Nuclei that have an odd number of protons or neutrons have a magnetic moment. This means that they act like small magnets. If a molecule that contains magnetic nuclei is placed in a powerful magnetic field, these magnetic nuclei will try to align with the magnetic field just as a compass needle aligns with the magnetic field of Earth.

If these aligned magnetic nuclei are irradiated with radio waves, they can absorb energy and become opposed to the external magnetic field. This absorption of radio waves can be recorded on a chart which is called a nuclear magnetic resonance spectrum. This technique is called nuclear magnetic resonance spectroscopy and is an important tool by which chemists study the structures of molecules.

Nuclear magnetic resonance is the basis of the medical diagnostic tool called Magnetic Resonance Imaging (MRI). Hydrogen atoms have magnetic moments and these are the nuclei that are observed in magnetic resonance imaging.

The magnetic resonance imaging technique produces a tomagraphic picture (slice) along any plane through the body. This picture is a map of hydrogen concentrations in the various tissues and fluids in this plane. Deviations from the normal pattern indicate medical anomalies such as tumors.

The subject is placed in a strong magnetic field and a spectrum is recorded from several angles. The signal intensity of a tissue in magnetic resonance imaging is a function of the hydrogen atom content in the tissue. Water and lipids (fats) are the major hydrogen containing components found in the body and these show up in the scan. The signal from protein is not as strong because the percentage of hydrogen in protein is not very large. Bone contains very little hydrogen and will not show up in the scan.

The multiple scans are then correlated into a single image by computer analysis. This new image will look like the following axial or sagital images. Magnetic resonance imaging is a technique that has far reaching utility in the medical field.

MRI of a Healthy Brain