TYPES OF MAGNET
The magnets used in MRI systems can be of two types: permanent and electromagnets.
The permanent magnets have a permanent magnetic field. They are composed of ferromagnetic substances such as magnetite. They do not require cooling but they do operate at a low field strength (less than 0.25 T).
The electromagnets create a magnetic field by passing an electric current through wire loops. They can be resistive or superconducting. The resistive electromagnets consist of copper loops. They need to be cooled by water and their magnetic fields are medium-low, with values less than 0.5T. The superconducting electromagnets consist of wire loops made of titanium-niobium and have the ability to lose their electrical resistance when cooled to temperatures of -273° C by liquid helium. They create very high magnetic fields from 0.5T to 4T. The most frequently used are between 1.0T and 1.5T.
 One tesla (T) is equal to 10,000 gauss (G). The earth's magnetic field is around 0.5G.