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DOI: 10.1148/radiol.2382050109
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(Radiology 2006;238:489-496.)
© RSNA, 2006

Experimental Studies

MR Imaging– versus Conventional X-ray Fluoroscopy–guided Renal Angioplasty in Swine: Prospective Randomized Comparison1

Reed A. Omary, MD, MS, James A. Gehl, BBA, Brian E. Schirf, MD, Jordin D. Green, PhD, Biao Lu, MD, F. Scott Pereles, MD, Jie Huang, ScD, Andrew C. Larson, PhD and Debiao Li, PhD

1 From the Departments of Radiology (R.A.O., J.A.G., B.E.S., J.D.G., B.L., F.S.P., A.C.L., D.L.), Biomedical Engineering (R.A.O., J.D.G., D.L.), and Preventive Medicine (J.H.), Northwestern University, Suite 700, 448 E Ontario St, Chicago, IL 60611. Received January 22, 2005; revision requested March 23; revision received April 15; final version accepted June 1. R.A.O. supported in part by NIH grant K08 DK60020. J.A.G. supported in part by the Radiological Society of North America Research & Education Foundation and the Northwestern University Feinberg School of Medicine Summer Research Program grant. Address correspondence to R.A.O. (e-mail: reed{at}northwestern.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Purpose: To test the hypothesis that the technical success rates, complication rates, and procedural times for magnetic resonance (MR) imaging–guided percutaneous transluminal angioplasty (PTA) and conventional (x-ray) fluoroscopy–guided PTA for treatment of renal artery stenosis are similar.

Materials and Methods: The study was animal care and use committee approved. After surgically inducing bilateral renal artery stenosis in 11 swine, the authors performed baseline digital subtraction angiography. They transferred each animal to a 1.5-T MR imaging unit and randomly decided which artery would be treated with MR-guided PTA. With MR imaging guidance, angioplastic devices were tracked by using active and passive techniques. Vascular depiction was achieved by using catheter-directed MR angiography. Stenotic vessels were dilated by using 5–6-mm-diameter balloon catheters. PTA was then performed in the contralateral artery by using conventional fluoroscopy–guided techniques. With the intention to treat, the authors compared the technical success (residual stenosis < 50%) rates, complication rates, and procedural times for each guidance method. They compared technical successes and complications by using the McNemar test and procedural times by using a paired t test, with P < .05 indicating a significant difference.

Results: The authors successfully dilated nine (82%) of 11 renal arteries with MR guidance and all 11 arteries (100%) with conventional fluoroscopic guidance. The difference was not significant (P = .5). Complications occurred in three (27%) arteries with MR guidance and in one (9%) artery with fluoroscopic guidance, with no significant differences (P = .5). The mean MR-guided PTA procedural time was 46 minutes longer than the fluoroscopy-guided PTA procedural time; this difference was significant (P = .01).

Conclusion: In a small cohort of swine, the authors did not observe a significant difference between MR imaging– and conventional fluoroscopy–guided renal artery PTA in terms of success and complication rates. However, no evidence of similarity between the techniques should be assumed. Procedural times differed significantly.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Renal vascular disease is a major contributor to hypertension and kidney disease. Of the approximately 60 million Americans with hypertension, 1%–5% have a renal vascular origin—most commonly atherosclerotic stenosis of the main renal artery (1). The clinical indications for treatment include renovascular hypertension (2,3) and ischemic nephropathy (4–6). The treatment of renal artery stenosis with endovascular stent placement and/or percutaneous transluminal angioplasty (PTA) performed with a balloon catheter is well accepted (7,8). Many patients with renal artery stenosis have diabetes mellitus, renal insufficiency, and/or a history of allergic reactions to iodinated contrast material. This group of patients is at greater risk for complications during conventional (x-ray) fluoroscopic interventional therapy (9).

Magnetic resonance (MR) imaging–guided PTA for treatment of renal artery stenosis has been shown to be feasible in swine (10). However, MR imaging guidance must be directly compared with conventional (x-ray) fluoroscopic guidance (hereafter referred to as fluoroscopic guidance) before it can be applied clinically in humans. Compared with interventions performed with fluoroscopic guidance, interventions performed with MR imaging guidance are associated with several perceived health benefits: There is a lack of exposure to ionizing radiation for the patient and the practitioner, the use of iodinated contrast material is avoided, concurrent tissue visualization is possible, and there is the potential to assess the success of the intervention by directly measuring renal function. The disadvantages of MR imaging guidance include the need to refine MR imaging techniques for catheter visualization, the requirement for advanced hardware and software systems, diminished spatial and temporal resolution compared with the resolution achievable with fluoroscopic guidance, and the unconfirmed safety of performing interventions with MR imaging guidance.

The purpose of our study was to test the hypothesis that the technical success rates, complication rates, and procedural times for MR imaging– and conventional (x-ray) fluoroscopy–guided (hereafter referred to as fluoroscopy-guided) PTA for treatment of renal artery stenosis are similar.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Animal Model
Our institutional animal care and use committee granted us approval to perform these experiments, which involved the use of 11 domestic swine (Oak Hill Genetics, Ewing, Ill) with a mean weight of 30 kg. We selected pigs as the animal model for these experiments because their size facilitates percutaneous vascular catheterization and their renal anatomy and physiologic characteristics closely resemble those of humans (11,12). The number of animals was selected on the basis of preliminary sample size calculations, which yielded an estimated power of 80% for the detection of a significant difference between fluoroscopy- and MR-guided renal artery PTA, with the assumption that such a difference existed.

We surgically induced bilateral renal artery stenosis in each animal (total of 22 arteries) by using reverse cable ties (13,14). This approach yields reproducible immediate stenoses that respond better to PTA than do the stenoses created in the more commonly used animal model of ameroid constrictors (13,15,16). The cable ties (Gardner Bender, Milwaukee, Wis) that we used, which have a bundle diameter of 1.5–20.6 mm, are widely available and require sterilization with ethylene oxide prior to their use in animals.

In a sterile surgical environment, we (B.E.S., B.L.) gained access to the renal arteries by using a flank retroperitoneal approach bilaterally. We aimed to produce 50%–75% stenosis by constricting one cable tie around each renal artery (Fig 1). Surgical incisions were closed by using standard three-layer suture techniques, and each pig was monitored until it recovered from anesthesia.


Figure 1
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  		Figure 1: Surgical placement by means of retroperitoneal approach involving reverse cable tie (arrow) around the left renal artery in a pig.

 
Digital Subtraction Angiography
Seven days after cable tie placement, the animals were transferred to the angiography suite, where they were tranquilized by using a combination of ketamine (10–15 mg per kilogram of body weight), xylazine (1–2 mg/kg), and atropine (0.05 mg/kg). The animals were administered isoflurane through a mask and then intubated for general anesthesia. After sterile preparation, we placed 8-F femoral sheaths with ultrasonographic (SiteRite; Bard, Murray Hill, NJ) guidance. Heparin (5000 U) was administered intravenously as an anticoagulant at the time of catheterization, and intravenous 1000-U doses of heparin were subsequently administered each hour. Transdermal nitroglycerine was administered to control vasospasms.

With fluoroscopic (PowerMobil; Siemens Medical Solutions, Erlangen, Germany) guidance, we advanced a 5-F multiple-side-hole angiographic catheter (Royal Flush II; Cook, Bloomington, Ind) into the suprarenal abdominal aorta. We (R.A.O., B.E.S, B.L.) then performed baseline digital subtraction angiography (DSA) by using an iodinated contrast agent (iohexol, Omnipaque 350; Amersham Health, Princeton, NJ) to visualize each stenosis and derive a reference standard for subsequent stenosis measurements.

After completing the DSA examination, we removed the catheter and transferred each animal to an adjacent 1.5-T MR imaging unit (Sonata; Siemens Medical Solutions). For each animal, we randomly selected the artery that would be treated with MR-guided PTA by flipping a standard two-sided American coin. If the coin showed "heads" after being flipped, MR-guided PTA was performed in the left renal artery; if the coin showed "tails," it was performed in the right renal artery. The contralateral artery was treated by using fluoroscopic guidance.

MR-guided PTA
We (R.A.O., B.E.S., B.L.) advanced a 0.030-inch-diameter loopless antenna guidewire coil (Intercept; Surgi-Vision, Gaithersburg, Md) and a 5-F aortic catheter filled with dilute 4% gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) through the existing femoral sheath with real-time MR imaging guidance. To our knowledge, this 0.030-inch-diameter guidewire is the smallest diameter guidewire that is commercially available for use in MR-guided interventions. The active guidewire was tracked by using steady-state free precession, and the catheter was passively tracked by using an inversion-recovery gradient-echo sequence (17,18). After the catheter was positioned in the suprarenal abdominal aorta, a small amount of dilute gadopentetate dimeglumine was intraarterially injected to verify catheter placement.

The inversion-recovery gradient-echo sequence parameters typically used were 2.3/1.15/50 (repetition time msec/echo time msec/inversion time msec), a 20° flip angle, a 206 x 300-mm field of view, a 74 x 256 acquisition matrix, and a 30-mm section thickness. With use of a sliding-window technique (19), 42 new lines were acquired during each acquisition period to yield an effective temporal resolution of 7 frames per second. The steady-state free precession sequence parameters typically used were 2.9/1.45 (repetition time msec/echo time msec), a 70° flip angle, a 206 x 300-mm field of view, a 70 x 128 matrix, and a 30-mm section thickness. We implemented the same sliding-window technique for steady-state free precession but acquired 40 new lines during each acquisition period. This yielded an effective temporal resolution of 9 frames per second.

As previously described, concentrations of intraarterially injected gadopentetate dimeglumine must be diluted according to local blood flow rates to balance competing T1 and T2* shortening effects (20). We measured local blood flow rates by using two-dimensional cine phase-contrast MR imaging (21), the results of which indicated that injected contrast agent concentrations of 6%–8% should be used (22,23). These concentrations were created by diluting full-strength (0.5 mol/L) gadopentetate dimeglumine with normal saline. The parameters typically used to perform phase-contrast MR imaging were 10.1/4.7, a 45° flip angle, a 240 x 110-mm field of view, a 5-mm section thickness, a 256 x 128 acquisition matrix, a 256 x 256 reconstruction matrix, and a velocity-encoding value of 300 cm/sec.

Once we verified the placement of the aortic catheter, we visualized the aortorenal segments by using catheter-directed MR angiography. By using three-dimensional (3D) gradient-echo imaging, we injected 40 mL of 6%–8% gadopentetate dimeglumine at 4 mL/sec by using an automated power injection system (Medrad, Indianola, Pa). This examination served as the preprocedural MR angiographic examination used to make subsequent vessel measurements. The 3D gradient-echo imaging parameters typically used were 4.3/1.52, a 50° flip angle, a 300 x 300-mm field of view, a 384 x 512 acquisition matrix, a single shot, six partitions interpolated to 12, a 48-mm slab thickness, and a 0.8 x 0.6-mm in-plane spatial resolution.

After removing the aortic catheter, we inserted through a guiding sheath a selective renal catheter that coaxially contained the same 0.030-inch-diameter active internal guidewire. We advanced the catheter and selected the appropriate renal artery by using the inversion-recovery gradient-echo catheter-tracking sequence described earlier. We confirmed the placement of the catheter by manually injecting a small amount of dilute gadopentetate dimeglumine intraarterially through the selective renal artery catheter. The guiding sheath was advanced over the selective catheter so that the tip of the sheath rested in the proximal portion of the renal artery. The selective renal catheter and active guidewire were withdrawn and replaced with a 5–6-mm-diameter conventional balloon catheter (Cordis Europa, Roden, The Netherlands) filled with dilute gadopentetate dimeglumine. The diameter of the balloon was based on estimations of the vessel diameter derived at 3D MR angiography. Dilation of the stenotic vessel was visualized in real-time by using the inversion-recovery gradient-echo catheter-tracking sequence. We performed another catheter-directed intraarterial injection by hand to initially assess whether technical success (ie, <50% residual stenosis) in dilating the stenotic vessel had been achieved.

After determining the presence of less than 50% residual stenosis, we replaced the balloon catheter with the active guidewire and the aortic catheter positioned in the suprarenal abdominal aorta. To visualize the vascular system after successful PTA by using the same 3D gradient-echo sequence, we injected 40 mL of 6%–8% dilute gadopentetate dimeglumine by using the automated power injection system. After MR-guided PTA, the swine were transferred back to the angiography suite for fluoroscopy-guided PTA in the contralateral artery.

Fluoroscopy-guided PTA
To confirm the success of the MR-guided procedure, we positioned a 5-F multiple-side-hole angiographic catheter in the suprarenal abdominal aorta for intraarterial injection of iodinated contrast material. After this injection, the angiographic catheter was replaced with a standard selective visceral catheter (cobra or multipurpose angled), and the artery randomly selected for PTA with fluoroscopic guidance was identified. We (R.A.O., B.E.S., B.L.) performed PTA by using conventional methods with a 5–6-mm balloon catheter (Cordis Endovascular, Warren, NJ). We used a 0.035-inch-diameter guidewire (TAD II; Mallinkrodt, Hazelwood, Mo), tapered to 0.018-inch distally, to cross the stenoses. After both the MR-guided and the fluoroscopy-guided procedures were completed, we performed DSA by using the same method used to perform preprocedural DSA. This examination yielded postprocedural images for subsequent reference-standard analysis. The renal arteries that could not be dilated with MR imaging guidance were then dilated with fluoroscopic guidance.

Data Analysis
To assess the success of the MR-guided interventions, 3D multiimage projections were generated from the pre- and post-PTA MR angiographic sequences. The pre- and post-PTA DSA images were analyzed to assess the success of the fluoroscopy-guided procedures. The pre- and post-PTA stenosis measurements were analyzed (J.A.G., J.D.G.) on a computer workstation (Dell, Round Rock, Tex) by using computer software (ImageJ 1.32j; National Institutes of Health, Bethesda, Md). Full width at half maximum values were used to calculate vessel diameters. Use of this technique allows one to limit the variance that could be introduced by window and level settings (24).

Each image was downloaded into the ImageJ software and interpolated to twice its original dimensions on the x- and y-axes. To visualize the renal arteries and obtain accurate and consistent measurements, we magnified the 3D MR angiographic multiimage projections to 200% and maintained the original magnification of the DSA images. For both fluoroscopy-guided and MR-guided PTA, we obtained measurements at the stenosis and proximal to the stenosis, which provided a reference of the normal lumen diameter. Relative changes between the pre- and post-PTA stenosis measurements were compared with measurements of the uninvolved proximal artery. Relative measurement changes accounted for the differences in spatial resolution and magnification between the DSA images and the 3D MR angiographic multiimage projections. Stenosis percentage was measured as follows: [1 – (FWHMsten/FWHMprox)]x 100, where FWHMsten is the full width at half maximum value at the stenosis and FWHMprox is the full width at half maximum value proximal to the stenosis.

Statistical Analyses
We assessed the technical success rate, complication rate, and procedural time for each MR- and fluoroscopy-guided procedure with the intention to treat. Technical success was defined as the post-PTA achievement of less than 50% stenosis. Complications were defined as untoward adverse events, including dissection, vessel rupture, renal infarction, and/or death. Procedural time was measured as the time that elapsed between the initial insertion of the renal catheter and the completion of post-PTA angiography. We used StatExact, version 6.0, software (Cytel Software, Cambridge, Mass) to perform the statistical analyses. We assessed success and complication rates by using the McNemar test and assessed procedural times by using a two-sided paired t test. Statistical significance was set at the P < .05 level.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
We were able to successfully visualize all angioplastic devices with MR imaging guidance by using active and passive tracking techniques. The combination of an active nitinol guidewire and dilute gadopentetate dimeglumine–filled catheters enabled us to track the devices and monitor the PTA procedure in real time (Fig 2).


Figure 2
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  		Figure 2a: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 

Figure 2
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  		Figure 2b: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 

Figure 2
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  		Figure 2c: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 

Figure 2
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  		Figure 2d: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 

Figure 2
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  		Figure 2e: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 

Figure 2
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  		Figure 2f: Bilateral stenosis treated with PTA in a pig. (a) Magnified frontal DSA image and (b) corresponding coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) show tight bilateral renal artery stenosis (arrows in a). (c) Coronal MR image (2.9/1.45, 70° flip angle) shows tracking of active guidewire across left renal artery stenosis. The guidewire is coaxially positioned through a balloon catheter. (d) Coronal MR image (2.3/1.15, 20° flip angle) shows expanded PTA balloon catheter (arrows) filled with dilute 4% gadopentetate dimeglumine. Radiofrequency interference from the in-room liquid crystal display monitor causes a horizontal band artifact (arrowheads). (e) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery successfully treated with PTA. (f) Corresponding frontal DSA image findings confirm successful MR-guided PTA.

 
Technical Success
We successfully dilated nine (82%) of 11 renal arteries with MR imaging guidance and all 11 (100%) renal arteries with fluoroscopic guidance. Using the McNemar exact test, the null hypothesis of the equality of the success rates associated with the two procedures was not rejected (P = .5). The difference in success rates between MR-guided PTA and fluoroscopy-guided PTA was –18%, with corresponding 95% and 90% exact confidence intervals of –53% to 13% and –47% to 9%, respectively. DSA results confirmed the technical success of the nine MR-guided procedures. It was not possible to dilate the two arteries with high-grade stenosis in which MR-guided PTA was attempted by using fluoroscopy-guided techniques. The mean percentages of stenosis in arteries before and after fluoroscopy-guided PTA were 71% ± 10 (standard deviation) (range, 58%–87%) and 23% ± 16 (range, 0%–48%), respectively. The mean percentages of stenosis in arteries before and after MR-guided PTA were 63% ± 10 (range, 50%–78%) and 19% ± 16 (range, 0%–39%), respectively. A renal artery successfully treated with MR-guided PTA is shown in Figure 3.


Figure 3
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  		Figure 3a: Successful MR-guided PTA performed in left renal artery of a pig. (a) Baseline frontal magnified DSA image shows tight stenosis (arrow). (b) Coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery before MR-guided PTA. (c) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery after successful MR-guided PTA. (d) Corresponding frontal DSA image findings confirm successful MR-guided dilation of the stenotic left renal artery.

 

Figure 3
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  		Figure 3b: Successful MR-guided PTA performed in left renal artery of a pig. (a) Baseline frontal magnified DSA image shows tight stenosis (arrow). (b) Coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery before MR-guided PTA. (c) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery after successful MR-guided PTA. (d) Corresponding frontal DSA image findings confirm successful MR-guided dilation of the stenotic left renal artery.

 

Figure 3
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  		Figure 3c: Successful MR-guided PTA performed in left renal artery of a pig. (a) Baseline frontal magnified DSA image shows tight stenosis (arrow). (b) Coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery before MR-guided PTA. (c) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery after successful MR-guided PTA. (d) Corresponding frontal DSA image findings confirm successful MR-guided dilation of the stenotic left renal artery.

 

Figure 3
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  		Figure 3d: Successful MR-guided PTA performed in left renal artery of a pig. (a) Baseline frontal magnified DSA image shows tight stenosis (arrow). (b) Coronal 3D intraarterial subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery before MR-guided PTA. (c) Coronal 3D subtracted MR angiogram (4.3/1.52, 50° flip angle) shows left renal artery after successful MR-guided PTA. (d) Corresponding frontal DSA image findings confirm successful MR-guided dilation of the stenotic left renal artery.

 
Complications
Complications, which included flow-limiting and non–flow-limiting dissections, occurred in three (27%) of 11 arteries during MR-guided PTA and in one (9%) artery during fluoroscopy-guided PTA. Using the McNemar exact test, the null hypothesis of the equality of the complication rates associated with the two procedures was not rejected (P = .5). The difference in complication rates between MR-guided PTA and fluoroscopy-guided PTA was 18%, with corresponding 95% and 90% exact confidence intervals of –13% to 53% and –9% to 47%, respectively. Complications were detected in real-time with MR imaging guidance and subsequently confirmed at DSA. A dissection that was detected in real time during MR-guided PTA and confirmed at DSA is shown in Figure 4.


Figure 4
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  		Figure 4a: Left renal artery dissection (arrow) in a pig, (a) initially detected on 3D intraarterial MR angiogram (4.3/1.52, 50° flip angle) and then (b) confirmed by findings on subsequently obtained frontal DSA image.

 

Figure 4
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  		Figure 4b: Left renal artery dissection (arrow) in a pig, (a) initially detected on 3D intraarterial MR angiogram (4.3/1.52, 50° flip angle) and then (b) confirmed by findings on subsequently obtained frontal DSA image.

 
Procedural Times
Mean procedural times were 31 minutes ± 18 (standard deviation) for fluoroscopy-guided PTA and 77 minutes ± 46 for MR-guided PTA. This 46-minute mean difference in procedural time was significant (P = .01). Although we successfully crossed nine stenoses with MR imaging guidance by using the 0.030-inch-diameter guidewire, we were unable to cross two high-grade stenoses with this technique. Figure 5 shows an example of a guidewire that buckled because its diameter was too large for it to cross one of these tight stenoses.


Figure 5
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  		Figure 5: Coronal MR image (2.9/1.45, 70° flip angle) shows guidewire buckling during unsuccessful attempt to cross a high-grade left renal artery stenosis in a pig. The guidewire ascended in the abdominal aorta but then buckled (arrows) as its tip attempted to cross the severe stenosis.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
In a prospective randomized study with swine, we did not detect a significant difference in success or complication rates between MR- and fluoroscopy-guided PTA of renal arteries; however, given the small sample, no evidence of similarity in outcomes between the two techniques should be concluded. We successfully performed MR-guided PTA in nine of 11 renal arteries, with each success confirmed at DSA. Complications occurred in three (27%) arteries during MR-guided PTA and in one (9%) artery during fluoroscopy-guided PTA. Each of these complications, which were detected in real time with MR imaging guidance, was confirmed at DSA. However, the mean procedural time with MR imaging guidance remained significantly longer than that with fluoroscopic guidance (77 vs 31 minutes).

There are limited reports of MR imaging being used to guide percutaneous treatment of renal artery stenosis (10,25,26). Omary et al (10) observed the feasibility of MR-guided renal artery PTA in four swine by using an ameroid model of renal artery stenosis. Le Blanche et al used MR imaging guidance to perform bilateral PTA for renal artery stenosis in rabbits (25). Wacker et al performed PTA and stent placement in the renal arteries of four swine (26).

Our randomized comparison between MR-guided and fluoroscopy-guided PTA advances the results of these previous studies because we performed direct comparisons of the technical success rates, complication rates, and procedural times between MR-guided and fluoroscopy-guided PTA in swine. With the exception of the Le Blanche et al study (25), in which a rabbit model was used, previous investigations were performed in uncontrolled case studies with small numbers of swine (10,26).

This study also reflects advances in technical proficiency in that we successfully implemented a combination of active and passive techniques for real-time guidance of an endovascular procedure by using MR imaging. In previous studies, only passive tracking without real-time monitoring (10) or susceptibility artifact imaging (25,26) was used to visualize devices. Real-time device tracking and procedure monitoring enable the detection of complications during the course of the procedure. In contrast to previous study investigators, we successfully verified the success of MR-guided PTA by using DSA in our study. We also confirmed each complication noted during the MR-guided procedures by using DSA.

Our study findings corroborate the results of previous studies that revealed intraarterial MR angiography and DSA to have similar accuracy in depicting renal artery stenosis (27,28). In no case did MR imaging lead to the misclassification of a residual stenosis as absent on the basis of our criteria for technical success. Also, in no instance did MR imaging facilitate failure to detect a procedural complication. These results are encouraging because they suggest that MR imaging might be able to be used as the sole method to guide PTA and monitor the procedure for complications in the future. However, we still recommend that initial human studies be performed in the context of hybrid fluoroscopy–MR imaging systems (29) until greater experience is acquired.

There were several important limitations to this study. First, our comparison was performed in an animal model of renal artery stenosis, which may not be generalizable to humans with renal artery stenosis. This model does not simulate the typical presentation of patients with heavily calcified ostial stenoses who are referred for renal interventions. Additionally, given the variable manifestations of postostial stenoses that occur within 2–3 cm of the renal artery segmental bifurcation, this animal model cannot represent all the types of renal artery stenoses that are encountered clinically. However, randomized studies in animals are required before translation to the clinical arena. Second, the MR-guided procedures have not been approved by the Food and Drug Administration. If the described intraarterial injections of gadopentetate dimeglumine were used in humans, they would represent an off-label route of administration of an approved contrast agent. The active guidewire has been approved for arterial plaque imaging—but not for use as a guidewire—in humans.

Third, if a significant difference in success and complication rates between the two guidance techniques does exist, it is possible that it would not be detected in our relatively small sample. Fourth, compared with the number of endovascular devices available for use with fluoroscopic guidance, there is a small armamentarium of MR-compatible endovascular devices available. The low availability of compatible guidewires in particular is a major device limitation for MR-guided procedures: To our knowledge, there are currently no commercially available MR-compatible 0.018-inch-diameter or smaller guidewires. If such guidewires were available, they would enable the use of lower profile balloon catheter systems that would potentially improve the success rate of treatments for high-grade stenoses. This limitation is among the factors that contribute to the longer procedural times for MR-guided PTA compared with the procedural times for fluoroscopy-guided PTA. If we encounter a high-grade stenosis with MR imaging guidance, we have few alternative lower-profile devices to attempt to cross the stenosis. Finally, the temporal and spatial resolution of MR imaging remains substantially lower than that of conventional fluoroscopy. Applications of fast MR imaging techniques such as parallel imaging (30,31) and projection reconstruction (32) might offer improvements in future studies.

Practical applications: In this cohort of swine, we did not observe a significant difference in technical success or complication rates between MR- and fluoroscopy-guided PTA. However, given our small sample, we do not think that any similarity in outcomes between the two techniques should be assumed. As expected, procedural times were significantly longer with MR imaging guidance. Before MR imaging guidance can be used clinically, improvements in MR-compatible devices, temporal resolution, and spatial resolution are required. Future steps include exploiting the ability of MR imaging to enable real-time assessment of end-organ function, such as the glomerular filtration rate, which might be used to predict the success of the endovascular intervention.


   ACKNOWLEDGMENTS
 
The authors thank Richard Tang, MD, for his expert animal assistance.


   FOOTNOTES
 

Abbreviations: DSA = digital subtraction angiography • PTA = percutaneous transluminal angioplasty • 3D = three-dimensional

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, R.A.O.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, R.A.O., J.A.G.; experimental studies, R.A.O., J.A.G., B.E.S., J.D.G., B.L., A.C.L., D.L.; statistical analysis, R.A.O., J.A.G., J.H.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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J. K. Park, T. K. Rhee, T. A. Cashen, W. Shin, B. E. Schirf, J. A. Gehl, A. C. Larson, P. V. Prasad, D. Li, T. J. Carroll, et al.
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