Magnetic shielding refers to the attempt to isolate or block the magnetic field of the MRI magnet.  This can be done to prevent unwanted interference from the MRI magnet on nearby electronic devices.  This is different from radiofrequency shielding, which is the attempt to prevent the unwanted interference of noise or radiofrequencies, often in order to avoid image distortion.

An example of where magnetic shielding is important would be when there are nearby devices that potentially can be susceptible to magnetic interference, such as cardiac pacemakers, or other sensitive pieces of electronic or medical equipment. It also becomes relevant when low frequency external magnetic fields are being used nearby and need to be shielded from the magnetic field or fringe field from the MRI magnet.  In general, the entire environment around the MR magnet needs to be protected from the fringe and magnetic fields.

A conventional marker of high magnetic field proximity is the 5 gauss line. Additional magnetic shielding may be required to protect devices that fall within the 5 G line, particularly if this extends beyond the MRI exam room. 5 G is the threshold required to avoid potential influence on pacemakers. Nearby medical imaging scanners can be influenced by magnetic field strengths as low as 1-3 G. 

The methods of magnetic shielding usually involve steel or copper placed in the walls of the magnet room. These metallic steel or copper plates capture the magnetic field based on their geometric make-up and result in a cancelation or blockade of the magnetic field. The magnetic shield essentially re-directs the magnetic field so that it protects the item being shielded. One of the relevant properties of the shield is that the magnetic field is attracted to the shielding material. The amount of material in the shield is also relevant, as the more material there is, the more magnetic field it can re-direct.