The same active and passive magnetic shielding methods used to reduce fringe fields also protects the main magnetic field from certain outside influences, such as the presence of stationary ferromagnetic material or slowly moving stray fields outside the magnet room. Magnetic shielding of this type does not guard against high frequency electromagnetic fields outside the scanner, however. This requires a different type of protection, called radiofrequency (RF) shielding. RF shielding methods will be described in several follow-up questions.
For most sites conventional magnetic shielding provides adequate protection against external field disturbances. Some MRI sites are challenged by their location near moving metal objects such as cars, buses, subways and elevators. This problem is being addressed in at least two ways.
Many newer magnets manufactured by Siemens employ an "External Interference Shield (EIS)." This active shielding modification uses a third set of electrically shorted superconductive coils in which current flows when the magnet senses a distortion in its fringe field.
An entirely different approach has been developed by ETS-Lindgren called the "Magnetic Active Compensation System (MACS)". This is a device external to the magnet, employing a 3-axis sensing magnetometer, a set of Helmholtz coils near the magnet and electronic feedback loop. The MACS sensors monitor the external magnetic field environment, and when an external field disturbance is detected (by a passing vehicle, for example), it drives current through the coils to create and equal, but opposing, magnetic field. This system dynamically cancels out the effect of transient environmental fluctuations, helping maintain homogeneity within the bore of the scanner.