The Magnet Is Always On!!!
Magnetic Resonance Imaging (MRI) is one of the least invasive of diagnostic imaging modalities and has no known biological risks, but, there are hazards that are inherent to and associated with high magnetic fields. The inadvertent introduction of a magnetic instrument or piece of equipment to the MR environment can result in a lethal outcome. Armed with a false understanding of MRI, "I didn't think the magnet was on!", staff, visitors and patients can become the bearers of potential projectiles. As such, the greatest biological hazard to be encountered in an MR scanner is anything made of magnetic materials.
A comprehensive reference on MRI safety can be found at the MRI Safety Website, authored by Frank G. Shellock, Ph.D., FACC, FACSM. “His primary research interests include investigations to study MRI safety for implants and devices, to evaluate electromagnetic field-related bioeffects, to develop kinematic MRI techniques to examine joint pathology, and to identify and implement new MR imaging applications." Registration is required to be able to fully utilize the site, but, Dr. Shellock has no time for junk mail, sent or received. He will also answer questions that you cannot find the answers to on the site. The site contains comprehensive list MR compatible and incompatible implants and devices.
Another excellent site is the Center for Biomedical Imaging, of NYU Medical Center, Department of Radiology, Center for Biomedical Imaging. It makes available a safety video and outlines operating procedure for its high field facility.
For those really interested in the workings of MRI, Joseph P. Hornak, Ph.D. has The Basics of MRI. An excellent, interactive teaching site.
For the even more technically disturbed, the FDA website for magnetic and radio frequency limits.