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Abstract

A magnet assembly suitable for use in magnetic resonance imaging has a first superconducting coil assembly for generating a first magnetic field and a second superconducting coil assembly for generating a second magnetic field. The first and second superconducting coil assemblies are connected in series and each generate magnetic fields whose corresponding components are of substantially the same order of magnitude. The assemblies are arranged so that a resultant, uniform magnetic field is generated in a working volume, and the second magnetic field substantially opposes the first magnetic field externally of the magnet assembly. The resultant magnetic field prevailing externally of the magnet assembly is maintained within preset limits during normal operation of the assembly in which the current in the superconducting coils is substantially constant. The assembly further includes at least one electrically conductive ring disposed outside of the working volume, which is sized and positioned so that current induced in the ring during a transient condition, in which the current in the superconducting coils is changing, such as in the event of a quench, creates a magnetic field which acts to oppose the magnetic fields created by the currents induced in conductive parts of the magnet assembly. This causes the resultant magnetic field prevailing externally of the magnet assembly to remain substantially within or close to the preset limits for normal operation.

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