High permeability structure used to concentrate the magnetic flux inside itself and thus decrease the magnetic fringe field and increase the field strength in the useful volume of a magnet.

This line specifies the perimeter around a MR scanner within which the static magnetic fields are higher than five gauss. Five gauss and below are considered 'safe' levels of static magnetic field exposure for the general public.
Due to the physical properties of magnetic fields, the magnetic flux, which penetrates the useful volume of the magnet will return through the surroundings of the magnet to form closed field lines. Depending on the magnet construction, the returning flux will penetrate large open spaces (unshielded magnets) or will be confined largely to iron yokes or through secondary coils (shielded magnets).

The region surrounding a magnet and exhibiting a magnetic field strength, which is significantly higher than the earth's magnetic field (typically 0.05-0.1 mT, depending on geographical location). Initially the most magnets had very extensive fringe fields. Magnets with iron have reduced the fringe field substantially (passively shielded magnets). At least, adding appropriate additional superconducting coils to superconducting magnets has resulted in a drastic reduction of the extent of the fringe fields (actively shielded magnets).
Due to the physical properties of magnetic fields, the magnetic flux, which penetrates the useful volume of the magnet will return through the surroundings of the magnet to form closed field lines. Depending on the magnet construction, the returning flux will penetrate large open spaces (unshielded magnets) or will be confined largely to iron yokes or through secondary coils (shielded magnets).
Fringe fields constitute one of the major hazards of MR scanners as these fields acting over extended distances outside the magnet produce strong attractive forces upon magnetic objects. These can thus 'fly' into the magnet when loose nearby acting like projectiles. Fringe fields also exert unwanted forces on metallic implants in patients.

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'

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.