The 5 Gauss line is the safety line drawn around the perimeter of the main magnet of the MR scanner, specifying the distance at which the stray magnetic field is equivalent to 5 Gauss (0.5 mT).
5 Gauss and below are considered 'safe' levels of static magnetic field exposure for the general public.
At 5 Gauss and above:
- cardiac pacemakers and other implanted electronic devices are at risk of being interfered with by the static magnetic field
- ferromagnetic materials may become projectile (and are thus prohibited from crossing the 5 Gauss line)