The main field magnet [Thomas, 1993] is required to produce an intense and highly uniform, static magnetic field over the entire region to be imaged. To be useful for imaging purposes, this field must be extremely uniform in space and constant in time. In practice, the spatial variation of the main field of a whole-body scanner must be less than about 1 to 10 parts per million (ppm) over a region approximately 40 cm in diameter. To achieve these high levels of homogeneity requires careful attention to magnet design and to manufacturing tolerances. The temporal drift of the field strength is normally required to be less than 0.1 ppm/h.
Two units of magnetic field strength are now in common use. The gauss (G) has a long historical usage and is firmly embedded in the older scientific literature. The tesla (T) is a more recently adopted unit, but is a part of the SI system of units and, for this reason, is generally preferred. The tesla is a much larger unit than the gauss — 1 T corresponds to 10,000 G. The magnitude of the earth’s magnetic field is about 0.05 mT (5000 G). The static magnetic fields of modern MRI scanners arc most commonly in the range of 0.5 to 1.5 T; useful scanners, however, have been built using the entire range from 0.02 to 8 T. The signal-to-noise ration (SNR) is the ratio of the NMR signal voltage to the ever-present noise voltages that arise within the patient and within the electronic components of the receiving system. The SNR is one of the key parameters that determine the performance capabilities of a scanner. The maximum available SNR increases linearly with field strength. The improvement in SNR as the field strength is increased is the major reason that so much effort has gone into producing high-field magnets for MRI systems.
Magnetic fields can be produced by using either electric currents or permanently magnetized materials as sources. In either case, the field strength falls off rapidly away from the source, and it is not possible to create a highly uniform magnetic field on the outside of a set of sources. Consequently, to produce the highly uniform field required for MRI, it is necessary to more or less surround the patient with a magnet. The main field magnet must be large enough, therefore, to effectively surround the patient; in addition, it must meet other stringent performance requirements. For these reasons, the main field magnet is the most important determinant of the cost, performance, and appearance of an MRI scanner. Four different classes of main magnets — (1) permanent magnets, (2) electromagnets, (3) resistive magnets, and (4) superconducting magnets — have been used in MRI scanners.
