2898Re: [buildcheapeeg] build your own treatment instrument
- Jun 5, 2003Optical Topography-Principles of Near Infrared Spectroscopy (NIRS)
Weak near infrared light of about 1.5mW from a laser diode is illuminated onto head from optical fibers attached to the scalp. This light passes through the skull and reaches the cerebral cortex. It penetrates to a depth of about 3cm and is scattered by hemoglobin in the blood. The light is partially reflected back through the scalp. The reflected light is detected 3cm from the original illuminated point. In adults, the light attenuates to a level as low as a hundred millionth or even a billionth of the irradiated light. The reflected light is sent through optical fibers and received with a highly sensitive avalanche photodiode detector.
The figure on the left is the molecular absorption coefficient of oxy- and deoxy-hemoglobin. Brain activity is characterized by the firing of neurons. This results in active energy metabolism and causes secondary increase in blood volume to supply glucose and oxygen to the tissues. Thus, the change in concentration of hemoglobin is an important index for the brain activity.
The reflected light is measured every 0.1 seconds, and the latter measured data is subtracted from the former to determine the concentration change of hemoglobin. The intensity of the reflected light depends on the respective concentration of the two kinds of hemoglobin. Two kinds of laser diodes (780nm and 830nm) are used to separate the two kinds of hemoglobin concentration changes independently.
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