MRA Evaluation of Intravascular Stents
MRA can be successfully utilized in evaluating intravascular stents, particularly in the case of newer nonferromagnetic stents.
The majority of atherosclerotic visceral artery stenosis, including that occurring in the renal arteries, is due to ostial plaque formation that requires stent implantation for optimal endovascular treatment. Hemodynamically significant restenosis after stent placement occurs in approximately 11% to 25% of cases.1 The evaluation of patency after stent placement may be performed using conventional catheter angiography, CTA, or duplex ultrasound. None of these techniques is optimal. Catheter angiography is inherently invasive, and both angiography and CTA require the administration of potentially nephrotoxic radiocontrast. Duplex sonography is operator dependent and often technically difficult due to patient respiration or body habitus.2
MRA has emerged as an easily performed and reliable imaging modality for native renal and visceral artery stenosis. However, MR imaging after intravascular stent placement is challenging. The evaluation of stent patency is limited by shielding from radiofrequency signals and susceptibility artifacts (ferromagnetic artifact), both of which cause loss of signal both within and adjacent to the stented vessel.3-12 Ferromagnetic stainless steel stents result in marked image degradation due to local magnetic field heterogeneity, essentially precluding reliable image interpretation.13
Although visceral artery stenting has traditionally been performed with stainless steel alloy stents, newer stent designs have emerged during the past several years. Nonferromagnetic stents manufactured of medical-grade cobalt-chromium, platinum, or nitinol are expected to produce less magnetic susceptibility and a markedly reduced degree of MRI image interference than traditional stainless steel alloy, balloon-expandable stents. To study this, we have conducted gadolinium-enhanced MR imaging in both static phantoms and human patients with cobalt-chromium stents (Racer, Medtronic, Santa Rosa, CA) and stainless steel stents (Herculink, Guidant Corporation, Indianapolis, IN) to determine comparative image quality and optimize MR techniques for evaluating stent patency.
Balloon-expandable cobalt-chromium and stainless steel stents were deployed to 6 mm in diameter in plastic straws filled with 6% gadolinium solution (3 mL in 50 mL saline) and suspended in a gelatin agar phantom model. Sequential coronal images of the phantom were then obtained using variable echo time (TE), flip angle, and slice thickness. Figures 1 and 2 show the effects of varying these imaging parameters on in-stent 3D MR visibility.
Patients who had previously undergone placement of a cobalt-chromium alloy stent in the renal artery or superior mesenteric artery were imaged. Racer stents were used in this series because they were the only balloon-expandable cobalt-chromium stents commercially available at the time of the procedures. Some of the patients also had a stainless steel stent implanted in the contralateral renal artery allowing for comparison. Contrast-enhanced MRA imaging was performed after intravenous injection of gadolinium. Fluoro-triggering was utilized to detect the arrival of the gadolinium bolus in the abdominal aorta. Finally, sequential acquisition of k-space was used to decrease the artifact caused by the metallic stent. Images are shown in Figures 3 through 7.
MRA is easily performed, operator independent, and is an attractive modality for imaging patients with visceral artery stenosis. However, traditionally utilized stainless steel stents preclude effective MR imaging due to signal loss caused by magnetic susceptibility effects and Faraday shielding.3-5,7 Although more recently available nonferromagnetic stents do not produce signal loss from magnetic susceptibility, shielding effects may still degrade image quality. To achieve optimal imaging, imaging parameters may be adjusted to enhance in-stent signal.4,10 The impact of varying different MR parameters is shown in Table 1.
Three-dimensional gadolinium-enhanced MRA generally employs short TR (repetition time) and TE.12 To minimize dephasing, the shortest possible TE should be used, with a bandwidth generally of 62.5 kHz. Additionally, to overcome some of the radiofrequency shielding limitations imposed by a stent, a high flip-angle (usually 60º) is needed. A small slice thickness (typically 2 mm to 3 mm) should be used to increase resolution.8
In most of our clinical cases, 40 mL of gadolinium was injected. Fluoro-triggering was utilized to detect the arrival of the gadolinium bolus in the abdominal aorta. Finally, sequential acquisition of k-space was used to decrease the artifact caused by the metallic stent. Imaging during the peak of gadolinium enhancement, using either fluoro-triggering or automatic bolus detection (such as SmartPrep, GE Healthcare, Waukesha, WI) is necessary for image optimization. Additionally, sequential ordering of k-space limits ringing artifact, which results when imaging starts too early.
Three-dimensional MRA should be obtained in the plane that limits the number of slices (to decrease time and breath-hold) but covers the entire area of interest. We have found that reading 3D MRA on a workstation is optimal because it allows quick and easy reconstructions in multiple planes. In our clinical practice, we utilize the MRA parameters listed in Table 2 to obtain high-quality diagnostic images.
The cobalt-chromium stent used in these studies produced diagnostic images in the majority of cases, despite incomplete acquisition optimization in several instances. Flow lumen images produced were superior in all cases to those seen with stainless steel stents. Understanding the MRI strategies to maximize in-stent visualization should allow quality MRI of cobalt-chromium stents in most patients.
The Racer stent was used in this study because it was the only commercially available cobalt-chromium platform at the time of investigation. Recently, another cobalt-chromium stent has become available (Palmaz Blue, Cordis Corporation, a Johnson & Johnson company, Miami, FL). In addition, there are various stents composed of nitinol and platinum alloys, which are also non-ferromagnetic. Although we have not evaluated the MRI transparency of each of these stents in either an MR phantom or clinical setting, other investigators have suggested that platinum stents have a low shielding effect and few MR artifacts.12 However, because image quality can be affected by a variety of factors other than the metal alloy employed, it is difficult to predict the ability of MRA to accurately image current clinically utilized visceral stents. Rather, the comparative effect of stent composition and geometry on MR image quality needs to be prospectively evaluated in future preclinical and patient models.
In summary, MRA of stented vessels can be successfully performed, particularly when nonferromagnetic stents are present. It is essential to identify patients with intravascular stents prior to MRI so that the protocol can be optimized. By changing the MRA parameters, high-quality imaging can be achieved.
Kevin W. Mennitt, MD, is Chief of Body MRI, Assistant Professor of Radiology, Weill Medical College of Cornell University, New York. He has disclosed that he is a paid consultant for Medtronic. Dr. Mennitt may be reached at firstname.lastname@example.org.
John H. Rundback, MD, is Director, Interventional Radiology, Holy Name Hospital, and Associate Professor of Radiology, Columbia University College of Physicians and Surgeons, Teaneck, New Jersey. He has disclosed that he is a paid consultant for Medtronic. Dr. Rundback may be reached at email@example.com.
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