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SCIENCE / MEDICINE : Shedding Light on Inner Workings of the Brain : Diagnosis: MEG, a new way of measuring magnetic fields, can be used to study epilepsy and other maladies.

February 26, 1990|T. A. HEPPENHEIMER | Heppenheimer is a free-lance writer.

Lovers, lawyers and politicians are among the people who would like to be able to tell what another person is thinking. Psychologists and neurologists have more modest goals: they would like to tell how a person thinks, to map the brain's activity.

For several years, medical imaging techniques have been giving astonishingly vivid and detailed interior views of the human brain. These include the CAT scan, an advanced form of X-ray; MRI (magnetic resonance imaging), which relies on signals emitted by the atoms in the brain itself; and PET (positron emission tomography), which uses a weak radioactive tracer.

Now a new technique--magnetoencephalography, or MEG--is being added to the arsenal. Though still in its preliminary research stages, MEG is giving the clearest views yet of the workings of the human brain by detecting the almost vanishingly weak magnetic fields that flash into existence when neurons fire momentarily. By detecting such fields, MEG determines which neurons are firing and measures the strength of their activity.

To do this, MEG makes use of the world's most sensitive magnetometers, called SQUIDs--Superconducting Quantum Interference Devices. A SQUID relies on a type of superconductivity that British physicist Brian Josephson won a Nobel Prize for discovering.

Superconductivity involves electric currents that flow with no resistance, at temperatures close to absolute zero, minus 460 degrees Fahrenheit. In the "Josephson effect," such currents are extremely sensitive to the presence of even a very weak magnetic field, falling off markedly if such a field is present.

In a SQUID, this sensitivity makes it possible to detect such fields even when they are a billion times weaker than the magnetism of the Earth, which swings a compass needle. That is the strength of magnetic fields produced in the normal brain.

These fields are produced by electrical flows within neurons, and such electric currents in the brain have been studied for decades using the technique of EEG, electroencephalography. But the skull and the brain's internal fluids distort the flow of these currents, making it difficult to interpret the measurements. Indeed, a common EEG technique has been to remove part of the skull and place electrodes directly on the surface of the living brain.

In MEG, by contrast, the magnetic fields emerge from the brain undistorted by nearby fluids or tissues. The technique is totally non-invasive; the patient does not even need to have his head shaved. The doctor simply presses a detector, which looks like a beer can and holds several SQUIDs, against the patient's head, then interprets the readings with the help of a computer.

Existing techniques allow no more than a few such measurements at any given moment, however. Thus, scans of the entire brain must be built up over several hours, with the detector being moved from place to place around the head. And the resulting scans are easiest to interpret when the neural activity being studied is concentrated in isolated spots within the brain, rather than being distributed throughout its volume.

Because of this, MEG has thus far been used particularly for studies of epilepsy. Epilepsy has been described as an electrical storm in the brain; it features the uncontrolled and violent firing of neurons, in ways that produce particularly strong magnetic fields. About 2.3 million Americans are epileptics; their seizures can produce uncontrolled movements of the arms or legs, abrupt memory lapses, loss of consciousness, or convulsions.

Most such people are helped with medication, but for as many as 50,000 patients, surgery is the only remedy, The neurosurgeon must cut away portions of brain tissue that trigger seizures. First, however, these portions must be located precisely.

Neurologist William W. Sutherling of UCLA said that by using MEG, it is possible to map magnetic field patterns directly onto photographs of the patient's head, graphically indicating areas of abnormal activity. MEG thus locates these areas with high accuracy.

For all its promise, however, MEG still is in the research stage. Only a few major hospitals have the needed instruments and facilities. The Food and Drug Administration has licensed the use of MEG for research on epilepsy, but not yet for its routine clinical use. And investigators continue to devote a great deal of attention to such basic issues as signal processing as they learn to extract the most information from their measurements.

At a conference held at New York University last year, other researchers showed that MEG can be used to diagnose Alzheimer's disease. Until now, Alzheimer's has been detected only by retrieving tissue from the brain for examination. But Rodolfo Llinas and his colleagues at NYU have found a very abnormal pattern of magnetic activity in Alzheimer's patients, in the regions of their brains that respond to hearing.

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