MR safety at 3T: Bare metal and drug-eluting coronary stents

Patients with coronary artery disease are often treated by percutaneous transluminal coronary angioplasty (PTCA). Renarrowing 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.

Considerable attention is focused on the use of drug-eluting stents to prevent the 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 with uncoated metallic stents.

Drug-eluting stents are now used in over 80% of patients with coronary artery disease.

Recently, MR safety information has been obtained for several bare wire and drug-eluting coronary stents, which have been reported to be safe for patients undergoing MR procedures at 3T or less (based on assessments of magnetic field interactions and MRI-related heating). These coronary artery stents include the following:

Endeavor Drug Eluting Coronary Artery Stent

Medtronic Vascular

Through nonclinical testing, the Endeavor stent has been shown to be MRI safe at field strengths of 3T or less and a maximum whole-body averaged specific absorption rate (SAR) of 2 W/kg for 15 minutes of MRI. The Endeavor stent should not migrate in this MRI environment. MRI at 3T or less may be performed immediately following the implantation of the Endeavor stent. Nonclinical testing has not been performed to rule out the possibility of stent migration at field strengths higher than 3T.

In this testing, the stent produced a maximum temperature rise of 0.5°C at a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MRI.

MR imaging quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the stent.

TAXUS Express Paclitaxel-Eluting Coronary Stent

Boston Scientific Corporation

Through nonclinical testing, the TAXUS Express stent has been shown to be MRI safe at field strengths of 3T or less and a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MRI. The TAXUS Express stent should not migrate in this MRI environment. MRI at 3T or less may be performed immediately following the implantation of the TAXUS Express stent. Nonclinical testing has not been performed to rule out the possibility of stent migration at field strengths higher than 3T.

In this testing, the stent produced a maximum temperature rise of 0.65°C at a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MRI. The effect of heating in the MRI environment was similar for overlapping bare metal stents (2 to 5-mm overlap at the ends), made of the same stainless steel material and having the same stent design. The effect of heating in the MRI environment on stents with fractured struts is not known. The temperature rise of 0.65°C for 15 minutes is calculated to result in an increase in cumulative drug release of 0.001% of the total dose.

MR imaging quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the stent.

Liberté Coronary Artery Stent (bare metal coronary artery stent)

Boston Scientific Corporation

The Liberté Stent has been shown to be MR safe at field strengths of 3T or less and a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MR imaging. The Liberté Stent should not migrate in this MR environment. MR imaging at 3T or less may be performed immediately following the implantation of the Liberté Stent.

In this testing, the stent experienced a maximum temperature rise of 0.65°C at a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MR imaging. The temperature rise was observed to be similar for comparable bare metal overlapping stents (2 to 5-mm overlap at the ends). Heating has not been determined for fractured struts.

MR imaging quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the stent.

This stent has not been evaluated to determine if it is safe in MRI systems with field strengths greater than 3T.

TAXUS Liberté Paclitaxel-Eluting Coronary Stent

Boston Scientific Corporation

Through nonclinical testing, the TAXUS Liberté stent has been shown to be MRI safe at field strengths of 3T or less and a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MRI. The TAXUS Liberté stent should not migrate in this MRI environment. MRI at 3T or less may be performed immediately following the implantation of the TAXUS Liberté stent. Nonclinical testing has not been performed to rule out the possibility of stent migration at field strengths higher than 3T.

In this testing, the stent produced a maximum temperature rise of 0.65°C at a maximum whole-body averaged SAR of 2 W/kg for 15 minutes of MRI. The effect of heating in the MRI environment was similar for overlapping bare metal stents (2 to 5-mm overlap at the ends), made of the same stainless steel material and having the same stent design. The effect of heating in the MRI environment on stents with fractured struts is not known. The temperature rise of 0.65°C for 15 minutes is calculated to result in an increase in cumulative drug release of 0.001% of the total dose.

MR imaging quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the stent.

(Note: "TAXUS" is the trademark name on the drug coating and refers to the drug-eluting coating addition to the bare metal stent. As such, when there is the addition of the drug-eluting coating, it will be referred to as the TAXUS and then the specific name of the stent. For example, TAXUS Express stent. The bare metal stent does not contain the TAXUS prefix.)

CYPHER Sirolimus-Eluting Coronary Stent

Cordis Corporation/Johnson and Johnson

Through nonclinical testing, a single and two overlapping CYPHER stents have been shown to be MRI safe at field strengths of 3T or less and a maximum whole-body averaged SAR of 4 W/kg for 15 minutes of MRI. Single and two overlapping CYPHER stents should not migrate in this MRI environment. Nonclinical testing has not been performed to rule out the possibility of stent migration at field strengths higher than 3T.

In this testing, single CYPHER stents up to 33-mm in length produced a temperature rise of less than 1°C, and two overlapped 33-mm length CYPHER stents produced a temperature rise of less than 2°C at a maximum whole-body averaged SAR of 4 W/kg for 15 minutes of MRI. The effect of heating in the MRI environment for stents with fractured struts is not known. The effect of heating in the MRI environment on the drug or polymer coating is not known.

MR imaging quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the stent.

(Note: This statement applies to all currently marketed CYPHER Stents in the U.S.)

Frank G. Shellock, Ph.D., FACC, FACSM, is an adjunct clinical professor of radiology and medicine at Keck School of Medicine, University of Southern California, and Institute for Magnetic Resonance Safety, Education, and Research.

www.MRIsafety.com

www.IMRSER.org

REFERENCE

Shellock FG, Forder J. Drug eluting coronary stent: In vitro evaluation of magnet resonance safety at 3-Tesla. J Cardiovasc Magn Reson 2005;7:415-419.