dark. MRI produces highly readable images in any plane desired, which is especially helpful for an irregularly shaped mass such as the brain. The contrast among various tissues is often stronger than with x-ray images, although the resolution of detail is not as fine, perhaps about 2 millimeters on the average. Because gray matter has many fluid-containing cell bodies, whereas white matter has more fatty tissue, MRI shows a clear distinction between gray and white matter. The technique is particularly suitable for diagnosing and monitoring the course of such a disease as multiple sclerosis, which affects the fatty myelin sheath enclosing the nerves. MRI is also good for defining the precise location and extent of tumors.

Similar to MRI, and using much of the same equipment and procedures, is magnetic resonance spectroscopy (MRS). Here the magnet and radio waves are “tuned” for atoms other than hydrogen. Phosphorous atoms, for example, form part of phosphate molecules, which are involved in energy metabolism. Phosphorous MRS can thus be used to measure changes resulting from diseases of muscle cells that involve energy metabolism. Under- or overactivity of some areas of the brain could ultimately be used to warn of a disorder detectable by MRS. After making a diagnosis, the clinician could continue to use MRS to keep an eye on the course of a disease and the effect of drug therapy. MRS has also been used as a research tool, yielding new information on Alzheimer's disease, schizophrenia, autism, stroke, and the normal development and aging of the brain.

One of the latest applications of magnetic signals is in magnetic source imaging, which takes advantage of fact that every discharge of electricity is accompanied by a magnetic field, albeit often a weak one. This technique locates the sources of the very weak fields that accompany the electrical firing of nerve cells in the brain. The extremely sensitive detectors of magnetic source imaging are arranged around the patient's head to track varying levels of activity in many locations of the brain. At present, magnetic source imaging is most effectively used to locate the point of origin of epileptic seizures. Precision is the chief concern here; often, further seizures can be prevented altogether by excision of the tiny area of the brain identified as the focal point. Surrounding areas are able to compensate for any loss of function.

One other imaging technique, ultrasound, has a rather spe-

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