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.
Advanced Discussion (show/hide)»
On rare occasions a person will enter a scanner room still wearing a watch with metallic gears and the watch will become magnetized and nonfunctional. This happened several years ago to one of our radiology residents who had a Rolex, and he was most upset!
If this situation should ever arise, you may be successful at "de-gaussing" the watch using the following technique. Take the watch back into the magnet room close to the scanner. Then slowly back away from the scanner continuously rotating the watch in a vertical plane as you exit the room.
I didn't believe this technique would work but it did. The resident's Rolex was returned to full function. No guarantees but it is certainly worth a try. Unfortunately, the same technique will not work for credit card erasure!
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.