Forces can result from the interaction of magnetic fields. Pulsed
magnetic field gradients can interact with the main
magnetic field during the
MRI scan, to produce
acoustic noise through the
gradient coil.
Magnetic fields attract
ferromagnetic objects with forces, which can be a lethal danger if one is hit by an unrestrained object in flight. One could also be trapped between the
magnet and a large unrestrained
ferromagnetic object or the object could damage the
MRI machine.
Access control and personnel awareness are the best preventions of such accidents. The attraction mechanism for
ferromagnetic objects is that the
magnetic field magnetizes the iron. This induced
magnetization reacts with the
gradient of the
magnetic field to produce an attraction toward the strongest area of the field. The details of this interaction are very dependent on the shape and composition of the attracted object. There is a very rapid increase of force as one approaches a
magnet. There is also a
torque or twisting force on objects, e.g. a long cylinder (such as a pen or an intracranial aneurysm clip) will tend to align along the magnet's field lines. The
torque increases with
field strength while the attraction increases with
field gradient.
Depending on the magnetic
saturation of the object, attraction is roughly proportional to object
mass. Motion of conducting objects in magnetic fields can induce
eddy currents that can have the effect of opposing the motion.
See also
Duty Cycle.
See also the related poll result: '
Most outages of your scanning system are caused by failure of'