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Published online before print September 13, 2002, 10.1148/radiol.2252011366
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(Radiology 2002;225:589-596.)
© RSNA, 2002

Technical Developments

Renal Arteries: Navigator-gated Balanced Fast Field-Echo Projection MR Angiography with Aortic Spin Labeling: Initial Experience1

Elmar Spuentrup, MD, Warren J. Manning, MD, Peter Börnert, PhD, Kraig V. Kissinger, RT, MS, René M. Botnar, PhD and Matthias Stuber, PhD

1 From the Departments of Medicine, Cardiovascular Division (E.S., W.J.M., P.B., K.V.K., R.M.B., M.S.), and Radiology (W.J.M.), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Mass; Department of Diagnostic Radiology, University Hospital, Technical University of Aachen, Pauwelsstrasse 30, 52057 Aachen, Germany (E.S.); Philips Research Laboratory, Hamburg, Germany (P.B.); and Philips Medical Systems, Best, the Netherlands (R.M.B., M.S.). Received August 13, 2001; revision requested October 9; final revision received March 4, 2002; accepted April 3. E.S. supported in part by German Research Council grant SP 634/1-1. W.J.M. supported in part by American Heart Association Established Investigator grant 9740003N. Address correspondence to E.S. (e-mail: spuenti@rad.rwth-aachen.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
A cardiac-triggered free-breathing three-dimensional balanced fast field-echo projection magnetic resonance (MR) angiographic sequence with a two-dimensional pencil-beam aortic labeling pulse was developed for the renal arteries. For data acquisition during free breathing in eight healthy adults and seven consecutive patients with renal artery disease, real-time navigator technology was implemented. This technique allows high-spatial-resolution and high-contrast renal MR angiography and visualization of renal artery stenosis without exogenous contrast agent or breath hold. Initial promising results warrant larger clinical studies.

© RSNA, 2002

Index terms: Magnetic resonance (MR), motion studies, 961.12942, 961.12943, 961.149, 961.721 • Magnetic resonance (MR), three-dimensional, 961.12942, 961.12943, 961.149, 961.721 • Magnetic resonance (MR), vascular studies, 961.12942, 961.12943, 961.149, 961.721 • Renal arteries, MR, 961.12942, 961.12943, 961.149, 961.721 • Renal arteries, stenosis or obstruction, 961.721


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
 Discussion
 REFERENCES
 
Several magnetic resonance (MR) angiographic techniques have been implemented for noninvasive imaging of the renal arteries. Images obtained with sequences that make direct use of the inflow of unsaturated spins for high intraluminal contrast (time-of-flight [1–3] or phase-contrast [4,5] MR angiography) can successfully depict the proximal renal arteries in healthy volunteers. Depiction of more distal segments is limited, however, and use of these techniques tends to result in overestimation of stenoses owing to local turbulent flow (6,7). Over the past decade, three-dimensional (3D) renal MR angiography has been introduced as a clinical tool that facilitates detection of renal artery stenoses (5,8,9). The primary advantage of MR angiography is the use of contrast media instead of inflow effects for intraluminal signal enhancement (7,8,10).

To obtain high contrast between the arteries and the surrounding tissue, data acquisition has to be performed within a brief (8) acquisition window during the first pass of the contrast agent (and before venous signal enhancement). Optimal bolus timing has been shown to be crucial (10), and all data have to be acquired in a single breath hold. However, patients may not tolerate the required breath-hold duration (11). Furthermore, the acquisition time constraints associated with a breath-hold limit spatial resolution in 3D MR angiography, while only one slab orientation can be acquired. Because of these time constraints, cardiac triggering is not used either, which somewhat restricts higher spatial resolution owing to motion of the proximal renal arteries during the cardiac cycle (2,12). The often observed enhancement of renal parenchyma may also lead to lack of interpretability of small or more distal branch vessels (13,14).

The purpose of our study was to evaluate renal projection MR angiography with a navigator-gated free-breathing cardiac-triggered 3D steady-state free-precession (balanced fast field-echo true fast imaging with steady-state precession [FISP]) pulse sequence. In this article, we report our initial experience.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Background
In projection MR angiography (15–21), spin tagging (spin labeling) is used upstream of the vascular bed of interest for contrast enhancement. During the labeling delay between the labeling pulse and the imaging sequence, labeled blood (with an inverted magnetization) washes into the imaging volume. The differences in magnetization at the time of imaging are used for selective visualization of the arterial lumen by means of subtraction of identical data sets, one with and one without the preceding labeling pulse. As a consequence, the arterial lumen appears bright on the projection MR images, whereas the signal intensity of the surrounding static tissue is suppressed.

MR Imaging
A navigator-gated free-breathing cardiac-triggered 3D balanced fast field-echo projection MR angiographic sequence was implemented with a commercial 1.5-T whole-body MR system equipped with a commercial cardiovascular software package (INCA2), high-performance gradient system (PowerTrak 6000 [23 mT/m, 219-µsec rise time]), and research pulse programming station (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands). A five-element synergy coil was used for signal reception.

Study Population
Results with renal projection MR angiography were investigated in eight clinically healthy adult subjects (three men and five women; age range, 19–53 years; mean age, 30 years) with normal blood pressure and no history of cardiovascular or kidney disease and in seven consecutive patients (four men and three women; age range, 43–78 years; mean age, 64 years) who were sent from the clinicians to the department of radiology for diagnostic imaging of the renal arteries (five stenoses, two occlusions). In only the patient study, 3D MR angiography (3.7/1.1 [repetition time msec/echo time msec]; field of view of 500 mm, to avoid fold over from the arms beside the body; 512 x 154 matrix; 0.2 mmol per kilogram of body weight gadopentetate dimeglumine [Magnevist; Schering, Berlin, Germany] administered intravenously with bolus timing) was performed for comparison. Results of digital subtraction angiography (within 48 hours after renal projection MR angiography) were available in five patients. In two patients, no conventional angiograms were acquired owing to a history of renal failure or allergy to conventional contrast media. Informed written consent was obtained from all participants, and the research protocol for MR examination was approved by the hospital committee on clinical investigation.

Scout Imaging
Initial coronal, sagittal, and transverse multistack scout images (free-running [no breath hold, no cardiac triggering] flow-compensated spoiled gradient-echo sequence with an inversion prepulse [20/6.9/1,800 {inversion time msec}], flip angle of 20°, five 5-mm-thick sections in each orientation, 256 x 128 matrix, field of view of 400 mm) were acquired to localize the abdominal aorta, the ostia of the renal arteries, and the dome of the right hemidiaphragm. For accurate targeting of the renal arteries, a subsequent scout image (transverse navigator-gated or breath-hold balanced fast field echo [3.8/1.9, 20 5-mm-thick sections]) was acquired.

Projection MR Angiography
A balanced fast field-echo sequence (true FISP: 3.8/1.9, 80° flip angle, field of view of 280 mm, 256 x 256 matrix, in-plane spatial resolution of 1.1 x 1.1 mm) was used for renal projection MR angiography. Thirty-two radio-frequency excitations with signal sampling resulted in a data acquisition window of 122 msec for each R-R interval and a 2 minute 17 second image time for a heart rate of 70 beats per minute and two signals acquired. To take full advantage of the balanced fast field-echo sequence, we sought to acquire data with steady-state conditions by performing 20 repetitive start-up cycles immediately preceding the imaging portion of the sequence (Fig 1). An oversampling factor of 1.3 was performed in the section-selection direction to account for suboptimal outer section profiles associated with 3D volume acquisitions. The most peripheral sections were rejected during reconstruction. The trigger delay for the data acquisition window was 500–600 msec after the R wave. With such timing, different interview flow velocities can be avoided, and high-spatial-resolution MR images can be obtained without motion or diameter changes in the proximal renal artery that originate from the aortic pulse wave (2,12). The remaining diastolic flow in the renal arteries supports the blood exchange for renal projection MR angiography. The acquired 3D volume included eight 3.8-mm-thick sections that were interpolated (with zero filling) to 16 contiguous 1.8-mm-thick sections.



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  		Figure 1. Schematic of the sequence elements for renal projection MR angiography. Twenty start-up cycles precede the 3D balanced fast field-echo (true FISP) portion of the sequence to obtain steady-state signal condition. The labeling delay is user defined. The local presaturation bands (REST) are preceded by the navigator for free-breathing data acquisition. SPOILER = radio-frequency spoiling, 2D = two-dimensional.

 
Two-dimensional Selective Navigator for Respiratory Gating
For respiratory motion suppression, prospective real-time navigator technology was used, as previously implemented for coronary MR angiography (22–24). A two-dimensional selective pencil-beam excitation pulse (25) was used for right hemidiaphragm position detection during normal breathing. The navigator-detected lung-liver interface was calculated by using a cross correlation (25) between the most recent navigator signal and an end-expiratory reference navigator profile obtained in a preparation phase that preceded acquisition of the image. If the navigator-detected interface position occurs in a user-specified gating window (4-mm interval), data are accepted; otherwise, the data are rejected and have to be remeasured in the subsequent R-R interval.

Two-dimensional Labeling Pulse
A two-dimensional selective spiral inversion pulse with a sinc-shaped radio-frequency excitation (26) and 12 cycles in k space was used for aortic spin labeling (27). The pencil-beam labeling pulse had a 35-mm diameter and was positioned parallel to the abdominal aorta. A labeling delay of 140 msec was chosen to allow time for subsequent wash-in of labeled blood into the renal arteries. The labeling angles for the nonlabeled and labeled acquisitions were changed during scanning between 0° and 180°.

Description of Experiments and Data Analysis
All images were acquired successfully without complications. MR examinations were performed with patients in the supine position. A hook and loop strap (20 cm wide) was wrapped tightly around the upper abdomen to ensure constant breathing patterns and to reduce abdominal wall excursions. Individual angulation for double-oblique balanced fast field-echo projection MR angiography was performed by using a three-point image planning tool (28). Both hila and the aorta at the level of the renal artery ostia were identified by means of interactive mouse clicks defined by the operator. Plane off center and angulation were translated automatically into a double-oblique section orientation. On the basis of these geometric data, a second (coronal) section orientation was planned that did not overlie the thoracic aorta or the heart, to avoid signal saturation and to facilitate imaging of potential accessory renal arteries. Two local presaturation bands were positioned ventrally to suppress signal from the intestine and the ventral abdominal wall. A third presaturation band was positioned caudally to suppress signal from inflowing blood of the inferior vena cava (29).

A projection MR angiogram was calculated by means of complex subtraction of the labeled and nonlabeled data obtained during acquisition of the same image (20,30). This image selectively displays the labeled blood in the aorta and renal arteries (Fig 2). In addition to the projection MR angiograms, anatomic images without labeling information were calculated to allow simultaneous display of the anatomic region of interest.



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  		Figure 2. Schematic depicts the basic principle of renal projection MR angiography (MRA). Two identical images acquired (A) without and (B) with aortic labeling pulse are complexly subtracted. As a result, only the labeled blood in the aorta and renal arteries contributes to the signal, whereas the venous signal and that of the surrounding tissue are suppressed. 2D = two-dimensional.

 
Data Analysis
Signal intensities (SI = mean value) were obtained in user-specified regions of interest that were placed by one investigator (E.S.) in the aorta (area range, 140–298 mm2), the proximal-middle portion of the renal arteries (area range, 54–114 mm2), the adjacent soft tissue (ellipsoid region of interest in the paravertebral muscle; area range, 200–406 mm2), and the renal veins (area range, 103–189 mm2). Regions of interest (ROIs) with identical size and position were placed in both the anatomic and projection MR angiograms. For the different ROIs, the signal-to-noise ratio (SNR) of the aorta, the renal artery (ra), and the renal vein (rv) were calculated subsequently as SNRROI = SIROI/SDROIa, where a is air, which was measured dorsally. Contrast-to-noise ratios (CNRs) were calculated as CNRra-rv = (SIROIra - SIROIrv)/SDROIa and CNRra-st = (SIROIra - SIROIst)/SDROIa, where st is soft tissue.

For qualitative image analysis, a parallel maximum intensity projection of the subtracted images was obtained with the system console. These maximum intensity projection reconstructions and the individual subtracted images were presented to two investigators (M.S., R.M.B.) to visually assess with consensus the image quality (diagnostic vs nondiagnostic) of the proximal, middle, and distal main renal artery and the more distal branching vessels. In the patient study, maximum intensity projection reconstructions of the renal projection MR angiogram, MR angiogram, and, if available, digital conventional angiogram were presented to two investigators for comparison (normal renal artery, low-grade [not relevant] stenosis, or relevant stenosis or occlusion).

Statistical Analysis
A normal distribution of the differences was assumed. A two-tailed paired t test was used for statistical analyses. A P value less than .05 was considered to indicate a statistically significant difference.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
In all 15 (100%) subjects, parallel maximum intensity projections of both the transverse and coronal double-oblique slab orientations could be obtained. In all renal arteries (except the two that were occluded), the proximal, middle, and distal portions could be visualized (Figs 3–6).



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  		Figure 3a. Transverse double-oblique renal projection MR angiograms in a healthy 36-year-old woman were obtained with the balanced fast field-echo sequence. (a) Anatomic image (single section), (b) subtracted image (single section), (c) and parallel maximum intensity projection, which shows both the main renal arteries and the branches with a high signal intensity (solid arrows). In b and c, signal from veins and the surrounding static tissue is completely suppressed; the superior mesenteric artery and the celiac trunk (dashed arrow) are also seen.

 


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  		Figure 3b. Transverse double-oblique renal projection MR angiograms in a healthy 36-year-old woman were obtained with the balanced fast field-echo sequence. (a) Anatomic image (single section), (b) subtracted image (single section), (c) and parallel maximum intensity projection, which shows both the main renal arteries and the branches with a high signal intensity (solid arrows). In b and c, signal from veins and the surrounding static tissue is completely suppressed; the superior mesenteric artery and the celiac trunk (dashed arrow) are also seen.

 


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  		Figure 3c. Transverse double-oblique renal projection MR angiograms in a healthy 36-year-old woman were obtained with the balanced fast field-echo sequence. (a) Anatomic image (single section), (b) subtracted image (single section), (c) and parallel maximum intensity projection, which shows both the main renal arteries and the branches with a high signal intensity (solid arrows). In b and c, signal from veins and the surrounding static tissue is completely suppressed; the superior mesenteric artery and the celiac trunk (dashed arrow) are also seen.

 


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  		Figure 4a. Double-oblique (a) transverse and (b) coronal balanced fast field-echo renal projection MR angiograms in a healthy 33-year-old woman. Main renal arteries and distal branches (arrows) are displayed with high signal intensity.

 


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  		Figure 4b. Double-oblique (a) transverse and (b) coronal balanced fast field-echo renal projection MR angiograms in a healthy 33-year-old woman. Main renal arteries and distal branches (arrows) are displayed with high signal intensity.

 


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  		Figure 5a. Arterial hypertension in a 64-year-old male patient. (a) Transverse double-oblique renal projection MR angiogram (balanced fast field-echo sequence) shows normal renal arteries (arrows) without stenosis. (b) Three-dimensional MR angiogram (transverse reconstruction) shows findings that confirm those in a.

 


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  		Figure 5b. Arterial hypertension in a 64-year-old male patient. (a) Transverse double-oblique renal projection MR angiogram (balanced fast field-echo sequence) shows normal renal arteries (arrows) without stenosis. (b) Three-dimensional MR angiogram (transverse reconstruction) shows findings that confirm those in a.

 


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  		Figure 6a. Arterial hypertension in a 68-year-old female patient suspected of having renal artery disease. Double-oblique (a) transverse and (b) coronal renal projection MR angiograms (balanced fast field-echo sequence) show occlusion of the left renal artery (arrowhead) and proximal stenosis (arrows) of the right renal artery close to the ostium. (c) Transverse and (d) coronal 3D MR angiogram reconstructions show consistent findings (labeled as in a).

 


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  		Figure 6b. Arterial hypertension in a 68-year-old female patient suspected of having renal artery disease. Double-oblique (a) transverse and (b) coronal renal projection MR angiograms (balanced fast field-echo sequence) show occlusion of the left renal artery (arrowhead) and proximal stenosis (arrows) of the right renal artery close to the ostium. (c) Transverse and (d) coronal 3D MR angiogram reconstructions show consistent findings (labeled as in a).

 


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  		Figure 6c. Arterial hypertension in a 68-year-old female patient suspected of having renal artery disease. Double-oblique (a) transverse and (b) coronal renal projection MR angiograms (balanced fast field-echo sequence) show occlusion of the left renal artery (arrowhead) and proximal stenosis (arrows) of the right renal artery close to the ostium. (c) Transverse and (d) coronal 3D MR angiogram reconstructions show consistent findings (labeled as in a).

 


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  		Figure 6d. Arterial hypertension in a 68-year-old female patient suspected of having renal artery disease. Double-oblique (a) transverse and (b) coronal renal projection MR angiograms (balanced fast field-echo sequence) show occlusion of the left renal artery (arrowhead) and proximal stenosis (arrows) of the right renal artery close to the ostium. (c) Transverse and (d) coronal 3D MR angiogram reconstructions show consistent findings (labeled as in a).

 
In Figure 3, representative transverse double-oblique anatomic (Fig 3a) and projection (Fig 3b, 3c) renal MR angiograms show high signal intensity in the proximal, middle, and distal portions of both renal arteries. Furthermore, branches embedded in the parenchyma can be seen clearly on the projection MR angiograms. The signal of the surrounding static tissue and renal veins is almost completely suppressed (Figs 3, 4). In comparison, in the anatomic MR image, renal arteries and renal veins, especially near the hila, show reduced signal intensity, and branches in the parenchyma cannot be seen.

Figure 5 depicts sharply defined main renal arteries and more distal branching vessels with image quality comparable to that in the volunteer study. Figure 6a and 6b depict an occlusion of the left renal artery and a stenosis of the proximal right renal artery. No signal void distal to the stenosis was observed. The findings at renal projection MR angiography were consistent with those at conventional renal MR angiography (Fig 6c, 6d). In Figure 7, findings at renal projection MR angiography were consistent with those at MR angiography and conventional angiography (Fig 7b, 7c, respectively). With the balanced fast field-echo renal projection MR angiographic technique, all stenoses (including low-grade [not relevant, n = 2] or hemodynamically relevant [moderate or high-grade, n = 3] stenoses), occlusions (n = 2), and normal (n = 8) renal arteries could be correctly diagnosed and classified.



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  		Figure 7a. Arterial hypertension in a 60-year-old male patient. (a) Transverse double-oblique renal projection MR angiogram (balanced fast field-echo sequence) shows moderate stenosis (arrowhead) of the proximal left main renal artery and high-grade (solid arrow) and low-grade (dashed arrow) stenoses of a right branch vessel. (b) Three-dimensional MR angiogram (transverse reconstruction) and (c) conventional MR angiogram show consistent findings (labeled as in a).

 


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  		Figure 7b. Arterial hypertension in a 60-year-old male patient. (a) Transverse double-oblique renal projection MR angiogram (balanced fast field-echo sequence) shows moderate stenosis (arrowhead) of the proximal left main renal artery and high-grade (solid arrow) and low-grade (dashed arrow) stenoses of a right branch vessel. (b) Three-dimensional MR angiogram (transverse reconstruction) and (c) conventional MR angiogram show consistent findings (labeled as in a).

 


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  		Figure 7c. Arterial hypertension in a 60-year-old male patient. (a) Transverse double-oblique renal projection MR angiogram (balanced fast field-echo sequence) shows moderate stenosis (arrowhead) of the proximal left main renal artery and high-grade (solid arrow) and low-grade (dashed arrow) stenoses of a right branch vessel. (b) Three-dimensional MR angiogram (transverse reconstruction) and (c) conventional MR angiogram show consistent findings (labeled as in a).

 
Signal-to-noise ratio and contrast-to-noise ratio data are shown in Figure 8. In the anatomic images, low contrast was found between the renal arteries and the veins (mean contrast-to-noise ratio, -4 ± 17 [SD]; range, -32 to 17). In contrast, in the projection MR angiograms, only the renal arteries and aorta appeared with high signal intensity, which resulted in a significantly improved contrast-to-noise ratio between the renal arteries and the renal veins (mean, 100 ± 31; range, 64–171; P < .001) (Fig 8). Static tissue is suppressed in the projection renal MR angiogram, which also results in increased contrast between the arteries and the surrounding soft tissue, such as renal parenchyma or muscle (mean signal-to-noise ratio, 100 ± 32 vs 36 ± 19; P < .001). Significantly higher signal-to-noise ratios for both aorta and renal arteries were found in the projection MR angiograms compared with those in the anatomic images (aorta mean, 116 ± 33 vs 90 ± 26; P = .005); renal artery mean, 109 ± 33 vs 84 ± 23; P = .002) (Fig 8).



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  		Figure 8. Bar graph depicts signal-to-noise ratio (SNR) of the aorta, renal arteries (ren art), and renal veins (ren ven) and contrast-to-noise ratio (CNR) of the renal arteries to the renal veins (ren art/ven) and renal arteries to the surrounding soft tissue (muscle) (ren art/soft) in the anatomic (white bars) and projection (black bars) MR angiograms. Error bars represent + 1 SD.

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
REFERENCES
 
In this study, balanced fast field-echo renal projection MR angiography was demonstrated with a two-dimensional pencil-beam aortic labeling pulse for selective renal MR angiography. This technique was implemented with a commercial MR imager and enabled high-spatial-resolution and high-contrast imaging of the renal arteries. The signal of static tissue or veins was completely suppressed to allow maximum intensity projection reconstruction. No exogenous contrast medium was needed. As a consequence of free-breathing navigator technology, no breath-hold time constraints for the data acquisition exist, and acquisition of multiple views is possible.

When compared with initial projection MR angiographic (15–18,20,21,29) or conventional MR angiographic (5,8,9,14,31) techniques, a series of technical refinements were implemented. These refinements included cardiac triggering, data acquisition during free breathing, and double-oblique targeting of the renal vessels. Cardiac triggering has been shown to be important to reduce flow artifacts (2,32) and motion artifacts of the proximal renal arteries that originate from the aortic pulse wave (12). Abdominal binding and end-expiratory right hemidiaphragmatic navigator gating provide sufficient respiratory motion suppression despite the large displacement of the distal renal artery during the normal respiratory cycle (8,12). With free-breathing navigator technology, there is no time constraint, which facilitates data acquisition for increased spatial resolution even in patients with reduced breath-hold capability.

Transverse slab orientations for renal MR angiography can be suboptimal because of the variable anatomic orientation of the right and left kidneys (33). This may be overcome by acquiring larger 3D slabs or by volume targeting of a slab that closely encompasses both renal arteries. In our study, we chose to use a three-point image planning tool (28) that allowed individual adaptation of a double-oblique section orientation parallel to the renal arteries. As a consequence, renal arteries, including more distal portions, could be imaged with a thinner slab, consistent with a reduced imaging time.

In early projection MR angiographic techniques (15–18,20,21,29), one-dimensional transverse or sagittal inversion (slab) pulses were used for blood labeling. Such an approach may interfere with the double-oblique 3D MR angiographic volumes and may thereby degrade the quality of the projection MR angiogram (eg, during maximum intensity projection reconstruction). This interference may be accentuated in the more coronal double-oblique projection MR angiogram, which was acquired as a second view to help detect potential accessory renal arteries. Finally, one-dimensional labeling pulse designs potentially invert blood in the left ventricular cavity. This labeled blood may wash into the imaging slab during the subsequent R-R interval, which would lead to reduced signal-to-noise and contrast-to-noise ratios in the anatomic images. The latter occurs because the labeling pulse of projection MR angiography potentially reinverts the (already inverted) spins, which would lead to reduced contrast after complex subtraction. The two-dimensional selective pencil-beam inversion pulse (19,25) overcomes the limitations of a one-dimensional labeling slab and can be freely angulated to selectively label aortic spins without requiring imaging of the entire heart. Because no periaortic tissue is labeled, image quality is not reduced in the projection MR angiogram despite the interference of the labeling beam with the imaging volume. Furthermore, the same labeling pulse geometry can be used for variable projection views.

An imaging sequence for projection MR angiography needs to be optimized for aortic spin labeling. Previous investigators (19–21,29,32) chose to use a reduced number of radio-frequency excitations in each acquisition, as in echo-planar imaging, or reduced flip angles. Multiple excitations with higher flip angles are less signal efficient for labeling techniques with which the labeled information is stored in the longitudinal magnetization. During the developmental phase of the protocol in this study, we investigated multiple imaging sequences with respect to signal-to-noise ratio, contrast-to-noise ratio, and artifacts. Superior image quality was obtained by using a balanced fast field-echo sequence.

The balanced fast field-echo sequence (34–36) was flow compensated in all three spatial coordinates and, therefore, inflow insensitive. Furthermore, data were acquired in a relatively brief (122-msec) acquisition window in late diastole with reduced blood flow velocities. By using such an inflow-insensitive imaging sequence, we could correctly identify and classify all renal artery stenoses. Our preliminary data indicate the potential for imaging of even high-grade stenoses (Fig 7) without distal signal void. Larger prospective clinical studies are needed to further define the clinical value of the technology used in this study.

Complex subtraction has multiple advantages compared with magnitude subtraction (20,30). After labeling with an inversion pulse has been performed, the magnetization of the labeled blood pool may still be markedly negative at the time of imaging (if a labeling delay much less than the T1 of blood [1,200 msec] is used). This results in a minor difference and a minor contrast with magnitude subtraction compared with those with complex subtraction (20). On the other hand, in case of negative magnetization at the time of imaging with complex subtraction, the signal intensity in the projection MR angiogram may be even higher than that in the original anatomic image. This may explain the slightly higher signal-to-noise ratios of the renal arteries in the projection MR angiograms compared with those in the anatomic images (Fig 8).

The projection MR angiographic technique used in this study was performed with healthy adult subjects and only a small group of patients suspected of having renovascular disease. Larger patient studies with more detailed grading of hemodynamically relevant stenoses and image quality are needed to define the clinical value of this approach.

In conclusion, the combination of cardiac triggering, a navigator-gated 3D balanced fast field-echo imaging sequence, and a two-dimensional pencil-beam selective aortic spin-labeling pulse enables selective projection MR angiography of the renal artery and its branches during free breathing and without the need for an exogenous contrast agent. A high contrast-to-noise ratio is obtained while signal from veins and static tissue is completely suppressed, and no breath holding is required. Our preliminary patient data demonstrate clinical promise for this approach and suggest that larger patient comparison studies with conventional 3D renal MR angiography should be performed.


   FOOTNOTES
 
Abbreviations: FISP = fast imaging with steady-state precession, 3D = three-dimensional

Author contributions: Guarantors of integrity of entire study, E.S., M.S., P.B.; study concepts, M.S., P.B.; study design, M.S., E.S.; literature research, E.S.; clinical studies, E.S., K.V.K.; data acquisition, E.S., K.V.K.; data analysis/interpretation, E.S., R.M.B.; statistical analysis, E.S., R.M.B.; manuscript preparation, E.S., W.J.M., M.S.; manuscript definition of intellectual content, E.S., M.S., P.B.; manuscript editing, E.S., M.S.; manuscript revision/review, M.S.; manuscript final version approval, W.J.M.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 

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