MRI Magnet

Photo courtesy NASA This image shows the tumor growth in a female's brain, sliced here in lateral view.

• Resistive magnets consist of many windings or coils of wire wrapped around a cylinder or bore through which an electric current is passed. This causes a magnetic field to be generated. If the electricity is turned off, the magnetic field dies out. These magnets are lower in cost to construct than a superconducting magnet (see below), but require huge amounts of electricity (up to 50 kilowatts) to operate because of the natural resistance in the wire. To operate this type of magnet above about the 0.3-tesla level would be prohibitively expensive.

• A permanent magnet is just that -- permanent. Its magnetic field is always there and always on full strength, so it costs nothing to maintain the field. The major drawback is that these magnets are extremely heavy: They weigh many, many tons at the 0.4-tesla level. A stronger field would require a magnet so heavy it would be difficult to construct. Permanent magnets are getting smaller, but are still limited to low field strengths.

• Superconducting magnets are by far the most commonly used. A superconducting magnet is somewhat similar to a resistive magnet -- coils or windings of wire through which a current of electricity is passed create the magnetic field. The important difference is that the wire is continually bathed in liquid helium at 452.4 degrees below zero. Yes, when you are inside the MRI machine, you are surrounded by a substance that is that cold! But don't worry, it is very well insulated by a vacuum in a manner identical to that used in a vacuum flask. This almost unimaginable cold causes the resistance in the wire to drop to zero, reducing the electrical requirement for the system dramatically and making it much more economical to operate. Superconductive systems are still very expensive, but they can easily generate 0.5-tesla to 2.0-tesla fields, allowing for much higher-quality imaging.