What types of magnets do MRIs use?

MRI systems use three basic types of magnets:

• Resistive magnets are made from many coils of wire wrapped around a cylinder through which an electric current is passed. This generates a magnetic field. When the electricity is shut off, the magnetic field dies. These magnets are lower in cost to make than a superconducting magnet (see below), but need huge amounts of electricity to operate because of the natural resistance of the wire. The electricity can get expensive when higher power magnets are needed.
• A permanent magnet is just that -- permanent. The magnetic field is always there and always at full strength. Therefore, it costs nothing to maintain the field. A major drawback is that these magnets are extremely heavy: sometimes many, many tons. Some strong fields would need magnets so heavy they would be difficult to construct.
• Superconducting magnets are by far the most commonly used in MRIs. Superconducting magnets are somewhat similar to resistive magnets - coils of wire with a passing electrical current create the magnetic field. The important difference is that in a superconducting magnet the wire is continually bathed in liquid helium (at a cold 452.4 degrees below zero). This almost unimaginable cold drops the wire's resistance to zero, dramatically reducing the electricity requirement for the system and making it much more economical to operate.