Magnetic field gradients are used to change the strength of the magnetic field Bo in a certain direction. Gradients are used in MR imaging with selective excitation to select a region for imaging and also to be able to encode the location of MR signals received from the object being imaged. The field strength is measured in Tesla per meter (T/m).

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'

The region surrounding a magnet and exhibiting a magnetic field strength, which is significantly higher than the earth's magnetic field (typically 0.05-0.1 mT, depending on geographical location). Initially the most magnets had very extensive fringe fields. Magnets with iron have reduced the fringe field substantially (passively shielded magnets). At least, adding appropriate additional superconducting coils to superconducting magnets has resulted in a drastic reduction of the extent of the fringe fields (actively shielded magnets).
Due to the physical properties of magnetic fields, the magnetic flux, which penetrates the useful volume of the magnet will return through the surroundings of the magnet to form closed field lines. Depending on the magnet construction, the returning flux will penetrate large open spaces (unshielded magnets) or will be confined largely to iron yokes or through secondary coils (shielded magnets).
Fringe fields constitute one of the major hazards of MR scanners as these fields acting over extended distances outside the magnet produce strong attractive forces upon magnetic objects. These can thus 'fly' into the magnet when loose nearby acting like projectiles. Fringe fields also exert unwanted forces on metallic implants in patients.

Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations describe the origin and behavior of the fields that govern these forces. Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in 'electromagnetism', or from the quantum-mechanical orbital motion (there is no orbital motion of electrons around the nucleus like planets around the sun, but there is an 'effective electron velocity') and spin of electrons, resulting in what are known as 'permanent magnets'.
The physical cause of the magnetism of objects, as distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale from the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus this motion can be considered as a current loop, resulting in an orbital dipole magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called the spin dipole magnetic moment.
Gauss (G) and tesla (T) are units to define the intensity of magnetic fields. One tesla is equivalent to 10 000 gauss.
Typically, the field strength of MRI scanners is between 0.15 T and 3 T.

See also Diamagnetism, Paramagnetism, Superparamagnetism, and Ferromagnetism.

This effect is an additional electrical charge generated by ions in blood (loaded particles) moving perpendicular to the magnetic field. At 1.5 T, no significant changes are expected; at 6.0 T a 10% blood pressure change is expected. A blood pressure increase is predicted theoretically for a field of 10 T. This is claimed to be caused by interaction of induced electrical potentials and currents within a solution, e.g. blood, and an electrical volume force causing a retardation in the direction opposite to the fluid flow. This decrease in blood flow-velocity must be compensated for by an elevation in pressure.
Static magnetic field gradients of 0.01 T/cm (100 G/cm) make no significant difference in the membrane transport processes. The influence of a static magnetic field upon erythrocytes is not sufficient to provoke sedimentation, as long as there is a normal blood circulation.

The magnetohydrodynamic effect which results from a voltage occurring across a vessel in a magnetic field, is irrelevant at the field strengths used.