MEDICAL IMAGING

Jan. 1, 1998
Near-IR spectroscopy locates epileptic sites Researchers at the Hitachi Central Research Laboratory (Tokyo, Japan) working with the Department of Neurosurgery at Tokyo Metropolitan Hospital have developed the first laser-based method of pinpointing the location of an epileptic focus in the brain. The technique involves noninvasive optical tomography, in which the head of a subject is irradiated with beams of very weak near-infrared (near-IR) light to obtain two-dimensional images of the brain s

MEDICAL IMAGING

Near-IR spectroscopy locates epileptic sites Researchers at the Hitachi Central Research Laboratory (Tokyo, Japan) working with the Department of Neurosurgery at Tokyo Metropolitan Hospital have developed the first laser-based method of pinpointing the location of an epileptic focus in the brain. The technique involves noninvasive optical tomography, in which the head of a subject is irradiated with beams of very weak near-infrared (near-IR) light to obtain two-dimensional images of the brain surface. These images are then analyzed to detect the type of brain activity that accompanies epileptic seizures.

The method locates increases in blood flow that occur during an epileptic episode, and the results are in close agreement with those obtained by conventional examinations. The technique will make it possible to carry out such examinations without subjecting a patient to stress and discomfort during diagnostic procedures.

Treatment of patients who do not have good results from drug therapy currently involves surgical removal of the epileptic focus, which requires pinpointing the exact location of the focus. Normally, this involves various electroencephalographic measurements. However, the low spatial resolution of such methods makes it difficult to establish the precise location.

Two methods are currently used to determine the location of an epileptic focus. One involves opening the cranium to place electrodes on the surface of the brain--or to insert electrodes into deeper portions of the brain--thereby enabling the measurement of discharges during seizures.

The other method is single-photon-emission computed tomography, during which a radioactive isotope is injected into the blood. This technique discloses the blood-flow increase at the epileptic focus during seizures. Both methods subject the patient to considerable stress, discomfort, and radioactive insult, so there has been a growing need for a less stressful procedure.

Noninvasive procedure

To achieve that goal, Tokyo Metropolitan Hospital and Hitachi started working together on a noninvasive method of locating the epileptic sites based on near-IR spectroscopy. Near-IR beams can pass relatively easily through the scal¥and cranium and are detected after being reflected by the cerebrum.

The prototype system takes advantage of differences in the absorption index of hemoglobin in blood, measures changes in blood flow on both sides of the brain arising during an epileptic seizure, and displays the information in real time. The multichannel intensity-modulation-spectroscopy method uses eight channels and dual wavelengths. Laser diodes emit at 780 and 830 nm, with their intensity modulated at 1.0 and 3.5 kHz.

The intensity-modulated light is mixed and distributed via two fibers to two different locations on the patient`s head. Scattered light passing through the head is received by four detection fibers. The eight signals--from scattered light with two wavelengths--are detected by an avalanche-photodiode array. A lock-in-amplifier array then separates the intensities of the scattered light for each wavelength. The incident and detection fibers are set 30 mm apart; the diameter of each fiber is 1 mm (see figure).

The system makes it possible to noninvasively determine whether an epileptic focus is on the right or left side of the head; it also has the potential to locate the focus within a brain hemisphere. Broader measurements involving the whole brain are also possible, replacing the invasive electrode technologies. "The ability to analyze time-based changes in cerebral blood flow during an epileptic seizure is something that was not possible with conventional methods, making this an important pathological tool," notes Hitachi researchers.

Hitachi plans to develo¥"optoencephalography" as a portable technique for the examination of other brain diseases and functions.

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