Magnetic shielding through the use of high permeability material. The iron provides a return path for the stray field lines of magnetic flux and so significantly decreases the flux away from the magnet.
Passive shielding (see also Faraday cage) significantly eases the problems of siting a MR imager in a confined space. Ferromagnetic objects are less prone to being attracted to the magnet, ancillary electronic equipment, credit cards and computer disks can be brought closer to the magnet and the MRI safety limit for pacemaker wearers (the 5 gauss line = 0.5 mT) is reduced from, typically, 10 m to 2 m from the magnet. A passive shield for a whole-body MRI magnet weights many tons. An alternative method of controlling stray field is active shielding.

See also Active Shielding, Magnetic Shielding, Self Shielding and Room Shielding.

Shimming by adjusting the position of suitable pieces of ferromagnetic metal within or around the main magnet of a MRI system. Typically, the iron is in the form of small plates which are held in removable trays. The magnetic field is plotted, the position and quality of the iron required is then calculated and the appropriate 'shims' put in place. Several repetitions of this process will normally be required in order to achieve the optimum arrangement.
Passive shimming does not replace adjustable shimming, but is installed during the commissioning of a unit to improve the homogeneity of the bare magnet.

A magnet whose magnetic field originates from permanently ferromagnetic materials (permanent magnets) to generate a magnetic field between the two poles of the magnet. There is no requirement for additional electrical power or cooling, and the iron-core structure of the magnet leads to a limited fringe field and no missile effect. Due to weight considerations, permanent magnets are usually limited to maximum field strengths of 0.4 T. The main disadvantages of a permanent magnet are the cost of the magnet itself and supporting structures and the varying changes in the magnetic field. Field homogeneity can be an on-going problem in permanent magnets.

Correction of inhomogeneity of the magnetic field produced by the main magnet of a MRI system due to imperfections in the magnet or to the presence of external ferromagnetic objects. May involve changing the configuration of the magnet or the using of shim coils (active shimming) or adding or removing steel from the magnets poles (passive shimming) to fine-tune the magnetic field.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

Quick Overview

Materials with magnetic susceptibility cause this artifact. There are in general three kinds of materials with magnetic susceptibility: ferromagnetic materials (iron, nickel etc.) with a strong influence and paramagnetic/diamagnetic (aluminium, platinum etc./gold, water, most organic compounds etc.) materials with a minimal/non influence on magnetic fields. In MRI, susceptibility artifacts are caused for example by medical devices in or near the magnetic field or by implants of the patient. These materials with magnetic susceptibility distort the linear magnetic field gradients, which results in bright areas (misregistered signals) and dark areas (no signal) nearby the magnetic material.

Use a spin echo or a fast spin echo sequence, because gradient echo sequences are more sensitve to susceptibility artifacts. A high bandwidth (small water fat shift) and a short echo time help also to reduce this artifact.
In some cases it is even beneficial to use a gradient echo sequence, e.g. a cavernom contains some iron-rich haemosiderin, which also causes a signal void on gradient echo sequences and for this purpose increases the diagnostic image quality.