Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a noninvasive imaging technique that uses no ionizing radiation and is without significant health hazard. It is based on the principles of nuclear magnetic resonance (NMR) discovered in the 1940's. The word nuclear was dropped from the name because of the negative connotation in the public mind associated with the word nuclear and to avoid confusion with other, unrelated phenomena such as nuclear power, nuclear energy and nuclear bombs.

MRI can display tomographic images of slices (cross sections) of the body. Unlike CT, MRI can acquire images in any plane selected by the physician without any degradation of image quality. Another advantage of MRI compared to CT is much greater contrast between different types of normal body tissues, and between pathologic tissues and normal tissues. Disadvantages include longer imaging time and the necessity to be in a slightly more closed space for imaging than CT, which may disturb patients with a tendency towards claustrophobia. Newer "open" design systems minimize this feeling of claustrophobia.

MRI is now the best imaging test for many body regions such as the brain, spine, bones, and joints. During an MRI examination, the patient is placed inside a very strong magnet. The hydrogen atoms within the patient's body align themselves with the magnetic field. The body area being examined is exposed to radio waves, which are first absorbed and then emitted. The emitted waves are the MRI signal. The signal is analyzed by a computer and processed into images of the body. The images are usually in the form of slices through the body, the way one would slice a loaf of bread. The slices can be taken in any plane chosen by the operator. Parts of the body such as fat with a high signal are displayed as white, those with the lowest signal such as air in the lung are black, and others such as muscle a shade of gray.