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This chapter explains some of the basic principles that are used in the construction of a homogeneous superconducting magnet for magnetic resonance, including the basic physics of magnetic field generation, as well as superconducting materials and their properties. Concentrating on magnet design for human MRI, the chapter describes how the stray-field of a magnet can be controlled and how the magnetic field is stabilised. The cryogenic design and practical considerations for the operation of a magnet system are included with a number of key safety recommendations. Finally, the article discusses future developments, in particular operation of human systems at field strengths in excess of 10 T, and cryogen-free systems.

Superconducting magnets have been used in magnetic resonance imaging (MRI) since the early 1980s when the first low-field 0.35 and 0.5 T systems were introduced. Since then, field strengths have gradually increased and, today, 3.0 T is recognized as a standard field strength for clinical diagnostic imaging while magnets are also now available with field strengths up to 11.74 T for research in biomedical imaging applications. Superconductivity is the physical phenomenon in which materials have zero electrical resistance below a critical temperature and a critical field. Most MRI magnets are constructed from niobium–titanium (NbTi) superconductor and sit in a bath of liquid helium boiling at 4.2 K. The critical current and magnetic field characteristics of the superconductor determine the maximum field strength a magnet can achieve, although operating at temperatures below 4.2 K provides enhancement of the superconductor properties allowing for higher field strengths to be achieved. Controlling the extent of the stray fields around MRI magnets has always been an issue, and various techniques have been employed utilizing both steel shielding (passive) and active shielding (reverse wound coils). Today, virtually all MRI magnets up to 7.0 T use active shielding technology to reduce the stray fields and help with magnet-siting issues. The liquid helium reservoir containing the magnet is housed in a cryostat that is designed to minimize the heat loads reaching the reservoir. The cryostat is an evacuated vessel with one or more radiation shields cooled by a cryorefrigerator. Today, most systems use a cryorefrigerator that provides sufficient cooling power at 4.2 K to allow the evaporated gas to be recondensed and operate with no loss of liquid helium. This article describes the current state of superconducting magnet technology used in MRI systems. Keywords: superconducting; magnet; cryostat; shimming; shielding; high field