS detection were 100%, 93%, 94%, and 100%, respectively, for observer 1 and 95%, 95%, 95%, and 99%, respectively, for observer 2. Corresponding patient-by-patient values were 100%, 92%, 94%, and 100%, respectively, for observer 1 and 100%, 95%, 96%, and 100%, respectively, for observer 2. Overestimation of stenosis grade with SSFP MR angiography resulted in six and four false-positive findings for readers 1 and 2, respectively. Mean maximal visible lengths of the renal arteries were 69.9 mm at contrast-enhanced MR angiography and 61.1 mm at SSFP MR angiography (P<.001). Both techniques yielded good to excellent image quality.

CONCLUSION

Slab-selective inversion-prepared 3D SSFP MR angiography had high sensitivity, specificity, accuracy, and NPV for RAS detection, without the need for contrast material. However, RAS severity was overestimated in some patients.

Incidental findings on renal MR angiography

OBJECTIVE

The purpose of this article is to assess the incidence of incidental vascular and nonvascular findings in patients undergoing renal MR angiography and to determine the extent to which these findings alter patient management.

MATERIALS AND METHODS

Reports from 380 consecutive renal MR angiography examinations performed at a single institution over a 12-month interval were examined. The presence of incidental vascular (i.e., nonrenal artery) and nonvascular findings was noted. Clinical records of patients with significant incidental findings were examined to determine whether additional imaging, biopsy, or surgery was performed.

RESULTS

Overall, 151 (40%) of 380 patients had one or more additional vascular findings not related to the renal arteries, and 221 (58%) of 380 patients had one or more additional nonvascular findings. Vascular findings included mesenteric artery stenosis or occlusion in 33% of patients, moderate to severe aortic atherosclerosis in 17%, aortic aneurysms in 7%, and aortic dissection in 2%. Incidental malignancies were detected in 10 patients (3%), and indeterminate lesions requiring follow-up imaging, biopsy, or surgery were noted in 18 patients (5%). Overall, management in 5% of patients was significantly altered (i.e., required biopsy, surgery, or other intervention) by incidental findings detected on renal MR angiography. Benign lesions not requiring additional imaging or follow-up occurred in 54% of patients and consisted predominantly of renal cysts.

CONCLUSION

Incidental findings on renal MR angiography are common. Most incidental lesions can be adequately detected and characterized with the addition of a few pulse sequences to the standard renal MR angiography protocol at a minimal cost in imaging time. The high incidence of incidental findings emphasizes the importance of performance and interpretation of these examinations by physicians with training in abdominal cross-sectional imaging.

Establishing a Cardiac Imaging Service

The tremendous progress in the field of cardiac imaging has drawn the interest of many radiologists, but has left them uncertain as to how obtain the training necessary to become expert practitioners. This review addresses the challenges in establishing a cardiac imaging service including equipment issues, training and credentialing issues, as well as referrals and reimbursement.

Efficacy and safety of gadodiamide (Gd-DTPA-BMA) in renal 3D-magnetic resonance angiography (MRA): a phase II study

PURPOSE

To determine the most efficacious dose of gadodiamide for three-dimensional (3D) contrast-enhanced (CE) magnetic resonance angiography (MRA) of the renal arteries on a patient level based on the sensitivity in detecting the main hemodynamically relevant (> or =50% or occlusion) renal artery stenosis (RAS) using intra-arterial digital subtraction angiography (IA DSA) as the gold standard.

MATERIALS AND METHODS

This prospective, randomized, double-blind, parallel-group, multicenter study included 273 patients referred to IA DSA for suspected RAS. Patients underwent 3D CE MRA after injection of 0.01, 0.05, 0.1, or 0.2mmol/kg of body weight gadodiamide (0.5mmol/ml). The images were assessed for location and degree of RAS by independent blinded readers (MRA: three readers, IA DSA: one reader). Hypothesis testing for a significant trend in sensitivity across dose groups was based on the one-sided Cochran-Armitage style trend test for each independent MRA reader.

RESULTS

The lowest dose group (0.01mmol/kg) proved non-efficacious in detecting hemodynamically relevant (i.e., > or =50% or occlusion) RAS. A statistically significant dose trend (p<0.001) was shown for each of the three independent readers. Depending on reader, the sensitivity obtained with 0.05, 0.1, and 0.2mmol/kg was 63.9-86.1%, 75.8-91.4% and 80.6-90.6%, the specificity was 66.7-73.9%, 59.3-75.0%, and 59.3-75.0% and accuracy was 67.8-78.9%, 75.4-77.4%, and 76.3-81.0%, for the three dose groups, respectively. There were eight non-severe adverse events (AEs). Three serious AEs occurring in one patient were judged not related to gadodiamide by the on-site investigator.

CONCLUSION

A significant dose trend between the four doses examined was observed. The lowest dose (0.01mmol/kg) differed significantly from those of the other three doses. Based on the analysis of the primary and secondary endpoints, 0.1mmol/kg gadodiamide appears to be the most suitable dose in diagnosing hemodynamically relevant RAS. The present study also demonstrated gadodiamide to be safe and well tolerated.

Magnetic resonance angiography of chest and abdomen at 3 T

During the past decade, contrast-enhanced magnetic resonance angiography (CE-MRA) has been proven to be a powerful tool to visualize the thoracoabdominal vasculature and, consequently, has become a widely accepted noninvasive imaging modality. With the more recent introduction of high-field whole-body magnetic resonance scanners, a further improvement of diagnostic accuracy can be expected. General considerations for performing high-resolution CE-MRA at higher field strength include the benefits of higher signal-to-noise ratio and an improved contrast between vascular and background tissues. Although there are many positive attributes for performing CE-MRA at 3 T, there are also some tradeoffs, such as static magnetic field inhomogeneity and increase in specific absorption rate. This review describes the main technical innovations of advanced CE-MRA techniques at 3 T, illustrated by characteristic cases.

Gadobenate dimeglumine-enhanced MR angiography: Diagnostic performance of four doses for detection and grading of carotid, renal, and aorto-iliac stenoses compared to digital subtraction angiography

PURPOSE

To determine the diagnostic performance of contrast-enhanced MR angiography (CE-MRA) with four doses of gadobenate dimeglumine for detection of significant steno-occlusive disease of the carotid, renal, and pelvic vasculature.

MATERIALS AND METHODS

Eighty-four patients with suspected disease of the renal (n = 16), pelvic (n = 41), or carotid (n = 27) arteries underwent CE-MRA (3D-spoiled gradient-echo sequences) at 1.5T. CE-MRA was performed with gadobenate dimeglumine at 0.025, 0.05, 0.1, or 0.2 mmol/kg (23, 24, 19, and 18 patients, respectively) administered at 2 mL/sec. Accuracy, sensitivity, specificity, and positive and negative predictive values (PPV and NPV, respectively) for detection of significant disease (>50% stenosis or occlusion for renal/pelvic arteries; >70% stenosis or occlusion for carotid arteries) was determined by three fully blinded, independent radiologists using conventional digital subtraction angiography (DSA) as reference standard. All comparisons were tested statistically (ANOVA, chi-square, and Mantel-Haenszel tests as appropriate) and reader agreement (kappa) was assessed.

RESULTS

Values for accuracy, sensitivity, specificity, PPV, and NPV on CE-MRA were consistently higher for 0.1 mmol/kg gadobenate dimeglumine (accuracy = 95.2-97.3%, sensitivity = 84.2% (all readers), specificity = 96.9-99.2%, PPV = 80.0-94.1%, NPV = 97.6-97.7%). The greater accuracy of the 0.1 mmol/kg dose was significant (P < 0.01, all readers) compared to all other dose groups. Agreement between the three readers was good for all dose groups (kappa >/=0.58), with the highest percent agreement (85.7%) noted for the 0.1 mmol/kg dose.

CONCLUSION

Significantly better diagnostic performance on CE-MRA of the renal, pelvic, and carotid arteries is achieved with a gadobenate dimeglumine dose of 0.1 mmol/kg bodyweight.

3D contrast-enhanced MR angiography

Safe, fast, accurate contrast arteriography can be obtained utilizing gadolinium (Gd) and 3D MR data acquisition for diagnosing vascular diseases. Optimizing contrast enhanced MRA (CE MRA), however, requires understanding the complex interplay between Gd injection timing, the Fourier mapping of 3D MR data acquisition and a multitude of parameters determining resolution, anatomic coverage, and sensitivity to motion artifacts. It is critical to time the bolus peak to coincide with central k-space data acquisition, which dominates image contrast. Oversampling the center of k-space allows reconstruction of multiple 3D acquisitions in rapid succession to time-resolve the passage of the contrast bolus. Parallel imaging increases resolution, shortens scan time and compresses the center of k-space into a shorter period of time, thereby minimizing motion and timing artifacts. Absence of ionizing radiation allows MRA to be repeated and combined with additional sequences to more fully characterize anatomy, flow, and physiology. Utilizing stepping table technology and thigh compression, whole body MRA is possible with a single contrast injection. As MR technology continues to advance, CE MRA becomes better and simpler to perform, increasing its efficacy in the diagnosis and management of vascular diseases.

Fibromuscular Dysplasia of the Main Renal Arteries: Comparison of Contrast-enhanced MR Angiography with Digital Subtraction Angiography

PURPOSE

To retrospectively evaluate the sensitivity and specificity of contrast material-enhanced magnetic resonance (MR) angiography by using digital subtraction angiography as the reference standard in patients with hypertension and renal artery fibromuscular dysplasia (FMD).

MATERIALS AND METHODS

Institutional review board approval was obtained, with waiver of informed consent. The results of renal contrast-enhanced MR angiography were retrospectively analyzed in 25 patients with hypertension (24 women, one man; mean age, 48 years +/- 19 [standard deviation]; age range, 18-72 years) who had FMD diagnosed on the basis of clinical and angiographic features. All examinations were performed at 1.5 T. Results were analyzed by two readers, and a third reader established a consensus in case of discrepancy. Sensitivity, specificity, and 95% confidence intervals (CIs) were calculated for FMD and for each possible type of FMD lesion ("string of pearls" appearance, stenosis, and aneurysm). A linear-weighted kappa statistic was calculated to determine agreement between digital subtraction angiography and contrast-enhanced MR angiography for the diagnosis of FMD and to determine inter- and intraobserver agreement regarding FMD diagnosis.

RESULTS

Fifty main renal arteries were analyzed, 35 of which demonstrated abnormal arteriographic features of FMD (stenosis, 22 arteries; string of pearls, 21 arteries; and aneurysm, four arteries). The sensitivity and specificity of contrast-enhanced MR angiography for the diagnosis of FMD were 97% (95% CI: 83%, 100%) and 93% (95% CI: 66%, 100%), respectively. Sensitivity was 68% (95% CI: 83%, 100%), 95% (95% CI: 74%, 100%), and 100% (95% CI: 40%, 100%) for the diagnosis of stenosis, string of pearls, and aneurysm, respectively. Linear-weighted kappa statistics for inter- and intraobserver agreement regarding FMD diagnosis were 0.63 and 0.92, respectively.

CONCLUSION

In patients with renal FMD, contrast-enhanced MR angiography can reliably facilitate diagnosis by demonstrating characteristic lesions.

Renovascular disease imaging: Today more than ever the invasive cardiologist may make the difference

Doppler ultrasound (US), computed tomography (CT) and magnetic resonance imaging (MRI) are gaining wide acceptance as gold standard for diagnosing peripheral artery disease at thoracic and abdominal aorta level and infrainguinal arteries. A different case is that of renal artery stenosis (RAS): noninvasive techniques, CT and MRI in particular, failed to become the new gold standard. Image interpretation and artifacts seems to invalidate MRI and CT sensitivity and specificity that remain quite low. The debacle of noninvasive imaging of renal arteries may have important reflexes on the invasive cardiology practice. In the light of the results of noninvasive tests for renal artery stenosis, the angiography of renal vessels performed at the time of coronary artery angiography may became a new tool for invasive professional to optimize the diagnosis of renal artery stenosis at least in patients scheduled for coronary angiography. The invasive cardiologist, today more than ever, has the chance to really impact the diagnosis and therapy of patients with renovascular disease and the prognosis of patients with both coronary artery and renal artery disease.

Utilizing SENSE to reduce scan duration in high-resolution contrast-enhanced renal MR angiography

PURPOSE

To evaluate the use of sensitivity encoding (SENSE) to reduce scan time and decrease detrimental artifacts arising from motion and bolus profile effects during contrast-enhanced MR angiography (CE-MRA) of the renal arteries (RAs).

MATERIALS AND METHODS

A direct comparison of conventional and SENSE (acceleration factor 2) CE-MRA protocols was performed on 20 patients. Each patient underwent both scans. Both protocols achieved the same resolution, but the SENSE protocol was 50% faster and utilized a faster injection than the conventional scan. Three radiologists graded the images for image quality, artifact levels, and reader confidence.

RESULTS

While the signal-to-noise ratio (SNR) decreased (26+/-5 vs. 30+/-10; P=0.04) with the SENSE protocol, the image-quality scores for four identified segments of the RAs increased or were unchanged. The largest improvements in image quality occurred in the more distal segments of the RAs. Parenchymal ringing (P=0.005) and RA blurring (P=0.006) were significantly reduced, and there was a trend toward improvement of RA ringing despite the increased injection rate.

CONCLUSION

The faster SENSE scan maintained nearly the same SNR (due to faster injection of Gd-chelate), reduced artifact levels, and improved image quality ratings for the distal renal vessels.

Estimation of the Ratio of Renal Artery Stenosis with Magnetic Resonance Angiography Using Parallel Imaging Technique in Suspected Renovascular Hypertension

BACKGROUND

Renovascular hypertension, which may lead to end-stage renal failure, necessitates prompt diagnosis and medication. Although various diagnostic tools exist for evaluation of renal arteries, magnetic resonance angiography (MRA), with the improvement of hardware and software systems, has become a very promising technique in screening patients with suspected renal hypertension. In this study, we aimed to assess renal artery stenosis on MRA in patients with suspected renovascular disease using a parallel imaging technique which allows faster scanning with higher resolution.

METHODS

Eighty-four patients with hypertension underwent MRA and digital angiography.

RESULTS

MRA detected renal artery stenosis with a sensitivity rate ranging from 69.3 to 100% and specificity rate ranging from 85.7 to 96%.

CONCLUSION

Contrast-enhanced MRA of renal arteries is very effective in the demonstration of renal artery stenoses and assessment of stenosis ratio. Furthermore, parallel imaging technology has improved this procedure by reducing the scan time. Renal MRA, as a diagnostic tool, can accurately direct patients with renovascular disease to intravascular treatment.

Renal MR and CT angiography: current concepts

During the past decade, noninvasive CTA and MRA imaging techniques have replaced catheter angiography for evaluation of the renal arteries. This article reviews techniques for optimizing renal MRA and CTA, assesses the advantages and limitations of MRA and CTA, and provides the current indications for renal vascular imaging including renal artery stenosis screening. New and future developments in these rapidly evolving techniques are also discussed.

Renal Magnetic Resonance Angiography at 3.0 Tesla Using a 32-Element Phased-Array Coil System and Parallel Imaging in 2 Directions

PURPOSE

The aim of the present study was to assess the feasibility of renal magnetic resonance angiography at 3.0 T using a phased-array coil system with 32-coil elements. Specifically, high parallel imaging factors were used for an increased spatial resolution and anatomic coverage of the whole abdomen.

MATERIALS AND METHODS

Signal-to-noise values and the g-factor distribution of the 32 element coil were examined in phantom studies for the magnetic resonance angiography (MRA) sequence. Eleven volunteers (6 men, median age of 30.0 years) were examined on a 3.0-T MR scanner (Magnetom Trio, Siemens Medical Solutions, Malvern, PA) using a 32-element phased-array coil (prototype from In vivo Corp.). Contrast-enhanced 3D-MRA (TR 2.95 milliseconds, TE 1.12 milliseconds, flip angle 25-30 degrees , bandwidth 650 Hz/pixel) was acquired with integrated generalized autocalibrating partially parallel acquisition (GRAPPA), in both phase- and slice-encoding direction. Images were assessed by 2 independent observers with regard to image quality, noise and presence of artifacts.

RESULTS

Signal-to-noise levels of 22.2 +/- 22.0 and 57.9 +/- 49.0 were measured with (GRAPPAx6) and without parallel-imaging, respectively. The mean g-factor of the 32-element coil for GRAPPA with an acceleration of 3 and 2 in the phase-encoding and slice-encoding direction, respectively, was 1.61. High image quality was found in 9 of 11 volunteers (2.6 +/- 0.8) with good overall interobserver agreement (k = 0.87). Relatively low image quality with higher noise levels were encountered in 2 volunteers.

CONCLUSION

MRA at 3.0 T using a 32-element phased-array coil is feasible in healthy volunteers. High diagnostic image quality and extended anatomic coverage could be achieved with application of high parallel imaging factors.

Technology insight: Clinical role of magnetic resonance angiography in the diagnosis and management of renal artery stenosis

Noninvasive modalities, including duplex ultrasonography, renal scintigraphy, CT angiography and magnetic resonance angiography (MRA), may usefully contribute to diagnosis and treatment planning in patients with suspected renal artery stenosis. Important technical developments have increased the accuracy and feasibility of MRA for the detection of renal artery stenosis. A number of different MRA techniques can be applied to the study of renal arteries, but contrast-enhanced MRA represents the most valuable approach; several studies corroborate the high diagnostic accuracy of this technique, especially for the detection of atherosclerotic renal artery stenosis. A combined MRA protocol, which might include angiographic information provided by contrast-enhanced technique in addition to renal flow information derived from phase-contrast imaging, could help in classifying patients appropriately. Limitations of renal MRA include low accuracy in the evaluation of renal fibromuscular dysplasia and in the assessment of patients who undergo stenting of the renal arteries. This review describes the MRA techniques applied to the study of renal artery stenosis, including the technical features of current approaches and forthcoming developments. An overview of the clinical role of MRA, in conjunction with the other diagnostic modalities, in the identification and management of patients with renal artery stenosis, is also presented.

Urine Attenuation Ratio: A New CT Indicator of Renal Artery Stenosis

OBJECTIVE

The purpose of our study was to evaluate the value of a ratio of the attenuation measurements of urine in each kidney (hereafter referred to as the urine CT attenuation ratio) in the detection and lateralization of significant renal artery stenosis (RAS).

SUBJECTS AND METHODS

In 33 patients with suspected renovascular hypertension and 43 normotensive patients, 5-mm-thick transverse CT scans of the kidney area were obtained 4 min after helical CT angiography (CTA). The attenuation of urine in each kidney was measured, and its ratio was calculated. All 76 patients underwent intraarterial digital subtraction angiography within 2 days after the CT examination. The results of angiography were correlated with the urine attenuation ratio of both kidneys.

RESULTS

Twenty-six patients showed significant RAS: unilaterally in 20 and bilaterally in six patients. Two patients showed moderate stenosis of renal arteries. The other patients with essential hypertension (n = 5) or normal blood pressure (n = 43) showed normal renal arteries. The CT attenuation of urine in each kidney was measured and its ratio calculated in all patients except four patients with unilateral RAS. The urine CT attenuation ratio in 22 patients with significant RAS ranged from 1.11 to 4.76 (mean, 2.07). The two patients with moderate RAS showed ratios of 1.83 and 1.23. The others (n = 48) had a urine CT attenuation ratio that ranged from 1.00 to 1.54 (mean, 1.07). The difference of the ratio between the RAS group and the normal group was statistically significant (p < 0.01). The mean urine CT attenuation ratio was 2.18 in patients with unilateral RAS (n = 16) and 1.75 in patients with bilateral RAS (n = 6). The difference of the ratio between the two groups was not statistically significant (p = 0.16). At a cutoff value of 1.22, the sensitivity, specificity, positive predictive value, and negative predictive value of the urine CT attenuation ratio in the diagnosis of significant RAS were 95%, 96%, 91%, and 98%, respectively.

CONCLUSION

The urine CT attenuation ratio is a simple and reliable indicator with which to detect and lateralize significant RAS and is a useful adjunct to helical CTA.

Angiografia por ressonância magnética na avaliação das artérias renais: achados de imagem

OBJETIVO: Descrever as indicações, os principais diagnósticos e os achados de imagem nas angiografias por ressonância magnética das artérias renais. MATERIAIS E MÉTODOS: Estudo retrospectivo, no período de 6/12/2001 a 11/3/2004, num total de 56 exames, totalizando 111 artérias renais estudadas. Os exames foram realizados em um equipamento de 1,5 tesla, segundo o protocolo do Serviço. RESULTADOS: Foi demonstrado que 55,4% (n = 31) pacientes eram masculinos e 44,6% (n = 25), femininos. O paciente mais novo tinha 12 anos e o mais velho, 88 anos. De um total de 25 diferentes indicações, a hipertensão arterial sistêmica com 26,7% (n = 15) foi a principal, seguida de dor abdominal e/ou lombar com 12,5% (n = 7), aneurisma da aorta abdominal com 10,7% (n = 6), estenose da artéria renal com 8,9% (n = 5), e outros. No que se refere às imagens, 43 (76,7%) exames tiveram algum tipo de alteração e 13 (23,2%) foram normais. Dentre os que tinham alterações, a maioria se deu no calibre, e dentre elas, as irregularidades parietais, aneurismas e estenoses foram as mais comuns. Na artéria renal direita as alterações mais comuns foram as irregularidades parietais com 17,87% (n = 10), e na artéria renal esquerda foram as estenoses com 25,45% (n = 14). CONCLUSÃO: A angio-RM mostrou-se excelente método no estudo das artérias renais, devido à sua sensibilidade e capacidade multiplanar para avaliar as estruturas vasculares.

MRA of abdominal vessels: technical advances

Magnetic resonance angiography (MRA) in general and MRA of the abdominal vessels in particular have undergone substantial improvements in the past 5 years triggered by the introduction and application of parallel imaging (PI), new sequence techniques such as centric k-space trajectories and undersampling, dedicated contrast agents and clinical high-field scanners. All of these techniques have the potential to improve image quality and resolution or decrease the image acquisition time. However, each of them has its own specific advantages and drawbacks. This review describes the main technical innovations and focuses on the impact these developments may have on abdominal MRA. Special consideration is given to the interaction of these various technical advances. The clinical value of advanced MRA techniques is discussed and illustrated by characteristic cases.