MRI Fringe Fields and Shielding

Fringe Fields and Shielding

Walls, floors and ceilings cannot contain static magnetic fields. The stray magnetic field outside the bore of the magnet is known as the fringe field. All magnets have a fringe field to some extent and these fields must be taken into account when installing a magnet.

Fringe fields can be compensated for by the use of magnetic field shielding which may be active or passive. Passive shielding is the more expensive alternative using iron plates to restrict the field lines. Some manufacturers offer actively shielded magnets that reduce the fringe field to about 30m². Active shielding partially cancels the field outside the main magnet coils thus reducing the magnitude of the fringe field. The 0.5mT isomagnetic line is taken as the critical cut-off limit.