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 MRImagnet weights many tons. An alternative method of controlling stray field is active shielding.
RF noise, which often appears as static on the image, can be caused by a medical device located anywhere in the MR procedure room.
RF noise is a result of excessive electromagnetic emissions from the device that interference with the proper operation of the MR scanner.
The interference is attenuated and aliased in the frequency direction.
Image Guidance
In the first case try to eliminate the factor of disturbance, if the problem persists screen the magnet- and room shielding.
Zipper artifacts appear as dashed lines. There are various causes for this MRIartifact.
Most of zipper artifacts result from inhomogeneities of the magnetic field caused by interferences with radio frequency from various sources ('your radio is working in the scanner room means your shielding is not working'). Software and equipment problems can also cause zipper lines in both directions.
Image Guidance
Scanning room door during the acquisition of images is closed, shielding is working, and all disturbing devices are removed?
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