Federal health officials have warned that patients who wear nicotine or other drug patches during MRI scans risk burns, because some patches contain tiny metal elements that can be heated by the device's huge magnet. Patients should consult their doctors on whether to replace or reuse patches after removal for scans, officials said. The alert is the latest in a string of safety warnings involving magnetic resonance imaging that have resulted from the unpredictable effects of the strong magnets used in MRI devices.