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Published online before print February 12, 2008, 10.1148/radiol.2471070565
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(Radiology 2008;247:228-240.)
© RSNA, 2008

Technical Developments

Isotropic High-Spatial-Resolution Contrast-enhanced 3.0-T MR Angiography in Patients Suspected of Having Renal Artery Stenosis1

Ulrich Kramer, MD, Jakub Wiskirchen, MD, Michael C. Fenchel, MD, Achim Seeger, MD, Gerhard Laub, PhD, Gunnar Tepe, MD, J. Paul Finn, MD, Claus D. Claussen, MD, and Stephan Miller, MD

1 From the Department of Diagnostic Radiology, University of Tuebingen, Hoppe-Seyler-Str 3, 72076 Tuebingen, Germany (U.K., J.W., M.C.F., A.S., G.T., C.D.C., S.M.); and Siemens Cardiovascular Center (G.L.) and Department of Radiological Sciences (J.P.F.), University of California Los Angeles, Los Angeles, Calif. Received March 28, 2007; revision requested May 23; revision received August 7; accepted August 28; final version accepted September 28. Address correspondence to U.K. (e-mail: ulrich.kramer{at}med.uni-tuebingen.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
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.

© RSNA, 2008


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
Renovascular hypertension has been recognized as an important cause of renal insufficiency and is believed to account for the refractory hypertension in about 1%–3% of patients with secondary hypertension (1–3). Timely diagnosis and treatment of this condition are important because renovascular hypertension is a progressive disease that results in end-stage renal disease (4). However, renal artery stenosis (RAS) is potentially curable with use of endovascular procedures, so the detection and precise quantification of RAS are important in treatment planning (5–8).

Contrast material–enhanced renal magnetic resonance (MR) angiography was established as a routine clinical imaging technique during the past decade and has become a widely accepted noninvasive test for the detection of RAS in patients with renovascular hypertension (8–13). Contrast-enhanced MR angiography is widely accepted as a safe and highly accurate technique compared with intraarterial catheter-based digital subtraction angiography (DSA), the traditional reference-standard examination. Several studies have revealed MR angiography to be comparable to computed tomographic (CT) angiography and superior to ultrasonography (US) (2,6,9,14). Although CT angiography is faster, is more cost-effective, and has the advantage of depicting the extent of atheromatous calcification, radiation exposure and the requirement for nephrotoxic contrast media are associated drawbacks. Because of the risk of nephrotoxicity from the iodinated contrast media used for DSA or CT angiography in patients with renal insufficiency, MR angiography has become the diagnostic modality of choice for patients suspected of having renovascular hypertension. Nevertheless, recent recognition of an epidemiologic association between the administration of gadolinium-based MR contrast agents and the development of nephrogenic systemic fibrosis has prompted caution in using these agents in the setting of renal disease (15,16).

Despite the convincing results of contrast-enhanced 1.5-T renal MR angiography in the assessment of RAS (1,3,9,17), reliable detection—of distal stenosis in particular—remains a challenge owing to the limited spatial resolution at 1.5 T (3). With the increasing availability of clinical MR units that operate at higher magnetic field strengths—3 T in particular—for routine abdominal MR angiographic applications, the resultant approximately doubled signal-to-noise ratio can be used to increase spatial resolution and overcome this limitation (11,18,19). To visualize vascular detail such as luminal narrowing, even in the distal or intrarenal part of the renal artery, high spatially resolved isotropic three-dimensional (3D) data sets are needed to avoid geometric distortions during image postprocessing and ensure accurate delineation of the vasculature. With these data sets, further improvements in diagnostic accuracy may be expected. High-field-strength MR angiography now enables the acquisition of 3D volumes during one breath hold of less than 20 seconds to image the renal arteries, with an isotropic spatial resolution of 1 mm3 or lower (19,20). Furthermore, by using parallel imaging techniques–which means acquiring only a fraction of the k-space lines and using coil sensitivity profiles to calculate the missing data—one can adjust the sequence parameters (eg, repetition time and flip angle) such that an optimal signal-to-noise ratio can be achieved without exceeding specific absorption rate limits (21–24). Thus, the purpose of our study was to prospectively evaluate the diagnostic performance of contrast-enhanced MR angiography performed at 3 T for assessment of RAS by using parallel data acquisition techniques with high acceleration factors (generalized autocalibrating partially parallel acquisition factor of three) and with DSA as the reference standard.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
An employee of Siemens Medical Solutions (Erlangen, Germany) (G.L.) assisted in the development of the described MR sequence and in the design of the imaging protocol. The other authors had full control of the data and information submitted for publication.

Patients and Reference-Standard Examination
Our study was approved by the institutional review board of the University of Tuebingen, and written informed consent was obtained from all patients. Between September 2005 and October 2006, a total of 29 consecutive patients (11 women, 18 men; age range, 20–82 years; mean age, 57.4 years ± 14.4 [standard deviation]) were prospectively enrolled in our study. In 15 of these patients (nine women, six men; age range, 41–82 years; mean age, 61.7 years ± 11.7), DSA was also performed because of a clinical indication or because contrast-enhanced MR angiographic results suggested the presence of an intrinsic vascular abnormality that required further assessment. DSA served as the reference standard in these patients. All patients were referred from the internal medicine outpatient department. Inclusion criteria for MR angiography were suspicion of RAS based on long-standing refractory hypertension (systolic blood pressure > 160 mm Hg), abnormal US findings, or failed US examination (due to obesity or intestinal gas interference); age between 18 and 85 years; and no history of allergic reaction to MR contrast media. Patients with contraindications to MR imaging, such as pacemakers or known claustrophobia, were excluded.

MR Imaging Technique
MR imaging was performed by using a 3-T whole-body system (Magnetom Trio; Siemens Medical Solutions) equipped with total imaging matrix coil systems. Parallel imaging capabilities were used in combination with phased-array surface coils (six elements). The gradient system operates with a maximum gradient strength of 45 mT/m and a maximum slew rate of 200 (mT · m–1)/msec. The phased-array coil was placed on the patient to encompass the vasculature from the proximal abdominal aorta to the level of the inguinal ligaments, including the renal and pelvic arteries. A 20–22-gauge intravenous catheter was inserted into an antecubital vein before the patient was positioned for imaging. Breath-hold acquisitions were performed at end expiration.

A two-dimensional fast low-angle shot MR sequence in the transverse and coronal orientations was used to acquire localizer images that depicted the abdominal aorta and the kidneys. After the individual circulation time was determined by using a coronal test bolus sequence—performed with a temporal resolution of 1.0 second and 2 mL of contrast material (gadopentetate dimeglumine, Magnevist; Bayer Healthcare, Berlin, Germany) administered at 2.0 mL/sec and followed by 30 mL of saline—the 3D MR angiographic volume was prescribed in a coronal orientation or, for better anatomic coverage during a reasonable breath hold, in a coronal-oblique orientation to encompass the course of the renal arteries from the aorta to the hilar region of the kidneys. Imaging volume positioning was performed in a sagittal plane and verified on additional transverse sections. The mean delay in the arrival of contrast material was 22 seconds ± 3 (standard deviation) (range, 17–32 seconds).

For high-spatial-resolution MR angiography, a centrically reordered 3D spoiled gradient-recalled-echo sequence (2.97/1.29 [repetition time msec/echo time msec], 23° flip angle) was performed after the administration of 0.1 mmol of gadopentetate dimeglumine per kilogram of body weight followed by 30 mL of saline (both at 2.0 mL/sec) with an automated power injector (MR Spectris Solaris; Medrad, Pittsburgh, Pa). Parallel imaging was performed with a generalized autocalibrating partially parallel acquisition algorithm, which is based on autocalibrating simultaneous acquisition of spatial harmonics and parallel acquisition (20,21). Depending on the patient's characteristics (eg, size and weight), the field of view ranged from 380 to 420 mm. Therefore, an in-plane resolution of at least 1.0 x 0.8 mm and a section thickness of 1.25 mm (interpolated to 1.0 mm) yielded a voxel volume of 1.0 mm3. An asymmetric k-space sampling scheme (partial Fourier factor, 6/8) and zero interpolation were applied to minimize the acquisition time (Table 1). MR angiographic volume acquisitions were repeated in the venous and equilibrium phases during separate breath holds. To reduce the signal intensity of the background tissue, a 3D data set that served as the mask image data and was subtracted from the arterial phase was obtained before the contrast material administration. If possible, the patient's arms were positioned over the head to facilitate a reduced field of view without causing wraparound artifacts.


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  		Table 1. Sequence Parameters for High-Spatial-Resolution T1-weighted Gradient-Echo MR Angiography

 
The sequence parameters that had to be changed during the examination to stay within specific absorption rate limits (eg, flip angle) were noted. In addition, for statistical analysis, the total examination time, individual circulation time, and age, sex, weight, body mass index, and serum creatinine level for all patients were recorded at the time of contrast-enhanced MR angiography.

DSA Examination and Image Interpretation
All DSA and interventional procedures were performed by two experienced interventionalists (J.W., G.T., 6 and 8 years experience in interventional radiology, respectively). Both interventionalists were aware of the MR angiography findings. DSA was performed by using an Axiom Artis system (Siemens Medical Solutions) equipped with a 40-cm image intensifier. Aortography was performed in the anteroposterior projection (occasionally complemented with a left anterior oblique projection) by using a 4-F pigtail or straight catheter (Cordis, Johnson and Johnson, New Brunswick, NJ). Then, a 4-F C2 cobra catheter (Cordis, Johnson and Johnson) was used to selectively examine each renal artery, with injection volumes (6–10 mL) and rates (2–5 mL/sec) varying according to vessel size. Digital subtraction angiograms were obtained to visually locate and count the renal arteries. Selective DSA depicted the renal artery and the distal vasculature. The minimal diameter was compared with the diameter of the vessel immediately proximal or distal to the stenosis. If this was not possible, the minimal diameter was compared with the diameter of the corresponding segment of the nonstenotic contralateral renal artery.

In six of the 15 patients who were also examined at DSA, pressure gradient measurements (PGMs) were performed by means of simultaneous blood pressure measurements in the 7-F introducer (45-cm RDC; Cordis, Johnson and Johnson) and in the 4-F catheter coaxially placed distal to the RAS to judge the significance of stenosis. Absolute values of systolic, diastolic, and mean blood pressure for the aorta and each renal artery were recorded. The gradient was calculated as the difference between the aortic peak systolic blood pressure and the renal artery peak systolic blood pressure. A pressure gradient of less than 15 mm Hg was considered to rule out significant stenosis (22).

In 11 (73%) of the 15 patients, MR angiography and DSA were performed within 24 hours of each other. In the remaining four (27%) patients, the examinations were performed within 3–13 days of each other. For all 15 patients, the mean time interval between the two examinations was 3.0 days ± 3.8 (standard deviation) (range, 0–13 days). The mean total examination time for contrast-enhanced MR angiography, including patient positioning, was 30 minutes ± 5. The mean total examination time for DSA, including PGM (in six patients), percutaneous transluminal angioplasty, and/or stent placement, was approximately 45 minutes ± 20.

Postprocessing of MR Data
After the data acquisitions, image processing was performed at a 3D workstation (Leonardo; Siemens Medical Solutions) by using a full-thickness rotational maximum intensity projection (MIP) algorithm. In addition, multiplanar reconstructions in transverse and sagittal planes or targeted (subvolume) MIPs were generated for evaluation of proximal renal artery disease. Because these image formats are most commonly used to detect RAS (4), there was no objective to perform a comparative analysis of the different postprocessing algorithms, such as volume rendering, in this study. The study coordinator (U.K.), who had 5 years experience in MR imaging and 3D reconstruction techniques but was not involved in the subsequent image analysis, performed all reconstructions. If RAS was doubtful in selected cases, additional targeted multiplanar reconstructions were generated perpendicular to the course of the renal artery. The postprocessing time was approximately 15 minutes per patient.

Image Analysis
MR angiographic and DSA data were independently reviewed off line by two experienced MR radiologists (M.C.F., S.M., 5 and 12 years experience, respectively). Postprocessed source and MIP MR angiograms were randomly evaluated. Both observers were blinded to the clinical data and to the results of the other imaging modalities and of the other reader when they analyzed MR angiographic data. For research purposes, the two radiologists performed the image readings separately. The image findings were interpreted for clinical care by an experienced vascular radiologist who was not involved in this study.

An advanced software tool (VesselView, Siemens Medical Solutions) performed quantification of vessel area stenosis by using the center-line technique. Dedicated 3D volume rendering was performed at a separate workstation (Vitrea; Vital Images, Plymouth, Minn). The observers were instructed to use postprocessed data in a first step, with source images and subtracted data available on request. Presence of hemodynamically significant RAS, visibility of the main segmental branches, and number of renal artery variants (ie, accessory renal artery, early branching) were noted; the left and right renal arteries were analyzed separately. Finally, the location of the RAS, the extent of stenosis, and the length of the vessel wall irregularity were recorded.

Each renal artery was analyzed for the presence of stenosis, which was graded on the basis of the most severe reduction in arterial diameter as compared with the diameter of an uninvolved renal artery segment proximal or distal to the stenosis. A renal artery was judged to be normal (grade 0), mildly stenotic (1%–49%, grade 1), moderately stenotic (50%–75%, grade 2), severely stenotic (76%–99%, grade 3), or occluded (100%, grade 4). A stenosis of 50% or greater was considered to be hemodynamically relevant. To avoid overestimation owing to eccentric stenosis, vessel narrowing had to be confirmed in two projections.

Semiquantitative Analysis
Coronal and transverse source images, as well as MIP images, were analyzed according to an ordinal four-point scale. General criteria included the impression of overall image quality, which was assessed as follows: Grade 1 indicated unsatisfactory vessel visibility, with a barely visible renal artery and inadequate evaluation; grade 2, average visibility, with compromised evaluation; grade 3, good visibility enabling adequate evaluation; and grade 4, very good visibility enabling detailed evaluation. Similarly, the presence, type, and severity of artifacts, such as ringing of the parenchyma or motion-induced blurring of the renal artery, venous overlay, inadequate bolus timing, and parallel imaging reconstruction artifacts, were recorded for each patient, with grade 0 indicating no artifacts; grade 1, artifacts that did not impair the diagnostic value; and grade 2, artifacts that impaired image reading. Overall reader confidence in the diagnosis of RAS was scored on a three-point scale, with a score of 1 indicating poor confidence; score 2, moderate confidence; and score 3, certainty. A standard form was used to collect all relevant data.

Statistical Analyses
The sensitivity, specificity, and predictive values of MR angiography as a diagnostic test for RAS were calculated on a per-artery (ie, the ability to correctly identify all stenotic arteries) basis by using cutoff levels of 49% stenosis and 75% stenosis to evaluate capability in the detection of various grades of stenosis. Accuracy, positive predictive, and negative predictive values were calculated. Comparative analysis was performed in 15 patients by using DSA as the reference standard.

Cohen {kappa} analysis was used to test agreement between the two observers regarding the MR angiographic findings. {kappa} Values were calculated in terms of the classification of the different stenosis grades (grades 0–4) and the presence (grades 2, 3, or 4) or absence (grades 0 and 1) of hemodynamically significant stenosis. To calculate sensitivity, specificity, and accuracy, the stenoses detected with MR angiography were categorized as absent (grade 0), mild (grade 1), moderate (grade 2), or severe (grade 3 or 4). For statistical analysis, the study population was then subdivided into group 1, comprising patients without significant stenosis (grade 0–1), and group 2 (grade 2–4), comprising patients with significant stenosis. MR angiographic estimates of mild or moderate stenosis (cutoff level, 49%) were considered to be negative results, and MR angiographic estimates of severe stenosis (cutoff level, 75%) were considered to be positive results. DSA stenosis estimates of greater than 75% were considered to be hemodynamically significant and were classified as positive results, whereas DSA stenosis estimates of 75% or less were classified as negative results.

{kappa} Statistics were used to assess agreement regarding image quality and vascular delineation. Interobserver agreement was considered to be slight ({kappa} = 0.20), fair ({kappa} = 0.21–0.40), moderate ({kappa} = 0.41–0.60), substantial ({kappa} = 0.61–0.80), or good ({kappa} = 0.81–1.00). No power analysis was performed. P < .05 indicated a significant difference. Statistical tests were performed by using JMP software (JMP Discovery Software 4.0.5; SAS Institute, Cary, NC).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
All MR angiographic examinations were considered to be technically sufficient for analysis; no examination had to be repeated. No adverse reactions or complications occurred during or after DSA and MR angiography.

Patient Characteristics
The patients with DSA-proved severe RAS (group 1) were significantly (P < .05) older (mean age, 61.7 years ± 11.7 [standard deviation]; age range, 41–82 years) than the patients without signs of RAS (group 2) (mean age, 52.9 years ± 15.6; age range, 20–81 years). The mean weights for groups 1 and 2 were 74.3 kg ± 7.6 and 84.8 kg ± 14.7, respectively. In group 1, six (40%) patients were men, and nine (60%) were women (Table 2); one of these patients (patient 14) had both adrenal adenoma, which was considered the leading diagnosis, and moderate left RAS. There was no significant difference in body mass index (BMI) between the group 1 (mean BMI, 26.0 kg/m2 ± 1.9) and group 2 (mean BMI, 27.3 kg/m2 ± 3.6; P = .33) patients. There also was no significant difference in age or body weight between the male and female patients. The serum creatinine level was significantly elevated in the patients referred for DSA (mean creatinine levels: 1.2 mg/dL ± 0.5 for group 1 vs 0.8 mg/dL ± 0.2 for group 2, P = .04).


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  		Table 2. Demographic and Clinical Characteristics of the Study Population

 
Imaging Findings
A total of 69 renal arteries—57 main and 12 accessory renal arteries—in 29 patients were evaluated with MR angiography (Fig 1). One patient had undergone nephrectomy owing to renal cell carcinoma several years ago. Three patients had accessory renal arteries on both sides, and each of six patients had one accessory renal artery on the left (n = 4) or right (n = 2) side. The origin of the renal arteries, as well as the course of the main renal arteries, was outlined clearly on all images. In all patients, the proximal parts of the renal artery were free of motion artifacts and were of diagnostic quality (Fig 2). In 14 (48%) of the 29 patients, no renovascular abnormality was found at MR angiography (Table 2). The remaining 15 patients were referred for DSA to confirm the MR angiographic findings and were treated with percutaneous transluminal angioplasty and/or stent placement (Fig 3). Thirty-four renal arteries—29 main and five accessory renal arteries—in 15 patients were also evaluated at DSA. MR angiography correctly identified all 29 (100%) main renal arteries and all five (100%) accessory renal arteries depicted at DSA. No additional renal artery that was not detected at DSA was detected by using MR angiography.


Figure 1
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  		Figure 1: Flowchart of the study design. Twenty-nine patients suspected of having renovascular disease were examined at contrast-enhanced MR angiography (CEMRA). Fifteen of these 29 patients were referred for DSA to confirm the MR angiographic findings and were treated with percutaneous transluminal angioplasty and/or stent placement.

 

Figure 2
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  		Figure 2: Three-dimensional volume-rendered images in, A, anterior, B, posterior, C, right anterior oblique, and, D, left anterior oblique projections derived from contrast-enhanced coronal MR angiographic data set in 63-year-old man with arterial hypertension. Accessory renal arteries (arrows) are seen on both sides. High spatial resolution is mandatory owing to the complex vascular anatomy.

 

Figure 3
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  		Figure 3: Multiplanar reformatted, A, transverse, and, B, C, cross-sectional source images from contrast-enhanced MR angiographic data set (2.9/1.14, 23° flip angle), and, D–F, corresponding DSA images obtained in 62-year-old woman. A, D, E, An atherosclerotic lesion is seen in the proximal right renal artery. Note the absence of stenosis in the left renal artery. Because of the eccentric location of the stenosis, the degree of narrowing could be assessed more precisely by using cross-sectional reformations perpendicular to the axis of the left renal artery (white lines in A) in the normal vessel segment (C) distal to the stenosis and in the stenotic segment (B). The degree of stenosis was confirmed on additional views (not shown). B, Region of interest 1 represents the outer contour of the vessel wall, and region of interest 2 is the perfused vessel lumen. C, The perfused vessel lumen (region 1) is circled. On the basis of the calculated areas within and distal to the stenosis, the cross-sectional area stenosis can be calculated. D, Nonselective DSA image in left anterior oblique projection, and, E, selective DSA image in right anterior oblique projection show the in-plane degree of stenosis to be only 44%. F, On the basis of the area stenosis calculation and PGMs (not shown), this patient was treated with stent placement.

 
Evaluation of RAS
In 11 (32%) of the 34 renal arteries that were also evaluated at DSA, no stenosis was found at either DSA or MR angiography. DSA revealed two accessory renal artery stenoses, which were also detected with MR angiography (Fig 4). Nine (26%) of the 34 renal arteries were judged to be mildly stenotic (grade 1), seven (21%) were judged to be moderately stenotic (grade 2), and the remaining seven (21%) were judged to be severely stenotic (grade 3). Both observers correctly identified all significant stenoses and occlusions (grades 3 and 4, respectively) on the MR angiograms (Table 3). However, observer 1 made two overestimations of a severe stenosis (grade 3) and one overestimation of a moderate stenosis (grade 2) and observer 2 made one underestimation of a mild stenosis (grade 1) and one overestimation of a moderate stenosis (grade 2). Linear regression analysis of all renal arteries revealed a significant correlation between the mean stenosis severity measured at DSA and that assessed at MR angiography (r2 = 0.86, P < .01). There was disagreement between the observers regarding the stenosis categorization of five (15%) of the 34 renal arteries.


Figure 4
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  		Figure 4: A–C, Contrast-enhanced MR angiograms (2.9/1.14, 19° flip angle), and, D–F, corresponding intraarterial DSA images obtained in 65-year-old woman. A, MIP image, and, B, multiplanar reconstruction source images show a high-grade stenosis of the lower left accessory renal artery (arrows). Note the delayed parenchymal enhancement (arrows) on the volume-rendered angiogram (C) and DSA image (F), indicating the potential hemodynamic significance of the lesion. G–I, By using an advanced software tool (VesselView), cross-sectional area measurements were performed to quantify stenosis degree: An area stenosis of 69% (minimal vessel area, 4.6 mm2; normal vessel area, 14.7 mm2) was calculated. G, Anterior three-dimensional volume-rendered image shows vessel path (green line) between the stenosis of the proximal part of the renal artery and the more distal part (middle third) of the left renal artery; this path was automatically generated by the software. Degree of stenosis was calculated on the basis of the area stenosis represented by the highest degree of luminal narrowing (red circle) and the normal vessel area in a nonaffected vessel segment (blue ring). Lettered cubes were automatically generated to represent anterior (A) and head (H) views. H, Cross-sectional, and, I, curved multiplanar reformations perpendicular to (H) and along (I) axis of left renal artery.

 

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  		Table 3. RAS Categories Assigned at DSA and Contrast-enhanced MR Angiography

 
All stenoses were located in the proximal third of the renal artery (Fig 5). The overall prevalence of 50% or greater stenosis was 41% (14 of 34 arteries). No lesions were found in the more distal parts of the renal arteries at either MR angiography or DSA.


Figure 5
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  		Figure 5: A, 3D coronal MR angiogram (2.9/1.14, 23° flip angle) obtained in 58-year-old man with arterial hypertension and suspected of having RAS shows stenosis (arrow) in proximal part of right renal artery; the stricture was correctly detected by both observers. B, Corresponding frontal DSA findings confirm extension of the vessel wall irregularity and length of the stenosis (arrow).

 
In 14 (93%) of the 15 patients examined with both MR angiography and DSA, hemodynamically significant RAS was proved with intraarterial PGM (n = 6) and/or DSA (n = 14) and was consequently treated by means of stent (Palmaz Genesis; Johnson and Johnson) placement. PGMs and corresponding DSA-proved degrees of stenosis are detailed in Table 4. No PGM was obtained when high-grade RAS (Fig 6) had been proved at DSA or guidewire placement was difficult. In one patient, percutaneous transluminal angioplasty was performed exclusively as a consequence of a mildly stenotic renal artery.


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  		Table 4. PGM Values

 

Figure 6
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  		Figure 6: A, B, Coronal full-thickness MIPs (2.9/1.14, 23° flip angle), C, curved coronal multiplanar reconstruction, and, D, targeted transverse multiplanar reconstruction reveal high-grade stenosis (arrow) of the main left renal artery in a 50-year-old woman. E–G, Findings on selective intraarterial DSA images in posteroanterior projection confirm the diagnosis. This patient was successfully treated with stent placement.

 
The overall sensitivity and specificity of contrast-enhanced MR angiography, as compared with DSA, for the detection of significant luminal changes (>75% stenosis) of the renal arteries were 94% (95% confidence interval: 68%, 98%) and 96% (95% confidence interval: 4%, 100%), respectively. The accuracy, positive predictive value, and negative predictive value of MR angiography are reported in Table 5. Interobserver agreement regarding the presence or absence of RAS was good: {kappa} values were 0.96 and 0.92 for DSA and MR angiographic findings, respectively.


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  		Table 5. Detectability of Moderate and Severe RAS at Contrast-enhanced MR Angiography

 
Image Quality
At 28 (97%) of the 29 MR angiographic examinations, all main renal branches, including the distal third of the renal arteries and the segmental branches, were of diagnostic quality (grades 3 and 4). For 26 patients, no venous and/or renal pelvic overlay was observed on the MR angiograms. In one (3%) patient, both observers rated the overall image quality to be grade 2 owing to inadequate bolus timing and motion artifacts caused by incomplete breath holding.

For presence and severity of image artifacts (graded on a three-point scale), observers 1 and 2 assigned a score of 0 (no artifacts) in 21 patients, a score of 1 (artifacts did not impair diagnostic value) in eight patients, and a score of 2 (artifacts impaired image reading) in no patients. There was no significant difference in artifact ratings between the two observers (P = .68); the {kappa} value for overall interobserver agreement was 0.84. Observer confidence in the diagnosis of significant RAS was high, with average confidence scores of 2.9 and 2.7 for observers 1 and 2, respectively.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 
In our study, the accuracy of 3D breath-hold contrast-enhanced 3-T MR angiography for the diagnosis of RAS was evaluated. There was good agreement between the MR angiographic and DSA grades of RAS. MR angiography at 3 T was shown to be highly sensitive and specific for the diagnosis of RAS. MR angiographic findings incorrectly suggested the presence of severe (76%–99%) RAS in only one renal artery, rendering a specificity of 96%. However, the negative predictive value was 98% and the sensitivity was 94%; these values are comparable to those in previous studies (12).

In general, contrast-enhanced MR angiography performed at 3 T offers several advantages over MR angiography at 1.5 T (the standard field strength), including increased signal-to-noise and contrast-to-noise ratios. Because the T1 of the contrast agent remains nearly constant at 3 T, prolonged T1 in background tissue leads to an increased contrast-to-noise ratio between contrast-enhanced vascular structures and the surrounding background tissue on T1-weighted images. This phenomenon reflects one of the most important benefits of high-field-strength MR angiography. In addition, the signal-to-noise ratio scales approximately linear with the magnetic field strength (18). Consequently, the higher signal-to-noise ratio at 3 T can be used to reduce the acquisition time or improve the spatial resolution (18,25–28).

Specific absorption rate limits might be a disadvantage of 3-T MR imaging because of the large deposit of energy in the patient during the examination. To avoid specific absorption rate limits in our study population, we had to adapt the flip angle (19°–23°) in 17 (59%) patients; however, this resulted in no substantial degradation of image quality. Second, inhomogeneity of the radiofrequency magnetic induction field caused by dielectric resonances is another potential source of image degradation at 3-T abdominal MR imaging, but this did not affect the performance of renal MR angiography in our study. Third, T2* dephasing can cause signal loss, which was another initial concern with contrast-enhanced MR angiography performed at a higher field strength. In the present study, however, we observed no T2* dephasing–related effects that compromised image quality. Although, to our knowledge, there are no available data on the T2* effects at abdominal MR angiography, it has been reported that the signal losses caused by T2* dephasing at 3-T imaging appear to be comparable to those that occur at 1.5-T imaging (29).

Owing to the improved signal-to-noise ratio at 3 T, we chose a high acceleration factor for parallel imaging to realize an acquisition time of less than 20 seconds and simultaneously achieve high spatial resolution. Consequently, depiction of the distal parts of the renal artery, as well as overall image quality, was improved, with a result of consecutively higher diagnostic confidence in detecting renovascular disease. Although we observed no stenoses in the distal parts of the renal arteries in our patient population, there have been several reports that improved spatial resolution is mandatory for the diagnosis of fibromuscular dysplasia (30), a far less frequent cause of renovascular hypertension. Moreover, multiple studies have revealed that one major limitation of 1.5-T MR angiography is the lack of capability to depict the distal main renal artery, mainly because of signal loss related to respiratory motion (31). By using parallel imaging techniques, the imaging duration can be reduced to overcome this limitation. In the same way, reducing the acquisition time may enable patients with limited breath-hold ability to comply with breath-holding requirements and therefore facilitate MR angiography with high diagnostic image quality.

With use of the described imaging protocol, high-spatial-resolution data sets with a voxel size of 1 mm3 or better can be acquired. Although this capability has improved visualization of the renal arteries, a more important effect is that isotropic data sets can now be reformatted in any imaging plane, and, therefore, vessel areas at sites of stenosis perpendicular to the course of the renal artery can be assessed. Schoenberg et al (32) recently found that even an eccentric area stenosis can be routinely measured with satisfactory precision. Using this approach, as compared with using traditional methods of assessing luminal diameter, was found to markedly improve RAS grading.

Nonetheless, most contrast-enhanced MR angiographic techniques still yield an in-plane resolution of approximately 1.5 mm or poorer and a voxel volume greater than 5 mm3 (1,5,10,17,33,34). This disparity in imaging parameters is reflected in the great variability of the sensitivities and specificities achieved when grading RAS with 1.5-T MR angiography. On the basis of the results of a meta-analysis of the detection and grading of RAS several years ago, Vasbinder et al (17) concluded that MR angiography and CT angiography are the best noninvasive screening modalities, with reasonable sensitivities and specificities. However, in a more recent multicenter trial (35), Vasbinder et al found that MR angiography was not suitable for ruling out RAS, with poor sensitivity, specificity, and interobserver agreement being responsible for the disappointing results. Eklöf et al (5) found MR angiography to have high sensitivity (96%) but moderate specificity (75%) when compared with DSA and PGM.

Another meta-analysis, performed by Tan et al (31) and involving 998 patients from 25 studies, revealed that contrast-enhanced MR angiography, as compared with DSA, had a high average sensitivity of 97% and an average specificity of 93%. When the MR angiographic examination was focused on the assessment of stenosis in accessory renal arteries, it failed to depict hemodynamically significant vascular abnormalities and even accessory renal arteries (12). The depiction of segmental or accessory renal arteries was impaired owing to limited spatial resolution and/or random motion-induced artifacts (3). In summary, several studies have revealed that the diagnostic accuracy of contrast-enhanced 1.5-T MR angiography, as compared with DSA, is affected by the overestimation of RAS, which results in 25%–32% of findings being false-positive (36). To our knowledge, all data published so far have been obtained at 1.5-T imaging.

In our opinion, these data can be further improved by using higher field strengths. At 3 T, visualization of the renal arteries was consistent, and the achieved spatial resolution might be adequate to depict even abnormalities in the distal parts or segmental arteries. Parenchymal overlay or random motion in the distal branches of the renal artery might compromise imaging to prevent depiction of the intrarenal branches at MR angiography, at least in some cases (37,38).

Our study was limited by the small number of patients (n = 15) examined with DSA for comparative image analysis. However, DSA was performed in only those patients with severe arterial stenosis as a part of the therapeutic intervention. This bias might have been partially responsible for the high sensitivity and specificity values achieved in our study. These values might be different in broader clinical settings. Nevertheless, the acceptance of any imaging modality is influenced not only by its accuracy but also by interobserver variability among different observers. In this respect, contrary to published 1.5-T imaging data, high interobserver agreement as a result of reproducible high image quality was observed.

Owing to its high sensitivity, contrast-enhanced renal MR angiography performed at 3 T can be used to detect RAS and thus has potential as a screening method in the diagnostic work-up of patients with arterial hypertension. With parallel imaging techniques, the measurement time can be reduced and high isotropic spatial resolution can be achieved without corruption of the signal yield.


   ADVANCE IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE
 References
 


   IMPLICATION FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
References
 


   FOOTNOTES
 

Abbreviations: DSA = digital subtraction angiography • MIP = maximum intensity projection • PGM = pressure gradient measurement • RAS = renal artery stenosis • 3D = three-dimensional

See Materials and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, U.K., J.W., S.M.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, U.K., S.M.; clinical studies, U.K., J.W., M.C.F., A.S., G.T.; statistical analysis, U.K., M.C.F.; and manuscript editing, M.C.F., G.L., J.P.F., C.D.C., S.M.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCE IN KNOWLEDGE
 IMPLICATION FOR PATIENT CARE
 References
 

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