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| Endovascular grafts, like W.L. Gore's TAG thoracic endoprosthesis, are among the "passive" implants subject to the new guidance.--Courtesy of W.L. Gore |
So far, the issue of MRI-safe devices has mostly focused on devices like pacemakers, and more recently cochlear implants, both of which use electronics. But the FDA just issued new guidance on the MRI compatibility--or in some cases incompatibility--of implants that do not require electrical power to service their function, such as cardiovascular stents, intracranial aneurysm clips, endovascular grafts and transprostatic tissue retractors.
The agency has three main concerns about patient safety with regards to these so-called "passive" implants in the MRI environment. They are: the displacement forces and torques on magnetic materials, which can turn devices into deadly projectiles; radio frequency heating that can burn patients; and the distorting effect of implants upon an MRI scan.
The FDA recommends four tests described by standard setting body ASTM International to address the three aforementioned safety issues, and provides additional details about the way those tests should be performed. For example, the guidance says that "the testing should encompass the range of sizes of the device you intend to market."
The results of the test should be submitted in product approval applications sent to the FDA.
MRIs are offered in various magnetic field strengths, measured in Teslas. "Although commercial 1.5T MR systems are currently the most common, 3T MR systems are becoming more common. A medical device that is MR Conditional in a 1.5T scanner may not be safe to scan in an MR system with a higher or lower field strength," the guidance warns.
The implant's labeling should describe the device as "MR Safe," "MR Unsafe," or "MR Conditional," as defined by ASTM International in standard F2503-13. Finally, the fourth category "Safety in MRI Not Evaluated," may be appropriate in some cases, but not for those passive implants that are known to present MRI compatibility issues, are a new device type or contain ferromagnetic materials.
- here's the guidance