Substances exhibiting paramagnetic properties are used as contrast agents in MR imaging. They have a small but positive magnetic susceptibility (magnetizability - tends to align along the magnetic field). Typical paramagnetic substances usually possess an unpaired electron and include atoms or ions of transition elements, rare earth elements, some metals, and some molecules including molecular oxygen and free radicals.

See also Paramagnetism.

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'

A small linear magnetic field applied in addition to (superimposed on) the large static magnetic field in a MRI scanner. The strength (amplitude) and direction of the gradient fields change during the scan, which allows each small volume element (voxel) within the imaging volume to resonate at a different frequency. In this way, spatial encoding may be performed.

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).

(MSI) The combination of biomagnetic field detection and MR imaging into a merged data set. Most applications of MSI involve the combined use of MRI and measurement of magnetic fields created by electric currents in the brain, so-called magnetoencephalography MEG.
MEG allows calculation of the source of the measured biomagnetic fields, and thereby localization of many regional brain functions, such as mapping of the sensorimotor, auditory and visual cortex and also localization of epileptogenic foci. The MEG coordinate system is defined by anatomical landmarks, which are easily identified also with MRI, making it possible to align the 3D MEG data with the 3D MR image data. The resulting magnetic source images show the spatial relationships between the functional area provided by MEG and the neighboring anatomy and pathology, both provided by MRI.
Cardiac applications of MSI are also being explored. The electric currents in the myocardium create extrathoracic magnetic fields and the source of these fields may be calculated by the same principles as those used in MEG. Possible cardiac applications include mapping of arrhythmogenic sites prior to ablation therapy.