**3.7. Magnetic field interactions**

The static magnetic field of an MR system is always on. No sound, sight, smells alerts per‐ sonnel to the presence or the extent of the invisible field surrounding the magnet in all direc‐ tions. The magnetic pull is strongest at the centre of the MR system and weakens with increased distance from the magnet, creating a spatial magnetic field gradient (Price, 1999). The distribution of the magnetic field outside the main magnet called fringe field is impossi‐ ble to see, but it is critical to safety in the MR environment because it can determine whether a ferromagnetic object could become a projectile. MR systems with large fringe field general‐ ly create the greatest hazards (Price, 1999). If the fringe strength decreases more gradually with distance from the magnet, the object's attraction to the magnet progressively strength‐ ens as it becomes closer to the magnet. Personnel within the MR room may notice an in‐ creasingly stronger pull on objects they are wearing or carrying as they walk closer to the MR system, permitting them to retreat from the MR system before an accident occurs (Kanal *et al.,* 2002).
