Coils, Filters, and Stents

Coils, stents, filters and vascular grafts have been evaluated relative to the use of MR systems. Several of these demonstrated magnetic field interactions in association with scanners. 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 now been evaluated at 3-Tesla (see below). There is at least one exception to this, however (see below). MRI-related may also be of concern for certain configurations or shapes for coils, stents, filters and vascular grafts.

Many of these implants are made from nonferromagnetic materials, such as the LGM IVC filter (Vena Tech) used for caval interruption and the Wallstent biliary endoprosthesis [Schneider (USA), 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 ex vivo testing (for specific information, see The List ).

Notably, it is unnecessary to wait an extended period of time after surgery to perform an MR procedure in a patient with a "passive" metallic implant that is made from a nonferromagnetic 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 using MR-guidance at 1.5-Tesla. The only exception may be if there are concerns associated with MRI-related heating.

The Guglielmi detachable coil (GDC) used for endovascular embolization, was evaluated for MRI issues. Because of the coiled-shape of the GDC, potential heating during MR imaging was suspected. Therefore, a study was performed using ex vivo testing techniques to determine the safety of the Guglielmi detachable coil with respect to magnetic field interactions, heating, and artifacts. The results indicated that there were no magnetic field interactions, the temperature increase was minimal during extreme MR imaging conditions, and the artifacts involved a mild signal void relative to the size and shape of the GDC. Other embolization coils made from Nitinol, platinum, or platinum and iridium have been evaluated and found to be acceptable for patients undergoing MR procedures.

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 1.5-Tesla or less (and some at 3-Tesla) without reported injuries or other problems.

Nevertheless, an MR procedure should not be performed if there is any possibility that implant is not positioned properly. Additionally, it should be noted that not all stents are safe for patients undergoing MR procedures, particularly since, to date, not all stents have undergone MRI testing (also, see information for the Zenith AAA Endovascular Graft below).

Furthermore, new coils, stents, filters and vascular grafts are developed on an ongoing basis. Therefore, there should never be a "general" policy to scan these implants in patients referred for MRI procedures.

A study by Taal et al. 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. In particular, "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."

Zenith Flex AAA Endovascular Graft
(excerpted from the IFU)

MRI Information
Non-clinical testing has demonstrated that the Zenith AAA Endovascular Graft is MR
Conditional. It can be scanned safely under the following conditions:

Static Magnetic Field
Static magnetic field of 3.0 Tesla or less.

Spatial magnetic gradient field of 720 Gauss/cm

MRI-Related Heating
1.5 Tesla Systems
Maximum whole body averaged specific absorption rate (SAR) of 2 W/kg for 15 minutes of scanning (i.e., per pulse sequence).

In non-clinical testing, the Zenith AAA Endovascular Graft produced a temperature rise of less than or equal to 1.4 degrees C at a maximum MR system reported, whole body averaged specific absorption rate (SAR) of 2.8 W/kg, as assessed by calorimetry for 15 minutes of MR scanning (i.e., per pulse sequence) in a 1.5 Tesla Magnetom, Siemens Medical Magnetom, Numaris/4 Software, Version Syngo MR 2002B DHHS MR Scanner. The maximum MR system reported, whole body averaged specific absorption rate (SAR) was 2.8 W/kg, which corresponds to a calorimetry measured value of 1.5 W/kg.

3.0 Tesla Systems, MRI-Related Heating
Maximum whole body averaged specific absorption rate (SAR) of 2 W/kg for 15 minutes of scanning (i.e., per pulse sequence).

In non-clinical testing, the Zenith AAA Endovascular Graft produced a temperature rise of less than or equal to 1.9 degrees C at a maximum MR system reported, whole body averaged specific absorption rate (SAR) of 3.0 W/kg, as assessed by calorimetry for 15 minutes of MR scanning in a 3.0 Tesla Excite, GE Electric Healthcare, G3.0-052B Software, MR Scanner. The maximum MR system reported, whole body averaged specific absorption rate (SAR) was 3.0 W/kg, which corresponds to a calorimetry measured value of 2.8 W/kg.

Artifacts
The image artifact extends throughout the anatomical region containing the device, obscuring the view of immediately adjacent anatomical structures within approximately
20 cm of the device, as well as the entire device and its lumen, when scanned in
nonclinical testing using the sequence: Fast spin echo, in a 3.0 Tesla, Excite, GE Electric Healthcare, with G3.0-052B software, MR system with body radiofrequency coil.

For all scanners, the image artifact dissipates as the distance from the device to the area of interest increases. MR scans of the head and neck and lower extremities may be obtained without image artifacts. Image artifacts may be present in scans of the abdominal region and upper extremities, depending on the distance from the device to the area of interest.

Clinical information is available for seventeen patients who received MRI scans after
stent-graft implantation. There have been no reported adverse events or device problems in any of these patients as a result of having received an MRI. Additionally, there have been approximately 100,000 Zenith AAA Endovascular Grafts distributed world wide, in which there have been no reported adverse events or device problems as a result of MRI.

MRI Information 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 mechanisms are sufficient to prevent them from posing a substantial risk to a patient or individual in the 3-Tesla MR environment. Thus, these issues warrant further consideration.

MRI Information 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 is placed in an effort to prevent restenosis. There is considerable attention focused on the use of drug eluting stents to prevent coronary artery restenosis that tends to occur in a substantial number of patients following stenting with "bare" devices. Studies have reported that drug eluting stents reduce the incidence of target vessel failure compared to uncoated metallic stents. As such, drug eluting stents are now used on a widespread basis in patients with coronary artery disease.

MRI information has been obtained for many bare wire 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). Please refer to The List for specific information.

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