Fringe Fields
What are fringe fields and why do they matter?
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Fringe field around a 1.5T superconducting scanner.
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The fringe field is the peripheral magnetic field outside of the magnet core. Depending on the design of the magnet and the room a moderately large fringe field may extend for several meters around, above, and below an MR scanner. Field plots (similar to the illustration right) are available from each magnet manufacturer. For cylindrical superconducting scanners, the fringe fields are largest along the direction of the axis bore (along the z-axis or Bo direction). In theory the strength of a magnetic fringe field is inversely related to the third power of the distance (1/r³) from the magnet isocenter. Thus moving twice as far away from the magnet, the fringe field should fall by a factor of approximately 1/2³ = 1/8. |
Because older pacemakers may be affected by magnetic fields exceeding 5 gauss (0.5 mT), the US Food and Drug Administration (FDA) has established guidelines to protect the unsuspecting public from exposure to fields of 5 G or higher. Smaller fields (1-3 G) may affect nearby CT and MRI scanners. Fringe fields of 10 G may affect computers while fields of 30 G may magnetize your watch and erase your credit cards!
References
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Magnetom Skyra Planning Guide. Siemens Medical Systems, 2013. Contains a detailed fringe field map for this 3.0T scanner and lists other siting requirements.
Turner BD. Shielding issues for medical products. Conformity, May 2007, pp 48-53.
Magnetom Skyra Planning Guide. Siemens Medical Systems, 2013. Contains a detailed fringe field map for this 3.0T scanner and lists other siting requirements.