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High-Spatial-Resolution Contrast-enhanced MR Angiography of the Renal Arteries: A Prospective Comparison with Digital Subtraction Angiography¹

¹ From the Magnetic Resonance Laboratory, Mayo Clinic, 200 1st St SW, Rochester, MN 55905. Received February 16, 2000; revision requested March 27; revision received May 1; accepted May 22. Supported by National Institutes of Health grants CA37993 and HL37310 and GE Medical Systems. Address correspondence to S.J.R. (e-mail: riederer@mayo.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To evaluate a high-spatial-resolution three-dimensional (3D) contrast material–enhanced magnetic resonance (MR) angiographic technique for detecting proximal and distal renal arterial stenosis.

MATERIALS AND METHODS: Twenty-five patients underwent high-spatial-resolution small–field-of-view (FOV) 3D contrast-enhanced MR angiography of the renal arteries, which was followed several minutes later by more standard, large-FOV 3D contrast-enhanced MR angiography that included the distal aorta and iliac arteries. For both acquisitions, MR fluoroscopic triggering and an elliptic centric view order were used. Two readers evaluated the MR angiograms for grade and hemodynamic significance of renal arterial stenosis, diagnostic quality, and presence of artifacts. MR imaging results for each patient were compared with those of digital subtraction angiograms.

RESULTS: The high-spatial-resolution small-FOV technique provided high sensitivity (97%) and specificity (92%) for the detection of renal arterial stenosis, including all four distal stenoses encountered. The portrayal of the segmental renal arteries was adequate for diagnosis in 19 (76%) of 25 patients. In 12% of the patients, impaired depiction of the segmental arteries was linked to motion.

CONCLUSION: The combined high-spatial-resolution small-FOV and large-FOV MR angiographic examination provides improved spatial resolution in the region of the renal arteries while maintaining coverage of the abdominal aorta and iliac arteries.

Index terms: Magnetic resonance (MR), contrast media, 96.12943 • Magnetic resonance (MR), rapid imaging, 96.12942, 96.12943 • Magnetic resonance (MR), vascular studies, 96.12942 • Renal angiography, 96.122, 96.12942 • Renal arteries, MR, 96.12942, 96.12943 • Renal arteries, stenosis or obstruction, 96.721

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Three-dimensional (3D) contrast material–enhanced magnetic resonance (MR) angiography of the renal arteries with an intravenously injected gadolinium-based contrast agent is becoming a commonly used technique (1–10). In early studies, acquisition and injection times up to several minutes long were used (11); but more recently, shorter 10–30-second acquisition times within the "first pass" of a rapidly injected bolus of gadolinium-based contrast agent were completed within a single breath hold (2–10,12,13).

For first-pass techniques, a 3D fast gradient-recalled-echo pulse sequence typically is used with one of several timing methods to correlate the peak of contrast enhancement to the acquisition of the MR angiograms (4,6,8). These timing methods include test bolus injection (4,8,14), real-time MR fluoroscopy (6) or line sampling (15), and time-resolved techniques (9,16). Timing methods that use real-time detection of contrast agent arrival typically use centric view ordering of k space to precisely capture the arterial phase of the bolus and minimize venous enhancement. The combination of MR fluoroscopic triggering with elliptic centric view ordering used in this study has been shown to provide reliably high-quality MR angiograms of the renal arteries with high arterial contrast and excellent suppression of venous signal intensity (6,17).

Despite the success of first-pass techniques, the in-plane spatial resolution of 3D contrast-enhanced MR angiography still remains inferior to the spatial resolution of conventional angiography and digital subtraction angiography. Authors of many studies have reported success in the detection of main renal arterial disease (1,3–5,7,18–20), but the reliable detection of distal stenoses remains a challenge (3,10). Spatial resolution improvements in 3D contrast-enhanced MR angiography may not only improve our ability to detect distal stenosis but also allow increased confidence in the evaluation and detection of proximal lesions.

Spatial resolution of 3D MR angiography is limited principally by the large voxel size of MR angiography compared with that of x-ray angiographic techniques. A large number of phase-encoding views typically are required to attain a small (<2 mm³) voxel size, which necessitates a long acquisition time. Given the limited duration of the bolus of contrast agent in the arteries, and the potential for motion during MR angiography, there is a practical lower limit on the useful sampled voxel size. This lower limit on spatial resolution improves with field-of-view (FOV) minimization because less time is required to reach the desired sampling resolution (21). Furthermore, interference from venous signal intensity and respiratory motion artifacts can be minimized by using the elliptic centric view order (22), which makes longer acquisition times feasible. Consistent with this thinking, a high-spatial-resolution MR angiographic technique was developed with reduced FOV and slightly extended acquisition time.

This study was performed to evaluate this high-spatial-resolution 3D contrast-enhanced MR angiographic technique for the detection of proximal and distal renal arterial stenosis, with digital subtraction angiography as the reference standard. Emphasis was placed on the detection of distal stenoses, including fibromuscular dysplasia, which typically occurs distally (23).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Patient Studies
From November 1998 to September 1999, more than 180 patients suspected of having renal arterial disease underwent imaging with a high-spatial-resolution 3D contrast-enhanced MR angiographic protocol. All imaging was performed at 1.5 T (Signa Echo Speed; GE Medical Systems, Milwaukee, Wis). All patients undergoing MR angiography of the renal arteries underwent high-spatial-resolution small-FOV MR angiography centered on the renal arteries, which was followed by the more conventional large-FOV MR angiography of the aorta and iliac vessels with a second bolus of gadolinium-based contrast agent. For both MR angiographic acquisitions, we used MR fluoroscopic triggering and the elliptic centric view order.

Of the more than 180 patients undergoing high-spatial-resolution 3D contrast-enhanced MR angiography, the 25 patients (eight women, 17 men; mean age, 65 years ± 16 [SD]; age range, 8–83 years) who composed the study population then underwent digital subtraction angiography. In 19 of these 25 patients, the MR angiogram was positive, and digital subtraction angiography was performed with the possibility of percutaneous transluminal angioplasty at the time of angiography. In the remaining six patients, the MR angiogram was either normal or any disease was considered to be not hemodynamically significant. However, the clinical suspicion was so high for renovascular disease that angiography was performed anyway.

The principal comparison in this study was between high-spatial-resolution small-FOV MR angiography and digital subtraction angiography, and both were performed in all 25 patients. In addition, large-FOV MR angiography was compared with digital subtraction angiography and high-spatial-resolution small-FOV MR angiography for only 23 of the 25 patients. In one patient, large-FOV MR angiography was not performed. In another, large-FOV MR angiography was not technically successful.

All examinations were performed for clinical purposes. This study was approved by our institutional review board, and the patients gave their informed consent for use of their medical records for research purposes.

Digital Subtraction Angiographic Technique
The images were acquired digitally (Multistar; Siemens Medical Systems, Erlangen, Germany) by using intraarterial injections of contrast agent, and they were viewed in subtracted and unsubtracted modes in all 25 patients. Specifically, intraarterial injections were performed by using iodinated nonionic contrast agent (Isovue 300; Bracco Diagnostics, Princeton, NJ) in 22 patients, carbon dioxide gas in two patients, and nonionic gadolinium-based contrast agent in one patient. Aortograms were obtained in the anteroposterior projection at 40- or 28-cm FOV by using 4- or 5-F flush catheters in the aorta; aortograms in oblique projections were also obtained, if necessary. Selective renal angiograms were obtained with a 5-F catheter in most patients, with contrast agent injection amount and rate adjusted appropriately for vessel size.

MR Angiographic Technique
For each MR examination, the high-spatial-resolution small-FOV MR angiogram was acquired first. The FOV was 26.0 cm (x axis [superior to inferior]) x 19.0 cm (y axis [right to left]) x 6.4 cm (z axis [anterior to posterior]) and covered the region of the renal arteries in a total acquisition time of 40 seconds. The large-FOV MR angiogram was then acquired by using a more conventional FOV of 40.0 cm (x) x 30.0 cm (y) x 6.8 cm (z) for broader anatomic coverage, including the aortoiliac vessels, in a total acquisition time of 25 seconds.

The high-spatial-resolution small- and large-FOV MR angiographic examinations were performed by using a 3D gradient-echo sequence with the elliptic centric view order in the y and z phase-encode directions, radio-frequency spoiling (24), and rewinding of both phase-encoding gradients (25). The flip angle for all studies was 45°. Repetition and echo times were 6.4 and 1.4 msec, respectively. Real-time bolus triggering was performed by an MR technologist using MR fluoroscopy, which is described elsewhere (6, 17,26). This method of bolus triggering allowed reliable precise timing of the 3D MR angiographic acquisition to the arrival of the bolus of contrast agent.

A phased-array torso coil composed of four elements was used for the high-spatial-resolution small-FOV study to offset the decrease in signal-to-noise ratio owing to the small voxel size of high-spatial-resolution MR angiography. Each of two sets of two overlapped coil elements was 30 cm (superior to inferior) x 26 cm (right to left); one overlapped set was placed dorsally, and the other was placed ventrally. Use of this coil also reduced phase wrap in the right-to-left phase-encode direction owing to the reduced sensitivity outside the FOV. No other efforts were made to reduce right-to-left phase wrap, which was negligible compared with arterial enhancement in all studies. For one very large patient, the small FOV was increased to 30.0 cm (x) x 24.0 cm (y) x 6.4 cm (z) to increase signal-to-noise ratio. The body coil was used for large-FOV MR angiography to provide broader coverage of the abdominal and pelvic vasculature. Signal-to-noise ratio was not a limiting factor for the large-FOV results.

A total dose of 0.2–0.3 mmol gadoteridol (Prohance; Bracco Diagnostics) per kilogram of body weight was delivered in two injections. The typical and maximum injection volume for high-spatial-resolution small-FOV MR angiography was 30 mL, which ranged as low as 20 mL for smaller patients, whereas the typical and maximum injection for large-FOV MR angiography was 20 mL, which ranged as low as 10 mL for smaller patients. Each bolus was delivered at 3 mL/sec by means of a power injector (Spectris; Medrad, Indianola, Pa), with a saline solution flush of 25 mL also at 3 mL/sec into the antecubital vein of the right or left arm.

Image Evaluation
The digital subtraction angiograms of the renal arteries were treated as the reference standard. MR angiograms were reviewed in three steps in a blinded fashion by two experienced MR angiographers (J.F.B., B.F.K.). First, the large-FOV MR angiograms were reviewed, and the diagnoses were recorded; the reviewers were blinded with respect to both the high-spatial-resolution small-FOV MR angiographic examination results and the digital subtraction angiographic examination results. Second, the high-spatial-resolution small-FOV MR angiographic examination results were then reviewed, and the diagnoses were recorded; the reviewers were blinded with respect to the digital subtraction angiographic examination results. Third, the digital subtraction angiographic examination results were reviewed, and the final diagnoses were recorded. The reviewers evaluated the examination results together, and consensus was required at each step. All MR angiograms were evaluated on the basis of the following features.

1. Presence and location of hemodynamically significant stenoses in the main and segmental renal arteries. "Significant" was defined as diameter reduction greater than 50%—all stenoses with grades of "moderate" or "severe or occluded" as defined in the next item.
   
2. Presence and grade of all stenoses in the main and segmental renal arteries. Stenosis not present was considered "normal"; stenosis of less than 50%, "mild"; stenosis of 50%–80%, "moderate"; and stenosis of more than 80%, "severe or occluded."
   
3. Relative diagnostic image quality of high-spatial-resolution small-FOV MR angiography relative to that of the more standard large-FOV MR angiography for each patient was scored as follows: 1, high-spatial-resolution small-FOV MR angiography was better and changed the diagnosis; 2, high-spatial-resolution small-FOV MR angiography was better and improved confidence in the diagnosis; 3, high-spatial-resolution small-FOV MR angiography was better but had no effect on the diagnosis; 4, high-spatial-resolution small-FOV MR angiography was equivalent to large-FOV MR angiography; and 5, high-spatial-resolution small-FOV MR angiography was worse than large-FOV MR angiography. A change in the diagnosis was defined as a case in which the stenosis either showed a change greater than 20% in degree of stenosis and/or a change to not significant or significant.
   
4. Depiction of the segmental renal arteries was scored as follows: 1, excellent visibility and more than adequate for diagnosis; 2, reduced signal intensity but visible and adequate for diagnosis; 3, barely visible and less than adequate for diagnosis; and 4, not visible.
   
5. Motion degradation of the renal arteries manifested by blurring and reduction of arterial signal intensity was scored as follows: 1, no visible motion degradation; 2, minimal motion degradation; 3, moderate motion degradation but diagnostic; and 4, severe degradation and nondiagnostic.
   
6. Venous enhancement was scored as follows: 1, no venous enhancement; 2, minimal venous enhancement; 3, visible venous enhancement but less than arterial enhancement; 4, equivalent venous and arterial enhancement; and 5, venous enhancement greater than arterial enhancement.
   
7. Overlying parenchymal enhancement of the kidney was scored as follows: 1, no parenchymal enhancement; 2, minimal parenchymal enhancement; 3, visible parenchymal enhancement but less than arterial enhancement; 4, equivalent parenchymal and arterial enhancement; and 5, parenchymal enhancement greater than arterial enhancement.
   

To facilitate depiction on the 3D MR angiograms, soft-copy review of multiplanar reformat images and maximum intensity projection images was performed for all examinations by the reviewing radiologists by using an MR-independent console (Signa version 5.7; GE Medical Systems, Milwaukee, Wis). Coronal, transverse, and sagittal reformatted images of the source images were reviewed, followed by interactive oblique maximum intensity projection images that focused on suspicious areas.

Several statistical tests were used. When directly testing the significance of differences between data sets consisting of ordinal evaluation scores, we used the Wilcoxon signed rank test (27). Similarly, when assessing the correlation between grade of stenosis measured by using two different techniques, we used the Spearman rank correlation (27), and each stenosis grade was assigned an ordinal score: 1, which was considered "normal," through 4, which was considered "severe or occluded." The statistical significance of the Spearman rank correlation was calculated by using the Student t test (27). The Student t test was also used to determine the statistical significance of differences between two Spearman rank correlations by calculating the desired CI for each rank correlation by using a bootstrapping algorithm written in the C programming language (28). All other statistical calculations were performed by using functions in a commercially available program (EXCEL; Microsoft, Redmond, Wash).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
A typical case in which the high-spatial-resolution 3D contrast-enhanced MR angiographic protocol was used is shown in Figure 1, in which the digital subtraction angiogram (Fig 1a) is shown with the high-spatial-resolution small-FOV MR angiogram (Fig 1b) and the large-FOV MR angiogram (Fig 1c). The acquisition parameters, matrices, and voxel sizes are summarized in Table 1.

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. Significant focal stenosis (short straight arrows) at the origin of the right main renal artery in a 59-year-old woman is clearly depicted in the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. (c) Full coronal maximum intensity projection image of the large-FOV MR angiogram shows that the proximal right main renal arterial stenosis appears to be normal at this spatial resolution. Note the blurring in the distal right segmental arteries (curved arrow in b) indicative of renal arterial motion during imaging. There is a 99% stenosis (long straight arrow in b and c) in the left main renal artery, which results in faint but visible enhancement of the left segmental renal arteries in b and c. The left renal arteries were not seen in a. Pulse sequence parameters are as summarized in Table 1.

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. Significant focal stenosis (short straight arrows) at the origin of the right main renal artery in a 59-year-old woman is clearly depicted in the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. (c) Full coronal maximum intensity projection image of the large-FOV MR angiogram shows that the proximal right main renal arterial stenosis appears to be normal at this spatial resolution. Note the blurring in the distal right segmental arteries (curved arrow in b) indicative of renal arterial motion during imaging. There is a 99% stenosis (long straight arrow in b and c) in the left main renal artery, which results in faint but visible enhancement of the left segmental renal arteries in b and c. The left renal arteries were not seen in a. Pulse sequence parameters are as summarized in Table 1.

View larger version (131K): [in this window] [in a new window] [Download PPT slide]   Figure 1c. Significant focal stenosis (short straight arrows) at the origin of the right main renal artery in a 59-year-old woman is clearly depicted in the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. (c) Full coronal maximum intensity projection image of the large-FOV MR angiogram shows that the proximal right main renal arterial stenosis appears to be normal at this spatial resolution. Note the blurring in the distal right segmental arteries (curved arrow in b) indicative of renal arterial motion during imaging. There is a 99% stenosis (long straight arrow in b and c) in the left main renal artery, which results in faint but visible enhancement of the left segmental renal arteries in b and c. The left renal arteries were not seen in a. Pulse sequence parameters are as summarized in Table 1.

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. (a, b) Anteroposterior x-ray angiograms obtained in a 73-year-old man reveal a severe stenosis (short arrow in a) in the proximal left main renal artery in (a) the unsubtracted view and a distal stenosis (curved arrow in b) in the right main renal artery in (b) the digitally subtracted view. Also, segmental stenoses (long straight arrow in a and b) are seen bilaterally. (c) Coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram shows excellent correspondence to the digital subtraction angiograms for the left main renal arterial stenosis (middle straight arrow) and distal right main stenosis (curved arrow). Bilateral segmental disease (outermost straight arrows) is also well depicted. (d) Magnified coronal subvolume of the large-FOV MR angiogram depicts the proximal left main renal arterial stenosis (middle straight arrow) well, but depiction of the segmental stenoses (outermost straight arrows) and distal right main renal arterial stenosis (curved arrow) is limited by inadequate spatial resolution and interference from parenchymal enhancement. Pulse sequence parameters are as summarized in Table 1.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. (a, b) Anteroposterior x-ray angiograms obtained in a 73-year-old man reveal a severe stenosis (short arrow in a) in the proximal left main renal artery in (a) the unsubtracted view and a distal stenosis (curved arrow in b) in the right main renal artery in (b) the digitally subtracted view. Also, segmental stenoses (long straight arrow in a and b) are seen bilaterally. (c) Coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram shows excellent correspondence to the digital subtraction angiograms for the left main renal arterial stenosis (middle straight arrow) and distal right main stenosis (curved arrow). Bilateral segmental disease (outermost straight arrows) is also well depicted. (d) Magnified coronal subvolume of the large-FOV MR angiogram depicts the proximal left main renal arterial stenosis (middle straight arrow) well, but depiction of the segmental stenoses (outermost straight arrows) and distal right main renal arterial stenosis (curved arrow) is limited by inadequate spatial resolution and interference from parenchymal enhancement. Pulse sequence parameters are as summarized in Table 1.

View larger version (120K): [in this window] [in a new window] [Download PPT slide]   Figure 2c. (a, b) Anteroposterior x-ray angiograms obtained in a 73-year-old man reveal a severe stenosis (short arrow in a) in the proximal left main renal artery in (a) the unsubtracted view and a distal stenosis (curved arrow in b) in the right main renal artery in (b) the digitally subtracted view. Also, segmental stenoses (long straight arrow in a and b) are seen bilaterally. (c) Coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram shows excellent correspondence to the digital subtraction angiograms for the left main renal arterial stenosis (middle straight arrow) and distal right main stenosis (curved arrow). Bilateral segmental disease (outermost straight arrows) is also well depicted. (d) Magnified coronal subvolume of the large-FOV MR angiogram depicts the proximal left main renal arterial stenosis (middle straight arrow) well, but depiction of the segmental stenoses (outermost straight arrows) and distal right main renal arterial stenosis (curved arrow) is limited by inadequate spatial resolution and interference from parenchymal enhancement. Pulse sequence parameters are as summarized in Table 1.

View larger version (125K): [in this window] [in a new window] [Download PPT slide]   Figure 2d. (a, b) Anteroposterior x-ray angiograms obtained in a 73-year-old man reveal a severe stenosis (short arrow in a) in the proximal left main renal artery in (a) the unsubtracted view and a distal stenosis (curved arrow in b) in the right main renal artery in (b) the digitally subtracted view. Also, segmental stenoses (long straight arrow in a and b) are seen bilaterally. (c) Coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram shows excellent correspondence to the digital subtraction angiograms for the left main renal arterial stenosis (middle straight arrow) and distal right main stenosis (curved arrow). Bilateral segmental disease (outermost straight arrows) is also well depicted. (d) Magnified coronal subvolume of the large-FOV MR angiogram depicts the proximal left main renal arterial stenosis (middle straight arrow) well, but depiction of the segmental stenoses (outermost straight arrows) and distal right main renal arterial stenosis (curved arrow) is limited by inadequate spatial resolution and interference from parenchymal enhancement. Pulse sequence parameters are as summarized in Table 1.

View larger version (178K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Fibromuscular dysplasia indicated by a string of beaded stenoses in a 59-year-old woman is clearly visible in the middle portion of the right main renal artery (straight arrow) on the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram but is less clearly depicted on the (c) magnified coronal subvolume maximum intensity projection image of the large-FOV MR angiogram. Note also the right accessory renal artery (curved arrow) is well depicted in b and is only faintly depicted in c owing to inferior spatial resolution. Pulse sequence parameters are as summarized in Table 1.

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Fibromuscular dysplasia indicated by a string of beaded stenoses in a 59-year-old woman is clearly visible in the middle portion of the right main renal artery (straight arrow) on the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram but is less clearly depicted on the (c) magnified coronal subvolume maximum intensity projection image of the large-FOV MR angiogram. Note also the right accessory renal artery (curved arrow) is well depicted in b and is only faintly depicted in c owing to inferior spatial resolution. Pulse sequence parameters are as summarized in Table 1.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 3c. Fibromuscular dysplasia indicated by a string of beaded stenoses in a 59-year-old woman is clearly visible in the middle portion of the right main renal artery (straight arrow) on the (a) anteroposterior digital subtraction angiogram and (b) full coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram but is less clearly depicted on the (c) magnified coronal subvolume maximum intensity projection image of the large-FOV MR angiogram. Note also the right accessory renal artery (curved arrow) is well depicted in b and is only faintly depicted in c owing to inferior spatial resolution. Pulse sequence parameters are as summarized in Table 1.

For three separate cases, MR angiography depicted severe, narrowly patent main renal arterial stenoses that appeared occluded at digital subtraction angiography. In one such case, both MR angiograms showed a severe 99% stenosis in the left main renal artery, with faint depiction of the left renal artery and segmental vessels (Fig 1b, 1c), as well as faint enhancement of the left kidney parenchyma. There was no depiction of the left main renal artery on the digital subtraction angiogram (Fig 1a). Despite the digital subtraction angiographic findings, efforts were made to recanalize the lumen but proved unsuccessful.

The degree of stenosis measured at high-spatial-resolution small-FOV MR angiography as compared with that measured at digital subtraction angiography had a Spearman rank correlation of 0.83 (P < .001). The closer the rank correlation is to one, the more perfect the correlation between the measurements of the degree of stenosis on the basis of MR angiography and digital subtraction angiography (27), and by inference, the more accurate the grading of stenoses at MR angiography. The large-FOV MR angiographic measurements relative to digital subtraction angiographic measurements had a smaller Spearman rank correlation of 0.72 (P < .001; 90% CI: 0.57, 0.83).

The upper limit of the CI for the large-FOV MR angiographic rank correlation was equal to the rank correlation of high-spatial-resolution small-FOV MR angiography. Therefore, the improvement in accuracy was not statistically significant (one-tailed test,  = .05), but the results were compelling. There was no apparent trend toward over- or underestimation of disease by using the high-spatial-resolution small-FOV MR angiographic technique given that the variations in the percentage of stenosis appeared evenly dispersed about the diagonal in the first four columns of Table 3. There was a trend toward underestimation of disease for the large-FOV MR angiographic technique, as suggested by the larger number of cases above the diagonal in the last four columns of Table 3.

View larger version (26K): [in this window] [in a new window] [Download PPT slide]   Figure 4a. (a) Bar graph shows that for high-spatial-resolution small-FOV MR angiography (white bars), depiction of the segmental renal arteries was adequate or more than adequate for diagnosis in 19 (76%) of 25 of the patients. Segmental arteries were depicted adequately at only four (17%) of 23 of the large-FOV MR angiographic examinations (gray bars). (b) Bar graph shows that degradation of image quality for segmental renal arterial depiction owing to motion is nearly identical for high-spatial-resolution small-FOV MR angiography (white bars) and large-FOV MR angiography (gray bars). Motion degradation is not visible or is minimal in more than 80% of patients.

View larger version (23K): [in this window] [in a new window] [Download PPT slide]   Figure 4b. (a) Bar graph shows that for high-spatial-resolution small-FOV MR angiography (white bars), depiction of the segmental renal arteries was adequate or more than adequate for diagnosis in 19 (76%) of 25 of the patients. Segmental arteries were depicted adequately at only four (17%) of 23 of the large-FOV MR angiographic examinations (gray bars). (b) Bar graph shows that degradation of image quality for segmental renal arterial depiction owing to motion is nearly identical for high-spatial-resolution small-FOV MR angiography (white bars) and large-FOV MR angiography (gray bars). Motion degradation is not visible or is minimal in more than 80% of patients.

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 5a. Example of an MR angiographic study with no visible motion in the region of the segmental renal arteries in a 78-year-old man. (a) Anteroposterior digital subtraction angiogram shows severe proximal main renal arterial stenoses (curved arrows in a-d) bilaterally, which are well depicted on the (b) coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. The segmental renal arteries are also well depicted on both the (c) anteroposterior selective digital subtraction angiogram of the left main renal artery and (d) magnified coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. A left accessory renal artery (straight arrows in a, b, and d) is also well depicted. Pulse sequence parameters are as summarized in Table 1. PRE = preangioplasty image.

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 5b. Example of an MR angiographic study with no visible motion in the region of the segmental renal arteries in a 78-year-old man. (a) Anteroposterior digital subtraction angiogram shows severe proximal main renal arterial stenoses (curved arrows in a-d) bilaterally, which are well depicted on the (b) coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. The segmental renal arteries are also well depicted on both the (c) anteroposterior selective digital subtraction angiogram of the left main renal artery and (d) magnified coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. A left accessory renal artery (straight arrows in a, b, and d) is also well depicted. Pulse sequence parameters are as summarized in Table 1. PRE = preangioplasty image.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 5c. Example of an MR angiographic study with no visible motion in the region of the segmental renal arteries in a 78-year-old man. (a) Anteroposterior digital subtraction angiogram shows severe proximal main renal arterial stenoses (curved arrows in a-d) bilaterally, which are well depicted on the (b) coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. The segmental renal arteries are also well depicted on both the (c) anteroposterior selective digital subtraction angiogram of the left main renal artery and (d) magnified coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. A left accessory renal artery (straight arrows in a, b, and d) is also well depicted. Pulse sequence parameters are as summarized in Table 1. PRE = preangioplasty image.

View larger version (101K): [in this window] [in a new window] [Download PPT slide]   Figure 5d. Example of an MR angiographic study with no visible motion in the region of the segmental renal arteries in a 78-year-old man. (a) Anteroposterior digital subtraction angiogram shows severe proximal main renal arterial stenoses (curved arrows in a-d) bilaterally, which are well depicted on the (b) coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. The segmental renal arteries are also well depicted on both the (c) anteroposterior selective digital subtraction angiogram of the left main renal artery and (d) magnified coronal maximum intensity projection image of the high-spatial-resolution small-FOV MR angiogram. A left accessory renal artery (straight arrows in a, b, and d) is also well depicted. Pulse sequence parameters are as summarized in Table 1. PRE = preangioplasty image.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The main goal of this study was to evaluate a high-spatial-resolution MR angiographic technique for the detection of both proximal and distal renal arterial disease such as fibromuscular dysplasia and segmental stenoses. In this regard, the high-spatial-resolution small-FOV technique depicted all distal stenoses in the sample of cases available in this study: three of three segmental stenoses and one of one case of fibromuscular dysplasia in the middle to distal main renal artery. We acknowledge that the larger number of men (17 men vs eight women) and older age (65 years ± 16) of the study population lower the probability of encountering fibromuscular dysplasia (23).

The renal 3D contrast-enhanced MR angiographic technique evaluated in this study made use of a high-spatial-resolution small-FOV centered on the renal arteries and a second, large FOV designed to evaluate the aorta, aortic bifurcation, and iliac arteries. In this dual-FOV MR angiographic technique, the first injection of contrast agent was used to acquire the high-spatial-resolution small-FOV MR angiograms focused on the renal arteries and including the celiac and superior mesenteric arterial origins. The small-FOV study was performed first, so it was uncontaminated by contrast agent in the renal parenchyma and pelvis. The second injection of contrast agent was used to acquire the large-FOV MR angiograms that covered the distal abdominal aorta and iliac arteries.

The combined technique provided improved confidence and accuracy for the detection and grading of proximal and distal renal arterial disease and still provided a complete study of the aorta and iliac arteries. The use of two intravenous injections of gadolinium-based contrast agent in this 3D contrast-enhanced MR angiographic protocol is analogous to conventional x-ray angiography in which multiple FOVs of the aorta and renal arteries are often imaged by using multiple injections of contrast agent.

The diagnosis and characterization of renal arterial disease demand high spatial resolution. The diagnostic accuracy of the conventional large-FOV MR angiographic technique in our study demonstrated a tendency toward underestimation of proximal and segmental stenoses (Table 3). This result suggests that the large-FOV images may be limited by the large acquired voxel size relative to that of the high-spatial-resolution small-FOV MR angiograms, which had a higher Spearman rank correlation and showed no tendency toward over- or underestimation of the degree of stenosis (Table 3). Although we expect this result on theoretic grounds (21), it is important to note that this study did not constitute a direct comparison of the small- and large-FOV techniques, since we reserved the first injection of contrast agent for the high-spatial-resolution small-FOV examination to obtain our best-quality image of the renal arteries in all patients.

The high-spatial-resolution small-FOV MR angiographic technique achieved a reduced acquired voxel size by increasing acquisition time from 25 to 40 seconds and decreasing FOV in all three directions (Table 1). The selection of these acquisition parameters was guided by the limit on spatial resolution imposed by the bolus profile, which was based on measured contrast enhancement curves (21). If patients are able to hold their breath for the duration of the 40-second acquisition, the sampled resolution will approach the spatial resolution imposed by the transient contrast enhancement. Therefore, when tolerable, longer breath holds are justified.

In many cases, the 40-second acquisition time for the high-spatial-resolution small-FOV examination was too long for patients to hold their breath. This brings up the possibility of adjusting the duration of the breath hold on a patient-by-patient basis. This approach would likely be impractical, partly because it would require additional time to obtain a test breath hold prior to prescribing the 3D contrast-enhanced MR angiographic examination and partly because this test breath hold might not be reproducible. Administering oxygen in all patients is likely to be a more successful strategy because it could be integrated into the imaging routine and might prolong the breath hold, even in patients with cardiovascular impairment. However, the elliptic centric view order is more tolerant of respiratory motion late in the acquisition (22), as was demonstrated by the fact that motion degradation was approximately the same for the large-FOV (25-second acquisition time) and high-spatial-resolution small-FOV (40-second acquisition time) examinations (Fig 4b).

Respiratory motion may become less important in the near future with the use of shorter repetition times, which will allow shorter breath holds. A shorter repetition time for fixed FOV will further decrease blurring due to the transient bolus profile (21). However, the means for reduction of repetition times will need to be balanced with considerations of the consequent decrease in signal-to-noise ratio.

Motion due to slow involuntary exhalation at the beginning of, rather than later in, the acquisition occasionally resulted in motion artifact that blurred the segmental renal arteries. This motion degradation was a problem in depicting the segmental renal arteries in three (12%) of 25 of the high-spatial-resolution small-FOV cases in our study (Table 5). In these three patients, poor depiction of the segmental vessels appeared to result from breathing motion during the first 10 seconds of the acquisition (ie, during the acquisition of the central views of k space). Because of this result, we have begun to graphically monitor the patient’s breath hold in real time with respiratory bellows prior to triggering and during the 3D MR angiographic acquisition.

We postulate that reduced or delayed flow to the segmental arteries may also impair depiction of the segmental renal arteries with this technique. For example, these three (12%) of 25 high-spatial-resolution small-FOV cases with poorly depicted segmental arteries showed advanced disease in the small vessels of the kidneys. Time-resolved 3D contrast-enhanced MR angiography may be useful for this particular set of patients (9,16). To better detect disease in the small vessels of the kidneys, a two-dimensional cine phase-contrast acquisition was routinely obtained in conjunction with the 3D contrast-enhanced MR angiography described here to assess diastolic flow in the renal arteries. The two-dimensional cine phase-contrast technique can be used to measure or estimate both systolic and diastolic flow patterns in a diseased artery and has been shown to help in assessing the hemodynamic significance of renal arterial stenoses (29).

For purposes of analysis, in this study we compared the high-spatial-resolution small-FOV and large-FOV MR angiographic renal studies to each other. Because it required a second injection of contrast agent, we recognize that the image quality of the large-FOV examination in our study was potentially compromised by residual contrast agent in the parenchymal tissues (Table 7). There were more large-FOV studies (13 [57%] of 23 patients) with clearly visible parenchymal enhancement compared with the number of high-spatial-resolution small-FOV studies (six [24%] of 25 patients). This increased parenchymal enhancement can interfere with the detection of segmental stenoses (9), as in Figure 2d, and likely played some role in the lower sensitivity of the large-FOV MR angiographic technique. However, venous enhancement residual from the first MR angiographic acquisition was generally not problematic (Table 6), despite a short mean delay of approximately 13 minutes between examinations. Good venous suppression in the large-FOV acquisition allowed the main renal arteries to be well seen (eg, Fig 1c).

We found the high-spatial-resolution small-FOV MR angiographic quality to be superior to x-ray angiographic quality for the three patients in whom iodinated contrast agents were not used—carbon dioxide was used in two and nonionic gadolinium-based contrast agent was used in one. This result suggests that the high-spatial-resolution small-FOV MR angiographic technique could supplant noniodinated x-ray angiography for diagnostic imaging of renal arteries to reduce the risk to the patient from catheterization and x-ray exposure.

In addition, three severely diseased arteries interpreted as occluded on x-ray angiograms were, in fact, high-grade stenoses (99%) on the MR angiograms. Although it is possible that these arteries were occluded sometime between MR angiography and x-ray angiography, it is also possible that low concentrations of gadolinium-based contrast agent at MR angiography are more readily detected at MR angiography than are low concentrations of iodinated contrast agent at x-ray angiography. This phenomenon warrants further study.

To summarize, the high-spatial-resolution small-FOV technique provides high sensitivity (97%) and specificity (92%) for the detection of proximal and distal renal arterial stenosis. No instances of distal disease were missed. In addition, high-spatial-resolution small-FOV MR angiography improves confidence and appears to improve accuracy in grading the degree of renal arterial stenosis compared with confidence and accuracy with conventional large-FOV MR angiography. The portrayal of the segmental renal arteries was adequate for diagnosis in 19 (76%) of 25 patients. Motion degradation was linked to impaired depiction of the segmental renal arteries in a small number of patients (12%) and appears to have been caused by involuntary slow exhalation during the start of the acquisition. Combining consecutive 3D contrast-enhanced MR angiographic acquisitions at a small and a large FOV within a single patient examination provided improved spatial resolution in the main and segmental renal arteries while maintaining coverage of the abdominal aorta and iliac arteries.

	ACKNOWLEDGMENTS

 
The authors thank W. Michael O’Fallon, PhD, for advice concerning the statistical analyses used for this study.

	FOOTNOTES

 
Abbreviations: FOV = field of view, 3D = three-dimensional

Author contributions: Guarantor of integrity of entire study, S.B.F.; study concepts, S.B.F., S.J.R., B.F.K., J.F.B.; study design, D.G.K., S.B.F., B.F.K., S.J.R.; definition of intellectual content, J.F.B., B.F.K., S.J.R.; literature research, S.B.F.; clinical studies, D.G.K., B.F.K., J.F.B., S.B.F.; data acquisition, S.B.F., D.G.K., B.F.K., J.F.B.; data analysis, S.B.F., S.J.R., B.F.K., J.F.B.; statistical analysis, S.B.F.; manuscript preparation, S.B.F., B.F.K., S.J.R.; manuscript editing, S.B.F., S.J.R., B.F.K., J.F.B.; manuscript review and final version approval, all authors.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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