MRI & Ferrite Materials

Before you have a magnetic resonance imaging scan, a nurse or technician will ask you to put your wallet, keys and any other metal items in a safe place before taking you to the MRI room itself. MRI machines have the most powerful magnets in common use. It attracts all ferrous (iron-bearing) metals and will pull them in from several feet away with great force. This can damage the machine and other equipment and possibly cause serious injury to the patient or staff.

MRI

• The MRI machine is a sophisticated diagnostic tool. Using it, doctors can obtain images of soft tissues and internal organs not possible with other methods. Radio waves probe the patient's body in a powerful magnetic field. A sensor array and computer read the radio signals and assemble them as finely detailed images.

Magnetism

• Every magnet produces a field of invisible force that attract ferrous metals and other magnets. For permanent magnets, the strength of this force depends on the materials they're made of. The strength of electromagnets depend on materials and electric current. An MRI machine has an expensive electromagnet that runs at extremely low temperatures (about 452 F). These magnets produce fields measured at between 1.5 and 3 tesla. By comparison, the strength of the Earth's magnetic field is .00005 tesla.

Precautions

• Since an MRI magnet is thousands of times stronger than other magnets we normally encounter, it pulls ferrous objects with greater force and from a greater distance. For this reason, the MRI must be operated in a separate room. Technicians must be careful not only about what patients wear, but also any metallic objects they might have in them, like surgical screws or pacemakers. The entrance to the MRI room may have a metal detector to ensure that no metal objects have been overlooked. Bystanders and other staff must also be carefully screened, as guns, floor polishers and oxygen tanks have all been pulled into the MRI's powerful magnet.

Injury

• Patients and staff have been injured or killed by flying metal objects that have been drawn into the MRI magnet's field. Ferrous metal may trap or crush limbs. Surgically implanted devices may become dislodged. Medicine patches can become hot in the magnetic field and can cause burns.

Damage

• In addition to personal injury, the MRI machine itself can be damaged by flying objects. The magnet is powerful enough to pull in large, heavy objects from several feet away. The force of impact will damage both the object and the MRI machine.

Shutdown

• An MRI machine has to go through a lengthy start-up process in which a technician lowers its temperature and puts a carefully controlled electric current into the magnet. Once it's operating, sudden changes to the magnet's temperature can cause an explosive release of energy. If the MRI's magnet traps a metal object, the only way to free it is to shut the machine down carefully. It takes several hours to shut the magnet off. Once it's off and the foreign object removed, the technician must take the machine through a time-consuming start-up procedure.