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.

Time-dependent electromagnetic fields are significantly attenuated by conducting media (including the human body); the skin depth gives a measure of the average depth of penetration of the RF field. A high power frequency tunable RF source can be rapidly switched on and off. This produces a large RF field perpendicular to the magnetic field. This RF field is focused by the body coil. The RF source and coils must be tunable in both frequency and impedance to 'match the impedance' of the patient's body.

The skin depth may be a limiting factor in MR imaging at very high frequencies (high magnetic fields). The skin depth also affects the Q of the coils.

Superconducting magnets are electromagnets that are partially built from superconducting materials and therefore reach much higher magnetic field intensity.
The coil windings of superconducting magnets are made of wires of a type 2 superconductor (mostly used is niobium-titanium - up to 15 Tesla the critical temperature is less then 10 Kelvin). These coils have no resistance when operated at temperatures near absolute zero (-273.15°C, -459°F, 0 K).
Liquid helium (4.2 K) is commonly used as a coolant (sometimes in addition with a second cryogen liquid nitrogen as an intermediate thermal shield to reduce the boil-off rate of liquid helium), which consequently conclude refilling (intervals: liquid helium ~ 3 month, liquid nitrogen ~ 2 weeks). There are cryogen-free superconducting magnets with a closed-cycle refrigerating system at the horizon. Superconducting magnets typically exhibit field strengths of greater than 0.5 T, operate clinically up to 3 T, and have a horizontal field orientation, which makes them prone to missile effects without significant magnetic shielding.
See also Quenching.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'

From FONAR Corporation;
in October of 2004, the company changed the product name of the Stand-Upâ„¢ MRI to the Uprightâ„¢ MRI. The Indomitableâ„¢, Uprightâ„¢ MRI is the only open MRI in the world that can perform positional MRI (pMRI), i.e. the Upright MRIâ„¢ scans patients in upright, weight-bearing positions, in addition to the conventional lie-down positions. The Uprightâ„¢ MRI is the only device that can scan patients in the position their symptoms occur, in their position of pain. In early clinical reports independently confirm the effectiveness and potential of positional MRI. In October 2000, Fonar received permission to market the Indomitableâ„¢ from the FDA.