Coils, stents, filters and vascular grafts have been evaluated relative to the use of MR systems. Several of these demonstrated magnetic field interactions. Fortunately, the devices that exhibited positive magnetic field interactions typically become incorporated securely in tissue within six weeks after implantation due to ingrowth and other mechanisms. Therefore, for most coils, filters, stents and grafts that have been tested, it is unlikely that these implants would become moved or dislodged as a result of exposure to MR systems operating at 1.5-Tesla or less. Additionally, many of these items have been evaluated at 3-Tesla (see below). MRI-related heating may also be of concern for certain configurations or shapes for coils, stents, filters, and vascular grafts. To date, there has been no reported case of excessive heating in association with MRI and these types of implants.
Many coils, filters, stents and grafts are made from nonferromagnetic materials, such as the LGM IVC filter (Vena Tech) used for caval interruption and the Wallstent biliary endoprosthesis (Schneider, Inc.) used for treatment of biliary obstruction. As such, these implants are acceptable for patients undergoing MR procedures relative to the use of the particular field strength utilized in the ex vivo testing (for specific information, see The List). Notably, it is unnecessary to wait after surgery to perform an MR procedure in a patient with a “passive” metallic implant that is made from a nonmagnetic material (see Guidelines for the Management of the Post-Operative Patient Referred for a Magnetic Resonance Procedure). In fact, there are reports in the peer-reviewed literature that describe placement of vascular stents or other similar devices using MR-guidance at 1.5-Tesla and 3-Tesla. Interestingly, some of these vascular implants (e.g., vascular stent grafts and stainless steel embolization coils) display high magnetic field interactions in association with 1.5- and 3-Tesla MR systems, yet have MR Conditional labeling approved by the Food and Drug Administration.
Patients with the specific coils, stents, filters and vascular grafts indicated in The List have had procedures using MR systems operating at static magnetic field strengths of 3-Tesla or less without reported injuries or other problems.
A study by Taal, et al. (1997) supports the fact that not all stents are safe for patients undergoing MR procedures. This investigation was performed to evaluate potential problems for four different types of stents: the Ultraflex (titanium alloy), the covered Wallstent (Nitinol), the Gianturco stent (Cook), and the modified Gianturco stent (Song) - the last two are made from stainless steel. Taal, et al. reported “an appreciable attraction force and torque” found for both types of Gianturco stents. Taal, et al. stated, “the Gianturco (Cook) stent pulled toward the head with a force of 7 g…however, it is uncertain whether this is a potential risk for dislodgment.” In consideration of these results, the investigators advised, “…specific information on the type of stent is necessary before a magnetic resonance imaging examination is planned.”
MRI at 3-Tesla and Coils, Stents, Filters and Vascular Grafts. Different coils, stents, filters and vascular grafts have been evaluated at 3-Tesla. Of these implants, two displayed magnetic field interactions that exceeded the American Society for Testing and Materials (ASTM) International guideline for safety (i.e. the deflection angles were greater than 45 degrees). However, similar to other comparable implants, tissue ingrowth and other mechanims are sufficient to prevent them from posing a substantial risk to a patient or individual in the 3-Tesla MR environment. Please refer to The List for specific information related to coils, stents, filters and vascular grafts.
MRI at 3-Tesla: Bare Metal and Drug Eluting Coronary Stents. Patients with coronary artery disease are often treated by percutaneous transluminal coronary angioplasty (PTCA). Re-narrowing at the angioplasty site, or restenosis, occurs in as many as 50% of patients following PTCA. Therefore, after coronary artery intervention, either a bare metal or drug eluting stent may be placed in an effort to prevent restenosis.
MRI information has been obtained for many bare metal and drug eluting coronary artery stents, which have been reported to be acceptable for patients undergoing MR procedures at 3-Tesla or less (i.e. based on assessments of magnetic field interactions and MRI-related heating). Of note is that, for these coronary artery stents, patients may undergo MRI procedures immediately after placement. Please refer to The List for specific information.
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