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DOI:10.2214/AJR.06.1138
AJR 2007; 188:W540-W546
© American Roentgen Ray Society

Original Research

Navigator-Gated MR Angiography of the Renal Arteries: A Potential Screening Tool for Renal Artery Stenosis

Jeffrey H. Maki1, Gregory J. Wilson2, William B. Eubank1, David J. Glickerman1, Juan A. Millan3 and Romhild M. Hoogeveen4

1 Department of Radiology (S113), University of Washington, Puget Sound VA Health Care System, 1660 S Colombian Way, Seattle, WA 98108.
2 UW Medicine at Lake Union–Vascular Imaging Lab, Seattle, WA.
3 Radia Inc., Everett, WA.
4 Philips Medical Systems, Cleveland, OH.

Received August 25, 2006; accepted after revision December 20, 2006.

 
Address correspondence to J. H. Maki (jamki{at}u.washington.edu).

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Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to determine how well unenhanced navigatorgated steady-state free precession (Nav SSFP) MR angiography (MRA) performs as a screening test for the detection of renal artery stenosis.

SUBJECTS AND METHODS. Forty patients referred to rule out renal artery stenosis were imaged using an optimized Nav SSFP MRA sequence before conventional contrast-enhanced MRA (CE-MRA). Two radiologists evaluated Nav SSFP for maximum stenosis measurement, and comparison was made with CE-MRA results.

RESULTS. Fifteen of the 40 patients had greater than 50% renal artery stenosis as determined on CE-MRA. Sensitivity for detecting renal artery stenosis with Nav SSFP was 100%; specificity, 84%; negative predictive value, 100%; and positive predictive value, 79%. The average mean stenosis difference between Nav SSFP and CE-MRA was 10% ± 9%.

CONCLUSION. Sensitivity and negative predictive value for the detection of renal artery stenosis using Nav SSFP were perfect, with an acceptable specificity of 84%. This suggests Nav SSFP is a promising technique for simple unenhanced screening for the detection of renal artery stenosis.

Keywords: kidney • MR angiography • MR technique • renal artery • renal disease • screening


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Renal artery stenosis is an important cause of morbidity and mortality and the number one cause of secondary hypertension; it accounts for up to 14% of the cases of ischemic renal disease [1] and 12–14% of new patients starting dialysis [2]. Most renal artery disease is secondary to atherosclerosis, most often in older patients with multiple cardiovascular risk factors and worsening of established hypertension or deterioration of renal function. A smaller fraction of renal artery stenosis is due to fibromuscular dysplasia [3]. However, renal artery stenosis is often clinically occult, as can be inferred from autopsy studies, in which a large necropsy study showed significant stenoses in 42% of patients older than 75 years [4]. Although generalized autopsy studies indicate only a small prevalence of renal artery stenosis (4%), the incidence increases dramatically (to as much as 43%) with comorbidities such diabetes mellitus, hypertension, abdominal aneurysm, and peripheral vascular disease [5–7]. Because the timely detection and treatment of renal artery stenosis can cure or improve hypertension and preserve renal function, there is definitely a role for an accurate and inexpensive screening test.

Although conventional digital subtraction angiography remains the gold standard for the diagnosis of renal artery stenosis, it is typically not used as a screening study because of its invasiveness and complication rate [8]. Of the less invasive alternatives, contrast-enhanced MR angiography (CE-MRA) has emerged as the most promising technique for evaluating the renal arteries [9–12], with a recent meta-analysis [13] showing it performs significantly better than sonography, captopril renal scintigraphy, or the captopril test. This same study showed CE-MRA and CT angiography (CTA) to be roughly equivalent, although CTA has the distinct disadvantages of ionizing radiation and nephrotoxic contrast material. Although Doppler sonography is usually considered the least expensive screening test, examination times are lengthy, the procedure is operator-dependent, and no anatomic vascular data are obtained. In addition, nondiagnostic examinations are common, with a recent study showing Doppler sonography failed to visualize the renal arteries in 22% of patients [14].

Because MRA is evolving as the preferred technique for detecting renal artery stenosis, consideration must be given to ways of minimizing examination time, discomfort, and cost in order to improve accessibility to the potentially large screening population. As is typical of screening populations, most screened individuals do not have renal artery stenosis. Two large studies [15, 16] both showed an approximately 20% prevalence of angiographically proven renal artery stenosis among appropriately screened patients by strictly applying standard clinically accepted clues (e.g., bruit; other vascular occlusive disease; malignant, accelerating, or sudden worsening of hypertension; unilateral small kidney; hypertension with unexplained impairment of renal function; hypertension in a child or young adult or refractory to a three-drug regimen; and impairment of renal function in response to an angiotensin-converting enzyme inhibitor) for the presence of renal artery stenosis [17].

Furthermore, a recent epidemiologic study developed a complex model of incremental cost-effectiveness ratio per life year gained to examine the cost-effectiveness of multiple imaging strategies (including digital subtraction angiography and MRA) for renal artery stenosis screening [18]. This group concluded that MRA (as it was in 1998) is only cost-effective if the pretest probability is at least 20% and the economic environment is such that MRA is approximately 70% the cost of digital subtraction angiography. This was not true at that time in the study country (The Netherlands, 1998; MRA, 141% the cost of digital subtraction angiography), and remains untrue at our academic teaching hospital (2006; MRA, 112% the cost of digital subtraction angiography). Thus, on the basis of these results, reduction in the cost of MR screening could make MRA more widely acceptable as a screening tool.

This article examines a recently introduced technique of unenhanced renal artery MRA using steady-state free precession (SSFP) [19–21], focusing primarily on whether it can be effectively applied to rapidly and accurately screen for renal artery stenosis without the need for the additional expense and potential discomfort of injecting MR contrast media and performing CE-MRA. We hypothesize that available SSFP techniques are of sufficient reliability and quality that they can be used to screen out the approximate 80% of normal renal arteries making up a "rule out renal artery stenosis" referral base, eliminating the need to administer contrast material and perform further imaging. Therefore, SSFP has the potential to be an excellent unenhanced screening tool for renal artery stenosis.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Forty consecutive patients (all men; 33–83 years old; median age, 67 years) at a Veterans Affairs Medical Center who had been referred for clinical renal MRA to rule out renal artery stenosis were enrolled. Twenty of these patients were previously reported in a feasibility study [22]. The study was approved by the institutional review board. All imaging was performed on a Philips Medical Systems 1.5-T Intera scanner with Master gradients (Philips Medical Systems) (30 mT/m amplitude, 150 mT/m/ms slew rate) running release 8.

The renal arteries were first localized using a fast water-selective SSFP coronal scout. After this, each patient underwent two unenhanced SSFP scans, the first a breath-hold, and the second a free-breathing navigator-gated sequence (Nav SSFP) using a pencil-beam navigator placed through the dome of the liver. Choice of these sequences was based on a previous analysis of different renal artery SSFP techniques performed as a precursor to this study, details of which are described elsewhere [22]. Relevant SSFP scanning parameters are shown in Table 1. All SSFP scanning was performed with saturation bands placed inferiorly and over the kidneys to suppress inferior vena cava and renal vein signals [19]. Navigator acceptance window was 5 mm. After SSFP imaging, each patient underwent routine 3D contrast-enhanced renal MRA (CE-MRA) using 20 mL of gadolinium chelate injected at 3 mL/s followed by a 25-mL saline flush at 2.5 mL/s. CE-MRA scanning parameters are also shown in Table 1. All pulse sequences were standard, commercially available clinical products.


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  		TABLE 1: Imaging Parameters for Breath-Hold Steady-State Free Precession (SSFP), Navigator (Nav) SSFP, and Contrast-Enhanced MR Angiography (CE-MRA) Studies

 

Two radiologists who were blinded to the other examination and the results of the other reviewer scored the degree of stenosis for all main and accessory renal arteries in the Nav SSFP and the CE-MRA examinations. In two cases in which the Nav SSFP sequence was not obtained, breath-hold SSFP was reviewed in lieu of the Nav SSFP. Analysis was performed on an offline workstation (ViewForum, Philips Medical Systems) equipped with multiplanar reformatting of source images and maximum intensity projections (MIPs). Degree of stenosis was determined on the offline workstation by measuring stenotic and normal distal renal artery diameters with electronic calipers having an accuracy of ± 0.1 mm and calculating maximal percentage of reduction in normal vessel diameter. A standard form was used to collect all relevant data.

Kappa statistics were used to evaluate interobserver variability with the following interpretation: poor, < 0.4; good, ≥ 0.4 but < 0.75; excellent, ≥ 0.75. Reviewer data were averaged, and statistical analysis of stenosis data was performed by stratifying each renal artery into positive or negative for stenosis using a 50% threshold for CE-MRA and a 45% threshold for SSFP. A nonparametric Spearman's rank correlation coefficient was also calculated.


Figure 1
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  		Fig. 1A —68-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show concordance for normal right renal artery (0% on CE-MRA, 11% on Nav SSFP MR angiography) and high-grade left renal artery stenosis (arrows) of 95% on CE-MRA and 82% on Nav SSFP. Note that there appears to be more than 11% stenosis on Nav SSFP in right renal artery (A) secondary to MIP artifact from overlapping signal in inferior vena cava. Stenosis measurements were obtained from thin-slice reformatted images rather than from MIPs.

 


Figure 2
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  		Fig. 1B —68-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show concordance for normal right renal artery (0% on CE-MRA, 11% on Nav SSFP MR angiography) and high-grade left renal artery stenosis (arrows) of 95% on CE-MRA and 82% on Nav SSFP. Note that there appears to be more than 11% stenosis on Nav SSFP in right renal artery (A) secondary to MIP artifact from overlapping signal in inferior vena cava. Stenosis measurements were obtained from thin-slice reformatted images rather than from MIPs.

 


Figure 3
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  		Fig. 1C —68-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show concordance for normal right renal artery (0% on CE-MRA, 11% on Nav SSFP MR angiography) and high-grade left renal artery stenosis (arrows) of 95% on CE-MRA and 82% on Nav SSFP. Note that there appears to be more than 11% stenosis on Nav SSFP in right renal artery (A) secondary to MIP artifact from overlapping signal in inferior vena cava. Stenosis measurements were obtained from thin-slice reformatted images rather than from MIPs.

 


Figure 4
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  		Fig. 1D —68-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show concordance for normal right renal artery (0% on CE-MRA, 11% on Nav SSFP MR angiography) and high-grade left renal artery stenosis (arrows) of 95% on CE-MRA and 82% on Nav SSFP. Note that there appears to be more than 11% stenosis on Nav SSFP in right renal artery (A) secondary to MIP artifact from overlapping signal in inferior vena cava. Stenosis measurements were obtained from thin-slice reformatted images rather than from MIPs.

 

Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The CE-MRA and breath-hold SSFP sequences were completed in all patients, and the Nav SSFP sequence was successful in 38 of the 40 patients. Examples of Nav SSFP and CE-MRA sequences are shown in Figures 1A, 1B, 1C, 1D, 2A, 2B, 2C, 2D, 3A, 3B, 3C, 3D and in the supplemental online Figures S1 and S2. In one patient, the Nav SSFP sequence was not attempted because of severe patient anxiety and the need to minimize scanning time. In a second patient, Nav SSFP failed due to technical problems in which the navigator would not consistently trigger despite being replanned several times. Navigator efficiency ranged from approximately 20% to 50%, meaning total Nav SSFP acquisition time ranged from approximately 3 to 8 minutes.


Figure 5
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  		Fig. 2A —47-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show agreement for nondiseased renal arteries. Note excellent depiction of small anterior right accessory renal artery on Nav SSFP (white arrow, A–D). Axial Nav SSFP MIP (C) is targeted to show this accessory renal artery and does not show main renal arteries. An inferior left accessory (black arrow, B) was missed on SSFP because it was out of imaging volume.

 

Figure 6
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  		Fig. 2B —47-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show agreement for nondiseased renal arteries. Note excellent depiction of small anterior right accessory renal artery on Nav SSFP (white arrow, A–D). Axial Nav SSFP MIP (C) is targeted to show this accessory renal artery and does not show main renal arteries. An inferior left accessory (black arrow, B) was missed on SSFP because it was out of imaging volume.

 

Figure 7
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  		Fig. 2C —47-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show agreement for nondiseased renal arteries. Note excellent depiction of small anterior right accessory renal artery on Nav SSFP (white arrow, A–D). Axial Nav SSFP MIP (C) is targeted to show this accessory renal artery and does not show main renal arteries. An inferior left accessory (black arrow, B) was missed on SSFP because it was out of imaging volume.

 

Figure 8
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  		Fig. 2D —47-year-old man with suspected renovascular hypertension. Coronal and axial subvolume maximum intensity projections (MIPs) from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A and C) and contrast-enhanced MR angiography (CE-MRA) (B and D) show agreement for nondiseased renal arteries. Note excellent depiction of small anterior right accessory renal artery on Nav SSFP (white arrow, A–D). Axial Nav SSFP MIP (C) is targeted to show this accessory renal artery and does not show main renal arteries. An inferior left accessory (black arrow, B) was missed on SSFP because it was out of imaging volume.

 

Figure 9
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  		Fig. 3A —33-year-old man with suspected renovascular hypertension. Coronal subvolume maximum intensity projections from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A) and contrast-enhanced MR angiography (CE-MRA) (B) with digital subtraction angiography correlation show injection of both aorta (C) and accessory (D) renal arteries. Nav SSFP and CE-MRA agreed that both main arteries were nondiseased. Arrows represent the accessory artery, which was seen and thought to be diseased on both Nav SSFP and CE-MRA. Accessory artery was believed to represent intimal fibroplasia (an atypical form of fibromuscular dysplasia) and to be responsible for patient's hypertension.

 

Figure 10
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  		Fig. 3B —33-year-old man with suspected renovascular hypertension. Coronal subvolume maximum intensity projections from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A) and contrast-enhanced MR angiography (CE-MRA) (B) with digital subtraction angiography correlation show injection of both aorta (C) and accessory (D) renal arteries. Nav SSFP and CE-MRA agreed that both main arteries were nondiseased. Arrows represent the accessory artery, which was seen and thought to be diseased on both Nav SSFP and CE-MRA. Accessory artery was believed to represent intimal fibroplasia (an atypical form of fibromuscular dysplasia) and to be responsible for patient's hypertension.

 

Figure 11
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  		Fig. 3C —33-year-old man with suspected renovascular hypertension. Coronal subvolume maximum intensity projections from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A) and contrast-enhanced MR angiography (CE-MRA) (B) with digital subtraction angiography correlation show injection of both aorta (C) and accessory (D) renal arteries. Nav SSFP and CE-MRA agreed that both main arteries were nondiseased. Arrows represent the accessory artery, which was seen and thought to be diseased on both Nav SSFP and CE-MRA. Accessory artery was believed to represent intimal fibroplasia (an atypical form of fibromuscular dysplasia) and to be responsible for patient's hypertension.

 

Figure 12
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  		Fig. 3D —33-year-old man with suspected renovascular hypertension. Coronal subvolume maximum intensity projections from navigator-gated steady-state free precession (Nav SSFP) MR angiography (A) and contrast-enhanced MR angiography (CE-MRA) (B) with digital subtraction angiography correlation show injection of both aorta (C) and accessory (D) renal arteries. Nav SSFP and CE-MRA agreed that both main arteries were nondiseased. Arrows represent the accessory artery, which was seen and thought to be diseased on both Nav SSFP and CE-MRA. Accessory artery was believed to represent intimal fibroplasia (an atypical form of fibromuscular dysplasia) and to be responsible for patient's hypertension.

 
Based on CE-MRA, three of the forty patients had codominant main renal arteries, providing a total of 83 main renal arteries for analysis. All main renal arteries were detected on SSFP. A total of 20 accessory renal arteries were seen on CE-MRA, 18 (90%) of which were detected with SSFP.

For the purposes of analysis, a renal artery stenosis of greater than 50% on CE-MRA was considered positive. Using these criteria, 15 (38%) of the 40 patient studies were positive (bilateral renal artery stenosis in five patients and unilateral renal artery stenosis in 10 patients), giving a grand total of 20 (24%) of 83 stenotic main renal arteries. A receiver operating characteristic (ROC) curve was constructed that showed 100% sensitivity when using a Nav SSFP stenosis threshold of 45% (Fig. 4).


Figure 13
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  		Fig. 4 —Receiver operating characteristics curve produced by using different positive stenosis thresholds for navigator-gated steady-state free precession (Nav SSFP) MR angiography. As can be seen, sensitivity of 100% was achieved using Nav SSFP stenosis threshold of 45%.

 
Using this cutoff point, sensitivity and specificity were 100% and 84% and 100% and 82% measured per patient and per renal artery, respectively. Confidence intervals were 75–99% and 59–92% per patient and 81–97% and 68–89% per renal artery, respectively. Positive and negative predictive values were 79% and 100% on a per-patient basis and 66% and 100% on a per-renal-artery basis. The nonparametric Spearman's rank correlation coefficient was 0.88, indicating excellent agreement between Nav SSFP and CE-MRA. The average absolute value of the measured stenosis difference between CE-MRA and Nav SSFP was 10% ± 9%. Examining only the Nav SSFP false-positive renal arteries (n = 11), the difference was 17% ± 6%. If a Nav SSFP threshold of 50% were chosen to match the CE-MRA threshold, sensitivity and specificity decrease to 87% and 84% and 90% and 84% per patient and per renal artery, respectively. This was due to two arteries, one measuring 49.5% on Nav SSFP and 79% on CE-MRA, and one measuring 47% on Nav SSFP and 54% on CE-MRA.

Stratifying stenosis into positive or negative (positive being > 50% for CE-MRA and > 45% for Nav SSFP), interobserver agreement for CE-MRA was excellent, with a simple kappa value of 0.77 (CI, 0.91–0.93). For Nav SSFP, the kappa statistic was 0.70 (CI, 0.54–0.86), indicating very good interobserver agreement.


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Although literature exists [22] clearly indicating SSFP is not on par with CE-MRA in terms of overall image quality, the navigator SSFP renal MRA protocol nonetheless performed extremely well in the context of a surrogate for the commonly used screening technique CE-MRA, with excellent agreement as characterized by a Spearman's rank correlation coefficient of 0.88. Nav SSFP detected all significant main renal artery stenoses (i.e., sensitivity, 100%) and had a negative predictive value of 100%. Furthermore, looking at disease on a per-patient basis (which is probably most relevant), specificity was 84%, with a positive predictive value of 79%. On a per-renal-artery basis, these values were 82% and 66%, respectively. These results indicate all patients negative for renal artery stenosis by Nav SSFP were in fact negative, and those classified as stenotic by Nav SSFP had a 79% chance of truly being positive.

On a per-patient basis for our 40 patients, 15 (38%) had significant stenoses, all of which were accurately detected by Nav SSFP. Of the remaining 25 patients, 21 were correctly called negative, and four were incorrectly classified as positive by Nav SSFP. Thus, had Nav SSFP been used to screen this particular population, 19 (48%) of the 40 would have been called positive and proceeded to further workup (consisting in this case of CE-MRA, of which 15 of 19, or 79%, would have been confirmed positive). The remaining 21 (53%) would not have required any further evaluation, thus saving the expense and added complication of gadolinium administration without missing any cases of renal artery stenosis.

Successful application of SSFP to the renal arteries is relatively new, with several recent articles showing the tremendous potential of this technique [19, 22–26]. A recent animal study [23] showed high accuracy comparing 2D SSFP techniques with digital subtraction angiography in a pig model. In recent clinical studies, Coenegrachts et al. [19] used parameters similar to our breath-hold SSFP to evaluate 25 patients using digital subtraction angiography as the gold standard. Their group reported results similar to ours, with sensitivity, specificity, and positive and negative predictive values of 100%, 98%, 80%, and 100%, respectively (renal artery stenosis prevalence, 22%). Katoh et al. [24] used a more sophisticated variant of navigator-gated SSFP with cardiac triggering and a slice-selective inversion prepulse. This small study was performed on 16 subjects and compared with CE-MRA. The authors state that all low-grade (n =3) and high-grade (n = 2) stenoses were verified by CE-MRA or digital subtraction angiography, implying sensitivity and negative predictive value were 100%. A recent expansion of this work compared SSFP MRA with digital subtraction angiography in 30 patients, 11 of whom were positive for renal artery stenosis [25]. That study used an SSFP threshold of 50% as positive and showed a 100% sensitivity, with specificities of 95% and 90% and accuracies of 92% and 96% for two reviewers. The most recent study, by Herborn et al. [26], compared breath-hold 3D SSFP with CE-MRA in 21 patients. Although that study was limited in that there were only two high-grade stenoses, both of those were accurately detected (sensitivity, 100%), and specificity was reported to be 81%.

Thus, our work, augmented by these similar studies, strongly suggests SSFP can be used to determine, at least as well as CE-MRA, the presence or absence of significant renal artery stenosis, thereby dismissing most screening patients who do not have disease while accurately identifying the minority with probable stenoses. The latter can immediately (in the same MRA examination) proceed with more definitive MRI, including gadolinium administration for CE-MRA. This approach has the potential to cut costs, eliminating the expense of contrast material and associated tubing sets and supplies in most cases.

In addition, depending on how efficiently an institution performs the CE-MRA examination and whether they perform additional imaging such as phase-contrast studies, Nav SSFP has the further potential to decrease the overall examination time. As an example, given a typical screening population prevalence of renal artery stenosis of 20% [15, 16], 80% of the screening population have nonstenotic renal arteries. Assuming our specificity of 84% is maintained, this translates to approximately 13% being incorrectly called positive; therefore, contrast administration and further imaging would occur in the true-positive 20% plus the false-positive 13%, or a total of 33% of the screening population. This directly translates to a 67% savings in contrast-associated costs. The issue of not using gadolinium may become even more relevant when evaluating patients with significant renal impairment because of recent concerns regarding the relationship between gadolinium use in patients with renal failure and nephrogenic systemic fibrosis [27].

Following the lead of some institutions that now offer low-cost limited cardiac MR screening, we propose that (after further validation), a limited lower-cost renal artery screening SSFP examination could be offered, with a more conventional fee charged for patients in whom contrast-enhanced studies and further pulse sequences are necessary. Although a careful cost analysis would need to be performed to corroborate this concept, such a practice would have the potential to make MRI more economically feasible as a widely accepted screening study for renal artery stenosis.

As implemented by our group, the Nav SSFP study requires only a rapid 30-second scout scan, a 30-second coronal SSFP renal artery localization scan, and then 3–8 minutes for the Nav SSFP (depending on respiratory rate and navigator efficiency). Including setup time, total on-magnet time is only 10–15 minutes. Furthermore, when a navigator is used, the patient need not breath-hold, increasing compliance and effectiveness in respiratory-compromised or sedated patients. As a bonus (depending on how one wished to set up the protocol), invasiveness might decrease, because most patients will not require an IV line or injection. Practicalities such as whether to preemptively place an IV catheter in all patients (using valuable magnet time) or only place one when necessary have yet to be determined, and could be well addressed by performing a time–cost analysis of the technique (which we did not do). As an added benefit, in positive cases in which CE-MRA is performed, the Nav SSFP allows excellent visualization of the renal arteries, which in turn aids in accurately and concisely planning the CE-MRA.

One generalized concern when screening for renal artery stenosis with MRA is the evaluation of accessory renal arteries; it is therefore important to analyze how well SSFP characterizes these (usually) small arteries. In our study, 20 accessory renal arteries were documented with CE-MRA, all but two (90%) of which were seen with Nav SSFP. No formal attempt was made to grade accessory renal artery stenosis. One missed accessory artery was moderately large and had an apparent moderate degree of proximal stenosis. The second was a small vessel just inferior to the Nav SSFP imaging slab (Fig. 2A, 2B, 2C, 2D). Although it occurred only once in this study (5%), accessories will not be completely visualized with SSFP if the origin arises outside the relatively small axially oriented SSFP imaging volume. Thus, because accessory renal arteries can be missed, the issue becomes whether this is clinically relevant. According to a recent study of 68 patients undergoing digital subtraction angiography for suspected renovascular hypertension [28], a hemodynamically significant accessory renal artery stenosis unaccompanied by a main renal artery stenosis occurred in less than 2% of patients. Furthermore, in patients with main renal artery stenosis discovered at screening, percutaneous intervention is usually warranted, and therefore a stenotic accessory renal artery would be identified at the time of digital subtraction angiography. Bude et al. [28] concluded that failure to detect accessory renal arteries should not negatively affect the usefulness of a noninvasive test for detecting renovascular hypertension, and we concur with this view.

Our study has several limitations. First, there was no correlation with conventional digital subtraction angiography. The standard used in this study was CE-MRA, and although it is known to be an imperfect standard, nonetheless it is an often-performed screening test for renal artery stenosis; therefore, comparing how well Nav SSFP performs with respect to CE-MRA is relevant and justified. Second, Nav SSFP is not always successful. On occasion, the navigator did not track well and had to be repositioned, adding a couple of minutes to the study. Only one of the 40 Nav SSFP sequences failed completely due to a technical navigator problem (and a second Nav SSFP was not attempted). In these two cases, we used the breath-hold SSFP data in lieu of the Nav SSFP data. Although the breath-hold SSFP data are not as high-quality as Nav SSFP data, Coenegrachts et al. [19] found essentially the same sequence to be effective, and thus we think using breath-hold SSFP as a backup in cases of failed Nav SSFP is justified.

Third, no patients with fibromuscular dysplasia (FMD) involving the main renal artery were included in this study, and it remains unclear how well Nav SSFP performs for detecting FMD. Anecdotally, we saw a small diseased accessory renal artery in one young patient that led to angiography (Fig. 3A, 3B, 3C, 3D), which showed normal bilateral main renal arteries (agreeing with SSFP and CE-MRA) and a diseased right accessory that was thought to represent focal intimal fibroplasia. This isolated case is encouraging as to the level of detail that can be achieved with SSFP. Because we are discussing Nav SSFP as a potential screening technique, it is important to consider the challenges of FMD. FMD tends to occur in young women and only infrequently causes hypertension or renal failure [29]. Because it typically involves the mid and distal renal arteries, FMD likely will not be well seen with Nav SSFP (as can be seen by the decrease in vessel image quality for more distal vessel segments [Fig. 2A, 2B, 2C, 2D]). On the other hand, FMD is frequently missed on CE-MRA as well, likely because of inadequate resolution and motion-related blurring in the distal renal arteries [15, 30]. Therefore, because of the rarity of symptomatic FMD in men older than 50 years, we believe FMD in men would only rarely be missed if we restricted Nav SSFP screening to those over age 50. Even if this leads to missing the occasional older woman with FMD, it would likely be acceptable as a screening examination. Therefore, we suggest Nav SSFP not be used exclusively to screen patients younger than 50 years or for any individuals suspected of having FMD. Further evaluation of Nav SSFP for the evaluation of FMD patients is clearly required.

Fourth, the screening concept being explored here requires a qualified observer to examine the SSFP data in real time to determine whether a stenosis is present, thus mandating further imaging and contrast administration. This should ideally be performed by an MR radiologist, although this is probably impractical and cost-prohibitive. With adequate training, this can likely be performed by an MR technologist, because very good Nav SSFP interobserver agreement ({kappa}, 0.70) suggests making this determination in real-time should not be too difficult. Finally, our study was relatively small (40 patients) and was performed in a single institution using a single MR platform. Clearly, a larger study is required to validate the concept of using Nav SSFP as a screening technique for renal artery stenosis, although the two other similarly sized SSFP studies discussed previously [19, 25] lend credible support to the apparent excellent screening potential of SSFP.

In conclusion, navigator SSFP performed perfectly in terms of sensitivity and negative predictive value (both 100%) for detecting significant renal artery stenosis as classified by CE-MRA. The positive predictive value of 79% is quite acceptable. These results are in keeping with several other recent studies examining SSFP for detecting renal artery stenosis, suggesting Nav SSFP can be used to save cost, contrast administration, and possibly time when used to screen for renal artery stenosis. Furthermore, because it is a navigated technique, it is applicable to patients who cannot or will not breath-hold.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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