Magnetic shielding by attaching a high permeability yoke to the magnet (passive shielding) or by incorporating additional magnetic field-generating coils designed to reduce the external field (active shielding).

See also Magnetic shielding, Active Shielding and Passive Shielding.

Magnetic shielding through the use of secondary shielding coils designed to produce a magnetic field that cancels the field from primary coils in regions where it is not desired. These coils may be inside the magnet cryostat. Active shielding can be applied to the main magnet or to the gradient magnetic fields.

See also Magnetic Shielding, Self Shielding and Room Shielding.

Magnetic shielding through the use of high permeability material. The iron provides a return path for the stray field lines of magnetic flux and so significantly decreases the flux away from the magnet.
Passive shielding (see also Faraday cage) significantly eases the problems of siting a MR imager in a confined space. Ferromagnetic objects are less prone to being attracted to the magnet, ancillary electronic equipment, credit cards and computer disks can be brought closer to the magnet and the MRI safety limit for pacemaker wearers (the 5 gauss line = 0.5 mT) is reduced from, typically, 10 m to 2 m from the magnet. A passive shield for a whole-body MRI magnet weights many tons. An alternative method of controlling stray field is active shielding.

See also Active Shielding, Magnetic Shielding, Self Shielding and Room Shielding.

There are different types of contraindications that would prevent a person from being examined with an MRI scanner. MRI systems use strong magnetic fields that attract any ferromagnetic objects with enormous force. Caused by the potential risk of heating, produced from the radio frequency pulses during the MRI procedure, metallic objects like wires, foreign bodies and other implants needs to be checked for compatibility. High field MRI requires particular safety precautions. In addition, any device or MRI equipment that enters the magnet room has to be MR compatible. MRI examinations are safe and harmless, if these MRI risks are observed and regulations are followed.

Safety concerns in magnetic resonance imaging include:

It is important to remember when working around a superconducting magnet that the magnetic field is always on. Under usual working conditions the field is never turned off. Attention must be paid to keep all ferromagnetic items at an adequate distance from the magnet. Ferromagnetic objects which came accidentally under the influence of these strong magnets can injure or kill individuals in or nearby the magnet, or can seriously damage every hardware, the magnet itself, the cooling system, etc.. See MRI resources Accidents.
The doors leading to a magnet room should be closed at all times except when entering or exiting the room. Every person working in or entering the magnet room or adjacent rooms with a magnetic field has to be instructed about the dangers. This should include the patient, intensive-care staff, and maintenance-, service- and cleaning personnel, etc..
The 5 Gauss limit defines the 'safe' level of static magnetic field exposure. The value of the absorbed dose is fixed by the authorities to avoid heating of the patient's tissue and is defined by the specific absorption rate. Leads or wires that are used in the magnet bore during imaging procedures, should not form large-radius wire loops. Leg-to-leg and leg-to-arm skin contact should be prevented in order to avoid the risk of burning due to the generation of high current loops if the legs or arms are allowed to touch. The patient's skin should not be in contact with the inner bore of the magnet.
The outflow from cryogens like liquid helium is improbable during normal operation and not a real danger for patients.
The safety of MRI contrast agents is tested in drug trials and they have a high compatibility with very few side effects. The variations of the side effects and possible contraindications are similar to X-ray contrast medium, but very rare. In general, an adverse reaction increases with the quantity of the MRI contrast medium and also with the osmolarity of the compound.

See also 5 Gauss Fringe Field, 5 Gauss Line, Cardiac Risks, Cardiac Stent, dB/dt, Legal Requirements, Low Field MRI, Magnetohydrodynamic Effect, MR Compatibility, MR Guided Interventions, Claustrophobia, MRI Risks and Shielding.

Means to confine the region of strong magnetic field surrounding a magnet; most commonly the use of material with high permeability (passive shielding) or by employing secondary counteracting coils outside of the primary coils (active shielding). The high permeability material can be employed in the form of a yoke immediately surrounding the magnet (self-shielding) or installed in the walls of a room as full or partial room-shielding. Unlike shielding ionizing radiation, for example, magnetic shielding can only be accomplished by forcing the unavoidable magnetic return flux through more confined areas or structures, not by absorbing it.

See also Radio Frequency Shielding Radio Frequency Shielding, and Faraday cage.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'