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Renal Artery Assessment with Nonenhanced Steady-State Free Precession versus Contrast-enhanced MR Angiography¹

¹ From the Departments of Radiology (R.W., A.B., M.A., L.B., P.S.), Internal Medicine (C.M., A.G., M.D.V.), and Radiotherapy (L.C.), Ospedale San Giovanni Bellinzona (EOC), CH-6500 Bellinzona, Switzerland; Department of Radiology, University of Aachen, Aachen, Germany (M.K.); Philips Medical Systems, Zürich, Switzerland (M.W.); University of Zürich and Institute of Biomedical Engineering, Eidgenössische Technische Hochschule, Zürich, Switzerland (M.W.); and Department of Radiology, Inselspital, University of Bern, Bern, Switzerland (P.V.). Received October 13, 2006; revision requested December 7; revision received December 20; final version accepted January 23, 2007. R.W. supported by a grant from the Swiss Heart Foundation. Address correspondence to R.W. (e-mail: rolf.wyttenbach{at}bluewin.ch).

	ABSTRACT

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

 
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 statistics were used to estimate interobserver agreement.

Results: One hundred eight renal arteries with 20 significant (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.

© RSNA, 2007

	INTRODUCTION

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

 
Three-dimensional (3D) contrast material–enhanced magnetic resonance (MR) angiography is widely considered the method of choice for noninvasive evaluation of renal artery stenosis (RAS), with greater than 90% sensitivity and specificity for the detection of RAS involving 50% or greater luminal narrowing reported in numerous publications (1–5). Gadolinium-based MR contrast agents, which are much less nephrotoxic compared with iodinated radiographic contrast agents, still have a nephrotoxic effect and contribute substantially to the high cost of the examination. At our institution, the costs related to contrast material administration for renal MR angiography account for approximately 20%–30% of the total expense of the examination, depending on the body weight of the patient. Contrast-enhanced MR angiography of the renal arteries requires acquisition of the data during breath holding, which limits the spatial resolution. In addition, the sharp visualization of the renal arteries may be negatively affected by continuous drift or irregular motion of the diaphragm (6) and by motion of the proximal renal arteries during the cardiac cycle (7).

A steady-state free precession (SSFP) technique, which is characterized by high blood signal intensity, has been used for breath-hold MR angiography of the renal arteries (8,9). This sequence allows visualization of the renal arteries without use of contrast material. However, the examination is still dependent on the breath-hold capacity of the patient.

In one study, Katoh et al (10) used a cardiac-triggered navigator-gated 3D SSFP technique in combination with a slab-selective inversion prepulse for selective visualization of the renal arteries in healthy volunteers and a small group of patients. The described imaging strategy enabled visualization of the renal arteries with high contrast and high spatial resolution without the use of contrast material. Thus, the purpose of our study was to prospectively assess the diagnostic accuracy of nonenhanced 3D SSFP MR angiography for the detection of RAS, with contrast-enhanced MR angiography performed as the reference standard.

	MATERIALS AND METHODS

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

 
Patients
For 15 months, 57 consecutive patients (31 male, 26 female; mean age ± standard deviation, 58 years ± 16) who had hypertension and were suspected of having RAS on the basis of clinical history, physical examination results, or Duplex ultrasonographic (US) findings were prospectively enrolled in the study. Each patient gave written informed consent, and the study was approved by the ethics committee of Southern Switzerland (Bellinzona). The patients' clinical characteristics were hypertension (n = 53), history of smoking (n = 20), diabetes mellitus (n = 8), elevated cholesterol level (n = 20), and family history of hypertension or coronary artery disease (n = 29). Both nonenhanced SSFP MR angiography and contrast-enhanced MR angiography were performed during the same imaging session, and digital subtraction angiography (DSA) of the renal arteries was performed in 12 patients a mean of 12 days (range, 1–28 days) after MR angiography.

MR Imaging
All MR angiography examinations were performed by using a 1.5-T whole-body MR system (Intera, release 11; Philips Medical Systems, Best, the Netherlands) with a gradient strength of 30 mT/m and a slew rate of 150 (mT·m^–1)/msec. A four-element phased-array body coil was used for signal reception. A coronal two-dimensional SSFP sequence was used to locate the course of the renal artery. Furthermore, a breath-hold two-dimensional phase-contrast sequence (5/3 [repetition time msec/echo time msec], 20° flip angle, 150 cm/sec velocity-encoding value) was performed to measure aortic blood flow at the level of the renal artery to determine the trigger delay for the subsequent SSFP MR angiography sequence. The optimum trigger delay was defined as the time between the R wave and the end of the main aortic pulse wave (Fig 1). At this point, most of the inflow into the renal artery has taken place, so the amount of inflow between the inversion pulse, which is applied before systole, and the readout pulse during diastole is maximized.

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Schematic illustration of elements for cardiac-triggered navigator-gated SSFP renal MR angiography sequence combined with a slab-selective inversion prepulse. The slab-selective inversion prepulse was applied before the data acquisition and after the aortic pulse wave by adjusting the trigger delay individually after two-dimensional (2D) phase-contrast flow measurement in the abdominal aorta. The inversion delay of 325 msec allowed suppression of the renal parenchyma and renal vein signals. Spectral fat saturation (SPIR) was applied, and use of an inferior rest slab (REST) suppressed the signal from the inferior vena cava. Prospective real-time navigator technology was used to suppress respiratory motion. ECG = electrocardiography, TI = inversion time.

A slab-selective inversion prepulse with an inversion delay of 325 msec was applied to suppress the signals from the renal parenchyma and the renal vein blood. Spectral fat saturation (11) and a saturation band inferior to the imaging volume were used to suppress the signals from fat and the inferior vena cava, respectively. Prospective navigator gating (12) was applied by using a gating window of 5 mm. This sequence can be performed on any Philips MR unit with navigator capability (release 8 or higher).

For contrast-enhanced MR angiography, the imaging delay was determined by using a test bolus: 1 mL of gadobenate dimeglumine (MultiHance; Bracco Diagnostics, Milan, Italy) at 2.2 mL/sec for patients weighing less than 70 kg and 2 mL at 2.5 mL/sec for patients weighing 70 kg or more administered intravenously with an MR-compatible injector (Injektron 82 MRT; Medtron, Saarbrücken, Germany), followed by a 30-mL saline flush at 2.5 mL/sec. Finally, a coronal 3D gradient-recalled-echo sequence was performed after the intravenous administration of gadobenate dimeglumine (0.15 mmol per kilogram of body weight)—at flow rates of 2.2 mL/sec for patients weighing less than 70 kg and 2.5 mL/sec for those weighing 70 kg or more—followed by a 30-mL saline flush at the same injection rate. Because the data acquisition time for contrast-enhanced MR angiography was constant for all patients, we used a higher injection rate in patients with a body weight of greater than 70 kg to avoid prolonging the duration of the bolus.

The following imaging parameters were used for 3D gradient-recalled-echo imaging: 3.8/1.3, 35° flip angle, 450-mm field of view, matrix of 416 x 270, 58 sections, acquired spatial resolution of 1.1 x 1.3 x 2.5 mm (reconstructed voxel size, 0.9 x 0.9 x 1.25 mm), acquisition time of 17.5 seconds, and 434-Hz bandwidth. Randomly segmented central k-space ordering (ie, contrast-enhanced timing-robust angiography, or CENTRA) was used (13). The 450-mm field of view was chosen to avoid fold-over artifacts since parallel imaging was used with a sensitivity-encoding acceleration factor of two. The patient's total in-room time for the MR imaging examination was approximately 35–40 minutes. This included the time for patient setup with positioning of the electrocardiographic leads (5–10 minutes), the nonenhanced MR angiography examination (20–25 minutes), and the contrast-enhanced MR angiography examination (5–10 minutes).

DSA Examination
DSA was performed only (in 12 patients) when it was required for patient clinical management—for example, when the noninvasive examination results were unclear and/or when percutaneous balloon angioplasty or stent placement was needed. Intraarterial DSA was performed by one interventional radiologist (M.A.) with 15 years experience by using a femoral approach with a 4-F Omniflush catheter (AngioDynamics, Queensbury, NY) or 5-F pigtail catheter (PBN Medicals, Stenløse, Denmark) and a conventional angiography unit (Integris V 3000; Philips Medical Systems). For assessment of the aorta and the renal artery, the catheter tip was positioned between the 12th thoracic and first lumbar vertebrae and a power injector was used to inject 15 mL of iodinated contrast material (meglumine ioxaglate, Hexabrix 320; Guerbet, Aulnay-sous-Bois, France) at a flow rate of 15 mL/sec. An anteroposterior view and at least two oblique projections of approximately 15° were obtained.

Postprocessing, Reference Standard, and Image Analysis
All data were transferred to a dedicated postprocessing workstation (View Forum, release 4.1; Philips Medical Systems) for evaluation. One radiologist experienced in 3D reconstruction techniques performed all standardized postprocessing procedures. This radiologist was not involved in the subsequent image analysis and was blinded to the patient data and DSA results. Fifteen maximum intensity projection (MIP) displays of the renal artery were generated around the superoinferior and right-left axes, with each data set ranging from –90° to 90°. In addition, targeted thin-slab MIP reconstructions of the renal artery in coronal and transverse orientations were generated. All reconstructions were performed in an identical manner for nonenhanced SSFP MR angiography and contrast-enhanced MR angiography during separate sessions, and the reconstructed images were stored for subsequent evaluation. Images were also evaluated for the presence of accessory renal arteries.

All nonenhanced SSFP and contrast-enhanced MR angiograms were analyzed independently by two board-certified radiologists with 9 (R.W.) and 8 (A.B.) years experience in MR angiography. Both readers were blinded to the patient data and DSA results. Corresponding SSFP and contrast-enhanced MR angiograms obtained in the same patients were assessed separately in random order, with an interval of at least 4 weeks between the readings. Data analysis was based on MIP reconstruction and source data. The readers were allowed to generate additional reconstructions, if necessary. Contrast-enhanced MR angiograms with differences in RAS grade between the two readers were additionally evaluated by consensus. Contrast-enhanced MR angiogram readings were used as the reference standard for nonenhanced MR angiogram readings.

For analysis, all renal arteries were divided into proximal, middle, and distal segments. The extent of RAS was graded as follows: Grade 1 indicated less than 20% luminal narrowing; grade 2, 20%–49% luminal narrowing; grade 3, 50%–74% luminal narrowing; grade 4, 75%–99% luminal narrowing; and grade 5, occlusion. Stenosis grading was performed by using an electronic caliper to measure the minimal luminal diameter at the optimal projection angle along the vessel axis. The percentage of stenosis was calculated as follows: [1 – (L/R)] x 100, where L and R are the diameters of the lesion and the reference site, respectively. Reference site was defined as the normal-looking portion of the stenotic vessel distal to the lesion. No subjective assessment of stenosis was performed. The presence or absence of accessory renal arteries was assessed, but no attempt to grade stenosis in these vessels was made. Renal arteries with stents were excluded from analysis. The maximal visible renal artery length was measured on the MIP reconstructions with both techniques.

The image quality of the MIPs was graded by using a four-point scale: Grade 1 meant excellent image quality (high homogeneous signal intensity within the vessel lumen, optimal delineation of the vessel border, evaluation possible with high diagnostic confidence); grade 2, good quality (good enhancement of the vessel lumen, incomplete delineation of the vessel border, evaluation possible with satisfactory diagnostic confidence); grade 3, moderate quality (low inhomogeneous signal intensity, incomplete delineation of vessel border, evaluation possible with low diagnostic confidence); and grade 4, nondiagnostic quality (no or minimal enhancement, inadequate for analysis).

The images were also assessed for motion degradation: Grade 1 meant no visible motion degradation; grade 2, minimal motion degradation; grade 3, moderate motion degradation with blurring of the vessel border, but diagnostic; and grade 4, severe motion degradation and nondiagnostic. Enhancement of the renal veins was graded as follows: Grade 1 meant no venous enhancement; grade 2, venous enhancement visible but less than arterial enhancement; grade 3, equivalent venous and arterial enhancement; and grade 4, venous enhancement greater than arterial enhancement. The enhancement data of patients younger than 50 years and those older than 50 years were assessed. DSA findings were evaluated on a picture archiving and communication system (Philips Medical Systems) by an experienced (15 years) vascular radiologist (M.A.) who was blinded to the MR angiography data.

Statistical Analyses
The sensitivity, specificity, accuracy, and negative predictive value (NPV) of nonenhanced SSFP MR angiography as a test for the diagnosis of RAS were calculated by using contrast-enhanced MR angiography as the reference standard on artery-by-artery and patient-by-patient bases. These same parameters were calculated for SSFP MR angiography and contrast-enhanced MR angiography with DSA as the reference standard, when possible. Because greater than 50% stenosis was considered hemodynamically significant, stenosis grades 1 and 2 were considered negative results for RAS and grades 3–5 were considered positive results.

Interobserver agreement regarding accurate determination of the presence or absence of stenosis and stenosis grading was assessed by using the Cohen test, in which a value greater than 0.75 corresponds to excellent agreement and a value of 0.50–0.75 corresponds to good agreement. Subjective blind evaluation of image quality was assessed by computing average scores for each anatomic level. For assessment of image quality, motion artifacts, and venous contamination, mean grades were calculated and the data obtained at nonenhanced SSFP and contrast-enhanced MR angiography were compared. For continuous variables, the paired two-tailed Student t test was applied, and for categorical variables, the marginal homogeneity test was used. Differences were considered statistically significant at P < .05. Statistical analysis was performed with 1998 SPSS, version 8.0 (SPSS, Chicago, Ill), and 2006 StatXact 7.0 (Cytel Software, Cambridge, Mass) software.

	RESULTS

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

 
Contrast-enhanced MR Angiography
Four of the 57 patients were excluded from the study because of claustrophobia (n = 3) or navigator malfunction (n = 1); thus, the contrast-enhanced and nonenhanced MR angiography data of 53 patients were available for comparison (Fig 2). All contrast-enhanced MR angiograms were considered to be technically adequate. One patient had two renal arteries on one side, and another patient had three renal arteries on one side, and all of these vessels were included as main renal arteries for evaluation. One renal artery had to be excluded from analysis because of the presence of a stent. Thus, a total of 108 main renal arteries were evaluated for stenosis, and 26 accessory renal arteries were detected with contrast-enhanced MR angiography. There were 22 main renal arteries with non–hemodynamically significant (20%–49%) stenosis. A total of 20 hemodynamically significant (50%) RAS lesions, including two occluded arteries, were detected in 15 patients.

View larger version (13K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Flowchart of patients with hypertension who were included in the study. MRA = MR angiography.

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 3a: Coronal (a) contrast-enhanced MR angiography (3.8/1.3, 35° flip angle) and (b) nonenhanced SSFP MR angiography (5.4/2.7, 85° flip angle) MIPs in 54-year-old patient with hypertension and normal renal arteries. Note the sharp delineation of the main and peripheral renal arteries in b due to suppression of cardiac and respiratory motion by means of cardiac triggering and navigator gating.

View larger version (42K): [in this window] [in a new window] [Download PPT slide]   Figure 3b: Coronal (a) contrast-enhanced MR angiography (3.8/1.3, 35° flip angle) and (b) nonenhanced SSFP MR angiography (5.4/2.7, 85° flip angle) MIPs in 54-year-old patient with hypertension and normal renal arteries. Note the sharp delineation of the main and peripheral renal arteries in b due to suppression of cardiac and respiratory motion by means of cardiac triggering and navigator gating.

View larger version (88K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 56-year-old patient with 80% stenosis of proximal left renal artery. (b) Transverse and (c) coronal nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient show correct grading of the RAS (arrow), which was confirmed on (d) the DSA image. In b and c, note the substantially decreased signal intensity distal to the RAS, which still enables visualization of the distal left renal artery.

View larger version (32K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 56-year-old patient with 80% stenosis of proximal left renal artery. (b) Transverse and (c) coronal nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient show correct grading of the RAS (arrow), which was confirmed on (d) the DSA image. In b and c, note the substantially decreased signal intensity distal to the RAS, which still enables visualization of the distal left renal artery.

View larger version (32K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 56-year-old patient with 80% stenosis of proximal left renal artery. (b) Transverse and (c) coronal nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient show correct grading of the RAS (arrow), which was confirmed on (d) the DSA image. In b and c, note the substantially decreased signal intensity distal to the RAS, which still enables visualization of the distal left renal artery.

View larger version (96K): [in this window] [in a new window] [Download PPT slide]   Figure 4d: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 56-year-old patient with 80% stenosis of proximal left renal artery. (b) Transverse and (c) coronal nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient show correct grading of the RAS (arrow), which was confirmed on (d) the DSA image. In b and c, note the substantially decreased signal intensity distal to the RAS, which still enables visualization of the distal left renal artery.

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 5a: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 72-year-old patient with 75% stenosis of left renal artery. On (b) coronal and (c) transverse nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient, note the overestimation of the stenosis with a distal signal intensity decrease (arrow). (d) DSA findings confirmed the presence of high-grade stenosis.

View larger version (23K): [in this window] [in a new window] [Download PPT slide]   Figure 5b: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 72-year-old patient with 75% stenosis of left renal artery. On (b) coronal and (c) transverse nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient, note the overestimation of the stenosis with a distal signal intensity decrease (arrow). (d) DSA findings confirmed the presence of high-grade stenosis.

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 5c: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 72-year-old patient with 75% stenosis of left renal artery. On (b) coronal and (c) transverse nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient, note the overestimation of the stenosis with a distal signal intensity decrease (arrow). (d) DSA findings confirmed the presence of high-grade stenosis.

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 5d: (a) Coronal contrast-enhanced MR angiography MIP (3.8/1.3, 35° flip angle) in 72-year-old patient with 75% stenosis of left renal artery. On (b) coronal and (c) transverse nonenhanced SSFP MIPs (5.4/2.7, 85° flip angle) in the same patient, note the overestimation of the stenosis with a distal signal intensity decrease (arrow). (d) DSA findings confirmed the presence of high-grade stenosis.

For reader 1, the image quality of the MIPs obtained at contrast-enhanced MR angiography was significantly better than that of the MIPs obtained at nonenhanced SSFP MR angiography (P < .001); there was a similar but nonsignificant trend for reader 2 (P < .2) (Table 2). The overall image quality with nonenhanced MR angiography, however, was still rated as good to excellent (mean grades: 1.83 [both renal arteries] for reader 1 and 1.67 [right renal artery] and 1.73 [left renal artery] for reader 2) by both observers. At evaluation of motion degradation, contrast-enhanced MR angiography and SSFP MR angiography had comparable results, with the exception of results for the right renal artery, which had fewer motion artifacts at contrast-enhanced MR angiography (P < .001) for reader 1 (P = .11 for difference between the two examinations for reader 2). With both techniques, renal vein enhancement was found to be stronger in younger (<50 years) patients than in older (50 years) patients (P < .01). No significant difference in venous enhancement between SSFP MR angiography and contrast-enhanced MR angiography was found in either age group (P = .5).

Comparisons with DSA
DSA was performed for clinical management in 12 patients with hemodynamically significant RAS at contrast-enhanced MR angiography, in whom 25 renal arteries were available for comparison between contrast-enhanced and nonenhanced SSFP MR angiography. One patient had three renal arteries of equal caliber on one side, all of which were included for evaluation. One renal artery was excluded from evaluation owing to the presence of a stent. DSA revealed 11 significant RAS lesions. Contrast-enhanced MR angiography (Table 3), as compared with DSA in this subgroup of patients with a high prevalence of significant RAS (11 of 25 arteries [44%]), had high sensitivity (91%), specificity (86%), accuracy (88%), and NPV (92%). The sensitivities, accuracies, and NPVs of SSFP MR angiography, as compared with DSA, were 91%, 72%, and 89%, respectively, for reader 1 and 82%, 80%, and 85%, respectively, for reader 2. The specificity of SSFP MR angiography for the detection of significant RAS was 57% for reader 1 and 79% for reader 2. In one case, both contrast-enhanced MR angiography and nonenhanced SSFP MR angiography led to an underestimated grade of a significant stenosis, which was confirmed with DSA, including intraarterial pressure measurements.

	DISCUSSION

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

Our study results show that consistently high-quality images can be obtained at SSFP MR angiography, which enabled visualization of even small branches within the renal parenchyma and subtle accessory renal arteries. The main reasons for the reduced image quality and blurred images in some patients were technical problems with the navigator-based detection of the diaphragm position, which were due to the long distance of the diaphragm from the isocenter. SSFP MR angiography of the renal arteries would therefore benefit from further improvements in the navigator technique.

It should be emphasized that SSFP MR angiography showed excellent concordance with contrast-enhanced MR angiography in the detection of hemodynamically significant (50%) RAS, and only one significant RAS was underestimated with SSFP by one of the readers. However, we observed a tendency of the RAS severity to be overestimated with nonenhanced MR angiography compared with the RAS severity determined with contrast-enhanced MR angiography, with resulting higher RAS grades in several patients. The reason for the overestimated RAS grades was severe signal loss distal to the stenotic lesion, which may have been due to reduced inflow and/or to the intravoxel dephasing often caused by fast or complex blood flow.

Adequate spatial resolution has been shown to be crucial to the accurate assessment of vessel dimensions at MR angiography (17). In our study, we further optimized a previously described SSFP MR angiography technique (10) by increasing the in-plane resolution and reducing the section thickness, which, despite the use of parallel imaging, resulted in a substantially smaller voxel volume compared with that achieved with the contrast-enhanced MR angiography protocol. These improvements in spatial resolution resulted in a reconstructed voxel size at SSFP MR angiography of 0.7 x 0.7 x 1.0 mm within an acceptable imaging time of approximately 4 minutes. One might be able to increase the spatial resolution of SSFP MR angiography even further—but at the cost of longer imaging times—since the data are acquired during free breathing with use of navigator technology.

Other nonenhanced techniques, such as time-of-flight and phase-contrast imaging, have facilitated limited success in the diagnosis of RAS in clinical practice owing to saturation of the signal of inflowing blood and long acquisition times, which lead to blurred images (18,19). Data cited in two more recent publications (8,9) show the feasibility of breath-hold 3D SSFP MR angiography for RAS assessment. Both studies revealed the potential of SSFP MR angiography for assessment of both normal and diseased renal arteries, although the numbers of patients (n = 25, n = 19) and RAS lesions (five and two) in these studies were small. Since breath-hold image acquisition was used in both these studies (8,9), the nonenhanced MR angiography examinations performed were subject to the limitations of breath-hold contrast-enhanced MR angiography, such as unsuitable breath-hold patterns, renal artery motion during the cardiac cycle, and limited spatial resolution due to time constraints.

We realize that in the near future contrast-enhanced MR angiography, with use of newer intravascular contrast agents and with longer intravascular enhancement, might also offer combined cardiac triggering and navigator gating. However, use of these newer agents will probably increase costs beyond the costs of currently used MR contrast agents. Moreover, with intravascular contrast agents, venous contamination may be problematic in the evaluation of RAS.

Our study had limitations. First, our use of contrast-enhanced MR angiography as the reference standard may be regarded as controversial (20). However, results of a meta-analysis showed contrast-enhanced MR angiography to have high accuracy and better performance than other noninvasive tests such as US (21). In another meta-analysis, Tan and colleagues (22) concluded that contrast-enhanced MR angiography may replace arteriography for most patients suspected of having RAS and has major advantages: It is noninvasive, does not involve ionizing irradiation, and involves the use of a minimally nephrotoxic contrast agent. Therefore, we did not perform DSA in patients who had normal arteries or non–hemodynamically significant RAS, because it did not seem ethical to perform an invasive diagnostic procedure after contrast-enhanced MR angiography yielded normal results that excluded significant RAS with high probability. However, we performed DSA in all patients in whom interventional treatment of RAS was an option. In this small subgroup, there was better concordance between DSA and contrast-enhanced MR angiography results than between DSA and SSFP MR angiography results; however, the validity of these data is limited by the relatively small number of renal arteries evaluated and should be further assessed by comparing SSFP MR angiography with DSA in a larger number of patients. DSA by itself may have substantial limitations in terms of accuracy when tortuous vessels and eccentric stenosis are present.

In addition, in our contrast-enhanced MR angiography protocol the spatial resolution was optimized by using parallel imaging and a high-relaxivity contrast agent in combination with elliptic-centric k-space acquisition. However, further reduction of the field of view would have facilitated even higher spatial resolution and thereby increased the risk of fold-over artifacts due to parallel imaging. Therefore, we believe that using contrast-enhanced MR angiography as the reference standard was reasonable. Another limitation was the restricted superoinferior field of view of our SSFP MR angiography sequence due to the use of a transverse orientation of the imaging volume. This probably explains the lower rate of detection of accessory renal arteries, especially polar arteries.

Both nonenhanced SSFP MR angiography and contrast-enhanced MR angiography are subject to the limitations inherent of anatomic imaging in the assessment of RAS. Ideally, the evaluation of RAS should involve combined morphologic and functional imaging techniques, such as cine phase-contrast MR flow measurements, for assessment of both the hemodynamic significance and the morphology of RAS (23). From a statistical standpoint, it should be mentioned that we excluded one patient from analysis owing to failure of the navigator, and this exclusion may have slightly altered the statistical performance of SSFP MR angiography.

In conclusion, compared with contrast-enhanced MR angiography, cardiac-triggered navigator-gated 3D SSFP MR angiography with slab-selective spin inversion has high sensitivity and high NPV in the detection of hemodynamically significant (50%) RAS without use of contrast material. The reproducibility of SSFP MR angiography results obtained by different readers is excellent. Normal SSFP MR angiography results exclude the presence of significant RAS. However, when RAS is present, the stenosis grade determined with nonenhanced SSFP MR angiography tends to be overestimated in some patients. Therefore, a possible algorithm for assessing RAS with MR imaging could start with nonenhanced MR angiography. If the results of this examination are normal, then further examinations might be stopped. If significant RAS is identified, contrast-enhanced MR angiography can be performed during the same imaging session, adding only a few minutes to the imaging time.

	ADVANCE IN KNOWLEDGE

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

 

• Compared with contrast-enhanced MR angiography, slab-selective inversion-prepared three-dimensional steady-state free precession renal MR angiography has 94% accuracy in the detection of renal artery stenosis (RAS) without use of contrast material; however, RAS severity may be overestimated in some patients.
  

	IMPLICATION FOR PATIENT CARE

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

 

• In most patients who undergo screening for renal artery stenosis, nonenhanced steady-state free precession MR angiography results may be sufficient to exclude hemodynamically significant stenosis.
  

	ACKNOWLEDGMENTS

 
The authors thank Romhild M. Hoogeveen, PhD, of Philips Medical Systems for technical assistance and helpful discussions.

	FOOTNOTES

 

Abbreviations: DSA = digital subtraction angiography • MIP = maximum intensity projection • NPV = negative predictive value • RAS = renal artery stenosis • SSFP = steady-state free precession • 3D = three-dimensional

Authors stated no financial relationship to disclose.

Author contributions:Guarantor of integrity of entire study, R.W.; 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, R.W., M.W., L.B., M.D.V., M.K.; clinical studies, R.W., A.B., M.A., L.B., P.S., M.D.V.; statistical analysis, R.W., L.B., L.C.; and manuscript editing, R.W., A.B., M.W., M.A., L.B., L.C., M.K., C.M., P.V., A.G.

	References

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

 

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