The fringe field is the peripheral magnetic field outside of the magnet core. It is important because it can cause interference with nearby electronic devices, such as pacemakers. Although the strength of the magnetic fields decreases with distance from the core of the magnet, the effect of the “fringe” of the magnetic field can still be relevant and have influence on external devices, depending on their susceptibility, and may need to be shielded.
MRI magnets will typically have a field plots which approximates the magnetic field strength at different distances from the magnet isocenter.
The magnetic field strength is inversely related to the third power of the distance from the magnet isocenter. This means that the magnetic field strength begins to decrease more slowly as you move further away from the magnet. In effect, this results in an elongated “tail” to the fringe magnetic field. As a consequence, the fringe field can often extend beyond the limit of the MRI scanning room, and needs to be accounted for and often shielded.