For the safety of people in the general vicinity of an MRI magnet, a controlled area is usually declared as defined by the 0.5 mT (5 Gauss) line. Magnetic fields higher than 0.5 mT should not be exceeded in a public area. Barriers, access control doors, warning signs and facilities for the safe keeping of watches, credit cards and other magnetically sensitive objects must be provided.

Further reduction of the fringe fields to 0.1 mT or even 0.05 mT (less than 1 Gauss) may be necessary to safeguard the performance of sensitive equipment such as computer monitors, image amplifiers and other medical equipment etc., or to satisfy local regulations.
For comparison, the magnetic field strength of the earth is approximately 0.04 mT (0.4 Gauss).
Picture above: Example of a fringe magnetic field
Magnetic shielding may be necessary to limit the fringe field of the MRI magnet at certain sites or to improve image quality by eliminating external influences due to, for example, ferro-magnetic masses.

Soft iron or magnetic sheet low carbon steel ( SAE 1006 Carbon Steel) or Silicon Steel (for weight reduction) are used to screen the fringe fields of MRI magnets.

Magnetic shields are engineered according to the MRI equipment manufacturer’s specification. Vector Field’s  Opera 3D computer code is used for the design and optimization of thicknesses of the steel plates A low cost magnetic shield is designed for any site minimizing the amount of steel to be used for containment of 5 Gauss or 1 Gauss fringe field. A detailed description of the magnetic shield configuration and the thicknesses of  steel plates are provided for fabrication  and construction.