The Science of Magnets in Identifying the Problems of the Body

At the heart of a magnetic resonance imaging scanner is a superconducting magnet, cooled by liquid helium to -452.4 degrees Fahrenheit and capable of producing a magnetic field 60,000 times stronger than that of the Earth. These medical devices were used to conduct 60 million scans of people worldwide in 2003, according to the National High Magnetic Field Laboratory. MRI scanners can produce images at any angle and depth without the need for surgery. Doctors use the images to help diagnose medical conditions such as multiple sclerosis, tumors and strokes.

Atomic Structure of Hydrogen

• Water molecules, which are found throughout the body, each contain two hydrogen atoms. The unique atomic structure of hydrogen enables MRI scanners to produce detailed images. Unlike other atoms, hydrogen has no neutrons in its center -- or nucleus -- only a single proton. This proton spins like a child's top and creates a magnetic field, which gives it a positive and negative pole like the ends of the needle on a compass.

Magnetic Attraction

• When hydrogen atoms are placed inside a larger magnetic field, like the one created by an MRI scanner, the protons line up with the larger field; this is similar to the way a compass needle lines up with the magnetic field of the Earth. Slightly more protons end up in a low-energy direction rather than a high-energy direction. Like hydrogen, all atoms with unequal numbers of protons and neutrons are affected by magnetic fields in the same way, but hydrogen is used for MRIs because water is very abundant in the body.

Radio Signals

• In addition to creating a large magnetic field, the MRI scanner sends out a radio frequency signal, similar to an FM radio signal. This "pulse" causes the extra low-energy protons to change to the high-energy direction. When the signal stops, these protons go back to the low-energy direction and give off a signal that is converted to an electrical current. The MRI computer analyzes the signals from each atom to create images of the body.

Layers and Slices

• When more water is present in a body tissue, there are also more hydrogen atoms that can give off signals. The computer uses this information to determine the density of the tissues and create an image that shows the structure of the body. MRI scanners can capture images in three planes: axial, from head to toe; sagittal, from side to side; and coronal, from front to back.