(ESR) Electron spin resonance is a spectroscopic technique to identify paramagnetic substances. This magnetic resonance phenomenon investigates the nature of the bonding within molecules by identifying unpaired electrons, e.g. in free radicals and their interaction with their immediate surroundings. The Larmor frequency are much higher than corresponding NMR frequencies in the same static magnetic field.
Nuclei with an odd number of neutrons and/or protons, because of their spin, react like tiny magnets and can be lined up in an applied magnetic field. Energy applied by alternating radio frequency radiation is absorbed when its frequency coincides with that of precession of the electron magnets. The spectrum of radiation absorbed as the field changes gives information valuable in chemistry, biology, and medicine since over 50 years.

Ferromagnetism is a phenomenon by which a material can exhibit a spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field. More recently: a material is ferromagnetic, only if all of its magnetic ions add a positive contribution to the net magnetization (for differentiation to ferrimagnetic and antiferromagnetic materials). If some of the magnetic ions subtract from the net magnetization (if they are partially anti-aligned), then the material is ferrimagnetic. If the ions anti-align completely so as to have zero net magnetization, despite the magnetic ordering, then it is an antiferromagnet. All of these alignment effects only occur at temperatures below a certain critical temperature, called the Curie temperature (for ferromagnets and ferrimagnets) or the Néel temperature (for antiferromagnets). Typical ferromagnetic materials are iron, cobalt, and nickel.
In MRI ferromagnetic objects, even very small ones, as implants or incorporations distort the homogeneity of the main magnetic field and cause susceptibility artifacts.

(G) An older unit of flux density. The currently preferred SI unit is the tesla (T).
Definition: 1 gauss is defined as 1 line of flux per cm². The Earth's magnetic field is approximately one half gauss to one gauss, depending on location. For the large magnetic fields used by MRI, the unit gauss (G) has been replaced by the more practical unit tesla (T), where 1 T = 10 000 G.

This term is commonly used for a particular kind of gradient coil, commonly used to create magnetic field gradients perpendicular to the main magnetic field. A golay coil (a special kind of saddle coils) produces a linear gradient in the x and y axes that requires wires running along the bore of the magnet. Such a coil produces a very linear field, but the linearity is lost rapidly away from the central plane. A number of pairs with different axial separations can be used to improve this.

(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.

See also Susceptibility Artifact.