Stents, Filters & Coils
If they're non-ferromagnetic, they are safe to scan immediately after implantation. If they're weakly ferromagnetic, they should be safe to scan after fibrosis has occurred (6-8 weeks after implantation). As usual, positive documented evidence must be obtained from the manufacturer confirming MR safety and/or compatibility before you can say either way.
The subject of stents is a good time to reiterate an important point: that it is essential to continually review and update policies and procedures pertaining to MR safety. In one journal article, Shellock states that "at least two prototype stents have been identified that display excessive magnetic field interactions at 1.5 Tesla, indicating that not all coils, stents and filters are safe in the MR environment".