Magnetic resonance MR --- also called nuclear magnetic resonance NMR --- is a physical property in the atomic nucleus: Every nucleus not having an even number of protons or an even number of neutrons has a spin. This spin generates a magnetic field, and the atomic nucleus is thus a little magnetic dipole moment. The spin can due to quantum mechanics only have some specific values. These are for the proton, the hydrogen H nucleus, + and --. The spins are normally aligned in all different directions, but when introduced in a strong magnetic field, they align themselves parallel and antiparallel with that field. The distribution between the parallel --- low energy --- and antiparallel --- high energy --- states is determined by the thermodynamics: the Boltzman law, with the lower energy states heavily outnumbering the higher.
Under the influence of the strong magnetic field, the nuclei begins to, what within the classical physical could be called precess. The frequency of this precession is called the Larmor frequency,and is dependent upon the strength of the magnetic field and the gyromagnetic ratio of the material. The precession of the nuclei are not in phase, but rather equally distributed over the cone of the precession.
Having the spins aligned in a strong magnetic field these can be excited by a electromagnetic impulse: The energy of the photon in the impulse (the Planck equation) must be in accordance with the difference in energy between the spin states. Those nuclei hit by the RF pulse will flip to the higher energy states. Because the RF pulse will interact differently with the different phases in the cone of precession, a magnetic field will be created orthogonal to the static field .
By the influence of the RF pulse, the ensemble magnetic field from a collection of nuclei, will now have a smaller parallel field (due to the exchange between the lower and higher energy states), and it will have an orthogonal component. Both will decay. The parallel field will decay (or rather restore) back to the thermodynamic equilibrium --- with a constant that has been denoted T This is a rather slow decay.
The decay of the orthogonal component is more rapid. It is caused by local differences in the field strength. These can come from inhomogeneities in the static field or the different magnetic properties of the ''material'' measured (the brain). The constant associated with the static field (we might denote it T is dominating over the material constant T, so the decay that is ###immediately observable is mostly a scanner specific constant T.
Table 3.1: Decay constants.
As the nuclei dephase, they emit an RF pulse with the decay of T. This is called the free induction decay --- FID. The amplitude of this signal is related to the spin density in the material. To acquire the interesting decay constants --- T and T --- different pulse sequences must be applied:
The fact that only odd proton-numbered or neutron-numbered atoms have spins, leaves out the important O and C, but keeps among clinical relevant nuclei P, C (1.1% isotope occurrence), Na, F, and most interesting the already mentioned H.
The strong static magnetic field can either be made with resistive, permanent or superconducting magnet. It is only the superconducting magnet that is able to come up in the Tesla area, while the others stays in the ''deciTesla'' area. To make the coil superconducting helium is used to cool it down. To be economical with the helium, this can be cooled down by nitrogen!
The magnetic field in connection with magnetic resonance seems not to be a safety problem. Weissman  mentions that the RF energy deposition can be a problem, especially in ''undervascularized, sensitive tissue'', such as the lens of the eye. He also draws attention to ''ballistic interaction'': Ferromagnetic material in the nearby area of the strong field interact very actively!