Shielding
Shielding refers to the magnetic field outside the magnet bore. Passive shielding is the placement of heavy ferromagnetic plating in the walls or floor of the magnet room or even the magnet itself. This concentrates magnetic flux lines, reducing the fringe field beyond the shielding.
Active shielding employs an extra set of superconducting windings in the scanner to oppose a portion of the main magnetic field.
Further reading on this topic:
Books: MRI From Picture to Proton p169-170, Q&A in MRI p60-61