Artificial Heart Valves
The short answer is: some are safe, and positive documented evidence must be obtained from the manufacturer confirming MR safety and/or compatibility.
The long answer is...
Many heart valves have been tested and are safe for MR imaging. Of these, most have displayed some mild translational attraction and/or rotational torque in the systems used for testing. But these forces are minimal compared to the force exerted by the beating heart. An often quoted exception to this used to be the Starr-Edwards Pre-6000 series valves, but these too have now been deemed acceptable for MRI. (This was after a revision of the estimate of the peak force exerted by the beating heart from 0.36 N to 7.2 N.) It should be noted that there has never been a report of a patient incident or injury related to the presence of a heart valve prosthesis from a clinical MR procedure. (Reported 2002.)