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(Radiology. 1999;211:693-697.)
© RSNA, 1999

Experimental Studies

Plain and Covered Stent-Grafts: In Vitro Evaluation of Characteristics at Three-dimensional MR Angiography1

Paul R. Hilfiker, MD, Harald H. Quick, MS and Jörg F. Debatin, MD

1 From the Institute of Diagnostic Radiology, University Hospital Zurich, Rämistrasse 100, CH-8091 Zurich, Switzerland. Received July 2, 1998; revision requested August 11; revision received September 1; accepted November 20. Address reprint requests to J.F.D.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To evaluate the characteristics of various plain and covered stents as regards demonstration of the contained lumen with simulated contrast material–enhanced fast three-dimensional (3D) magnetic resonance (MR) angiography.

MATERIALS AND METHODS: Six stents (Easy Wallstent, Cragg, Palmaz, Cragg EndoPro System 1, Corvita, Passager) were implanted in plastic tubes and integrated into a closed-tubing circuit driven by a pulsatile roller flow pump. The circulating water was spiked with gadopentetate dimeglumine. Three-dimensional MR angiograms were obtained with an echo time of 1.4 or 2.1 msec and with partial (0.5 signal acquired) or full (one signal acquired) k-space sampling. The size of the stent-associated artifact was determined relative to the tube diameter.

RESULTS: The Easy Wallstent and Palmaz stent caused complete obliteration of the stent lumen with all sequences. The Cragg, Cragg EndoPro System 1, and Passager stents allowed good visualization of the stent lumen. The Corvita stent demonstrated major artifacts. The magnitude of the stent-associated artifact was related to the echo time (P < .01) but not to the type of k-space sampling (P = .35).

CONCLUSION: The luminal patency of selected plain and covered stents can be assessed with contrast-enhanced 3D MR angiography.

Index terms: Blood vessels, stenosis or obstruction, 9*.702 • Magnetic resonance (MR), artifact, 9*.12942, 9*.12943 • Magnetic resonance (MR), vascular studies, 9*.12917, 9*.129412, 9*.12942 • Stents and prostheses, 9*.1268


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Intravascular stents are widely used to improve vessel patency after balloon angioplasty (1,2). The therapeutic benefit of stent placement has been proved in the iliac and renal arteries (3,4). In the femoral arteries, where less flow and excessive neointimal hyperplasia can lead to early reocclusion (5), use of covered stent-grafts has been attempted with some success (6). Because of a need for close follow-up, several techniques have been used to monitor vessel patency after stent placement (3,7,8). Digital subtraction angiography remains the standard of reference for detecting restenosis (8).

Contrast material–enhanced three-dimensional (3D) magnetic resonance (MR) angiography is rapidly emerging as an attractive noninvasive alternative to digital subtraction angiography in the preinterventional assessment of the thoracic and abdominal aorta, its branches, the iliac arteries, and most recently the peripheral vessels (9–14). Although the appearances of endovascular stents at MR imaging with various two-dimensional spin-echo or gradient-echo sequences have been described (15–17), the appearances at contrast-enhanced fast 3D MR angiography have remained unexplored. Thus, we performed a study to evaluate the characteristics of various plain and covered stents at fast 3D MR angiography as regards demonstration of the contained lumen.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Experimental Setup
Various 6- or 10-mm-diameter, noncovered or covered, commercially available stents, all of which are routinely used at our institution, were studied (Table 1). Table 2 summarizes the underlying materials of the stents. For imaging, the stents were implanted in plastic tubes that had an inner luminal diameter of 6 or 10 mm and simulated the femoral or external iliac artery, respectively. The tube was integrated into a closed-tubing circuit driven by a pulsatile roller flow pump. The pump was programmed to provide a flow rate of 700 mL/min with 60 pulses per minute. To simulate the signal intensity inherent in the arterial lumen on 3D MR angiograms, the circulating water was spiked with gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) in a concentration of 1:20. This concentration was chosen on the basis of the results of prior dilution series, which simulated the intraarterial signal intensity on contrast-enhanced 3D MR angiograms. Each stent was assessed separately by placing the tube containing the stent into a water-filled plastic container (Fig 1).


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  		TABLE 1. Stents Evaluated with 3D MR Angiography
 

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  		TABLE 2. Underlying Materials of the Stents
 


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  		Figure 1. Experimental setup. Photograph shows a 10-mm-diameter tube containing a stent positioned in a plastic container filled with water. A pulsatile roller flow pump was connected to the tube in a closed circuit to simulate blood flow.

 
MR Imaging
MR imaging was performed with a 1.5-T imager (Signa EchoSpeed; GE Medical Systems, Milwaukee, Wis); an anteroposterior phased-array surface coil was used for signal reception. For imaging, the plastic tube containing the stent was positioned in the center of the bore parallel to the main magnetic field (B0). After acquisition of axial multiphase gradient-echo localizing images, a 3D Fourier transform fast spoiled gradient-echo acquisition consisting of 25 contiguous 2.0-mm-thick coronal sections was performed. Fifty overlapping sections were reconstructed by means of zero interpolation. The sequence had a sampling bandwidth of ±62.5 kHz and a flip angle of 20°. Three-dimensional data sets were obtained by using partial echoes (6.8/1.4 [repetition time msec/echo time msec]) or full echoes (7.7/2.1). Each sequence was assessed with partial k-space sampling (0.5 signal acquired) or full k-space sampling (one signal acquired). A 35 x 35-cm field of view and a 256 x 192 matrix provided an in-plane resolution of 1.3 x 1.8 mm.

Characterization of Stent Appearance
Source images of the stents were displayed as subvolume (targeted) maximum intensity projection images. For each stent, the ability to assess the stent lumen with each of the four sequences was evaluated by a single observer (P.R.H.). The size of the stent-associated artifact (signal void) was determined relative to the diameter of the "vessel" (plastic tube). Measurements of the diameter of the stent lumen were based on signal intensity plots drawn orthogonal to the long axis of the stent at three locations: 10 mm inside from both stent ends and in the middle of the stent. The luminal stent diameter was defined as the distance between the two inner points of half maximum or minimum signal intensity. The worst-case planes were chosen from one artifact ridge to the opposite one. Measurements were rounded to the nearest millimeter. The luminal diameter was compared with the known tube diameter for all stents and imaging parameters.

Statistical analysis was performed with the paired Student t test. A P value of less than .05 was used to indicate a statistically significant difference.


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The degree of luminal obliteration due to stent-associated artifact is summarized in Table 3. Both sizes of the Easy Wallstent caused extensive artifact with all four sequences; demonstration of the stent lumen was not possible. In contrast, the stent lumen was well demonstrated with the Cragg, Cragg EndoPro System 1, and Passager stents independently of the stent diameter. The appearance of these stents was characterized by only minor artifacts, which consisted of thin, dark, indistinct ridges that appeared to indent the stent lumen and were directly related to the stent filaments (Fig 2). Both sizes of the Palmaz stent caused complete obliteration of the lumen; no signal was observed within the stent. The lumen of the Corvita stent appeared severely narrowed, a finding that reflects considerable artifacts; evaluation of the stent lumen was not possible.


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  		TABLE 3. Extent of Artifactual Stent-related Luminal Narrowing Relative to Actual Tube Diameter
 


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  		Figure 2. MR angiograms of stents implanted in a 10-mm-diameter tube. Signal loss due to a Palmaz stent (arrow) and Easy Wallstent (double arrow) simulates complete obliteration of the stent lumen. A Cragg stent (arrowhead) and Passager stent (double arrowhead) cause only minor stent-related artifact. With these stents, the contained lumen remains clearly visible. Although the vessel lumen covered by the Corvita stent (*) remains visible, the associated artifact is too large to exclude a high-grade stenosis.

 
The length of the echo time did influence the size of the stent-associated artifact (Table 4). Artifacts were significantly less pronounced in data sets collected with the partial-echo technique than in those acquired with the full-echo technique (P < .01). The shorter the echo time, the less pronounced the artifacts. The use of partial k-space sampling did not adversely affect stent-related artifact size; differences between the two sets of sequences were not statistically significant (P = .35).


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  		TABLE 4. Effect of Imaging Parameters on the Extent of Luminal Obliteration
 

   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
Arteries in which metallic endoprostheses are placed are subject to intimal hyperplasia and thrombus formation, which threaten vascular patency. Different imaging modalities are used to monitor vessel patency after intravascular stent placement (7,18). Enthusiasm for the use of intraarterial digital subtraction angiography, which provides an excellent overview of the stent configuration, is tempered by the invasiveness of the technique (19). Duplex ultrasonography (US) has the disadvantages of difficulty in penetrating the stent cage, a problem that complicates assessment of the vascular lumen and wall, as well as operator dependence (8). Spiral computed tomographic (CT) angiography, which is being used for evaluation of the abdominal aorta after stent placement (19), is less capable than other techniques of demonstrating the peripheral vasculature, where most stents are placed.

Recent years have seen the rapid application of contrast-enhanced 3D MR angiography to the assessment of virtually all arterial territories (9–11). The technique is based on the availability of high-performance gradient systems, which are capable of reducing data collection times sufficiently to acquire an entire 3D image set within the confines of a comfortable breath hold (10). The underlying principle of contrast-enhanced 3D MR angiography, which exploits the intraarterial phase of imaging with intravenously administered extracellular T1-shortening contrast material, is similar to that of CT angiography. However, contrast-enhanced 3D MR angiography has several advantages. Besides the absence of ionizing radiation and the ability to acquire images in nonaxial planes, imaging with paramagnetic contrast material has a more favorable safety profile with respect to anaphylactic reactions. In addition, paramagnetic contrast agents are not nephrotoxic (20,21), a quality that permits repeated use even in patients with impaired renal function. Contrast-enhanced 3D MR angiography would thus constitute an alternative for the close follow-up required after implantation of selected "MR imaging–compatible" stents.

Evaluation of the MR imaging appearances of intravascular devices has been limited mainly to two-dimensional gradient-echo or conventional spin-echo sequences (15–17). Laissy et al (16) concluded that two-dimensional time-of-flight MR angiography should not be used for assessment of the vascular lumen contained by a stent regardless of the stent type. However, citing the short echo time inherent in contrast-enhanced 3D MR angiography, these authors speculated that their recommendation may be different for stent imaging with 3D MR angiography. Conversely, Thurnher et al (22) expected 3D MR angiography to be associated with severe stent-related artifacts. The data of our in vitro study demonstrate that this concern should not be generalized. The short echo times inherent in fast 3D gradient-echo sequences limit susceptibility artifacts sufficiently to permit assessment of the luminal patency of selected commercially available stents with contrast-enhanced 3D MR angiography. The results of a recently published clinical study of patients who underwent endovascular treatment of abdominal aortic aneurysms support this observation (23).

MR imaging artifacts associated with endoprostheses are related to the geometry of the stent and the underlying metal composing the stent (16,24). The effect of the latter dominates when imaging the cobalt-based alloy Easy Wallstent or the stainless-steel Palmaz stent. These prostheses caused large signal voids on 3D MR angiograms; as a result, assessment of the stent lumen and thus stent patency was impossible. The Corvita stent is constructed of the same cobalt-based alloy as the Wallstent. However, the Corvita stent also has a tantalum core. The presence of this core reduced the associated artifact and thereby maintained the possibility of assessing luminal patency. The artifacts were too extensive, however, to exclude the presence of even high-grade (>50%) stenosis.

Conversely, stents made of nitinol (Cragg, Cragg EndoPro System 1, and Passager stents) caused only minor artifacts. With these stents, the luminal diameter was artifactually reduced by only 0%–20% in most cases (Table 3). The nitinol frame filaments of the stent were identifiable on the individual imaging sections and reformation images as distinct areas of signal void, an appearance that allowed detailed assessment of stent structure. The stent lumen could be assessed with ease. Luminal patency can thus be established, and hemodynamically significant stenosis (that causing >50% luminal narrowing) can be excluded with certainty. However, intimal hyperplasia causing less than 50% luminal narrowing would likely remain undetected.

The signal voids associated with the stents primarily reflect susceptibility artifacts (16). These "black hole" artifacts reflect magnetic field distortions. The magnitude of these effects is directly dependent on the echo time (25). Thus, it was not surprising that the partial-echo technique allowed better assessment of the stent lumen than did the full-echo technique.

The introduction of metallic conducting materials into the imager can potentially lead to hazardous ferromagnetic or thermal effects (26–28). Many authors have concluded that the induction of electric currents and tissue heating are negligible for small implants at clinically used magnetic field strengths (29,30). In addition, numerous reports have suggested that metallic implants cause little harm in an MR environment as long as they are nonferromagnetic (30–33). All of the evaluated stents can thus be considered safe for MR imaging (17,29,31,34,35).

Contrast-enhanced 3D MR angiography can thus be used for monitoring the vascular lumen after stent placement if the correct stent was used. To avoid misinterpretation of the signal void associated with some stents as stent occlusion, manufacturers should be asked to label stents as suitable or unsuitable for 3D MR angiography. Such labeling may actually influence the choice of stent product, although this effect will depend on the type of vascular practice.

Practical application: The ability to evaluate stents with contrast-enhanced 3D MR angiography is dependent on the type of stent. Provided nitinol-based noncovered or covered stents are used, 3D MR angiography may be considered an attractive alternative to digital subtraction angiography, US, or CT angiography for assessing the stent lumen.


   Footnotes
 
9*. Vascular system, location unspecified Back

Abbreviation: 3D = three-dimensional

Author contributions: Guarantor of integrity of entire study, J.F.D.; study concepts and design, J.F.D., H.H.Q., P.R.H.; definition of intellectual content, P.R.H.; literature research, P.R.H.; experimental studies, P.R.H., H.H.Q.; data acquisition and analysis, P.R.H., H.H.Q.; statistical analysis, P.R.H.; manuscript preparation and editing, J.F.D., P.R.H.; manuscript review, J.F.D.


   References
TOP
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 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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K. Bertschinger, P. C. Cassina, J. F. Debatin, and S. G. Ruehm
Surveillance of Peripheral Arterial Bypass Grafts with Three-Dimensional MR Angiography: Comparison with Digital Subtraction Angiography
Am. J. Roentgenol., January 1, 2001; 176(1): 215 - 220.
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R. Tello, K. R. Thomson, J. F. Debatin, and P. R. Hilfiker
MR Angiography of Intravascular Stents Drs Debatin and Hilfiker respond:
Radiology, April 1, 2000; 215(1): 305 - 305.
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N. M. Rofsky and M. A. Adelman
MR Angiography in the Evaluation of Atherosclerotic Peripheral Vascular Disease
Radiology, February 1, 2000; 214(2): 325 - 338.
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R. Ahmadi, M. Schillinger, T. Maca, and E. Minar
Femoropopliteal Arteries: Immediate and Long-term Results with a Dacron-covered Stent-Graft
Radiology, May 1, 2002; 223(2): 345 - 350.
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T. Kuehne, M. Saeed, P. Moore, K. Gleason, G. Reddy, D. Teitel, and C. B. Higgins
Influence of Blood-Pool Contrast Media on MR Imaging and Flow Measurements in the Presence of Pulmonary Arterial Stents in Swine
Radiology, May 1, 2002; 223(2): 439 - 445.
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