About NIRS (Principle of Operation and How It Works)
What Is NIRS (Near-Infrared Spectroscopy)?
The human brain has about 100 billion neurons. Humans take in the information associated with sight, hearing, touch, smell, and taste from sensory organs (eyes, ears, etc.) and convert this information to electrical signals which are then sent to the brain. Neurons in the brain process this information through mutual exchange of the signals to determine the next action. During this process, oxygenated hemoglobin (oxyHb) supplies oxygen via capillary vessels. NIRS technology can analyze the functional localization of the brain by measuring this reaction in real time using near infrared light.
Principle of NIRS (Near-Infrared Spectroscopy) - Why use near infrared light? -
The blood component hemoglobin scatters light, and the ratio of infrared light absorbed to that scattered changes depending on the degree of hemoglobin binding with oxygen. NIRS measures this rate of change and the change in oxygenated hemoglobin concentration.
Near infrared light having a wavelength of 700 to 900 nm is used in the optical measurement of living organisms. Visible light (wavelength 400 to 700 nm) is substantially absorbed by hemoglobin and other component organic matter, while absorption by water increases at wavelengths longer than near infrared light. Thus, light in these wavelength ranges cannot internally penetrate living organisms. Thus, the near infrared wavelength region is also referred to as "a window to living organisms" because near infrared rays easily penetrate living organisms.
Though absorption of light in this wavelength region is caused mainly by oxygenated hemoglobin (oxyHb) and deoxygenated hemoglobin (deoxyHb), both of these have differing absorption spectra, and the isosbestic point is in the vicinity of 805 nm, as shown in the figure on the right. For this reason, if the mol molecular absorption coefficient of oxyHb and deoxyHb is known, the change in oxyHb and deoxyHb concentration can be calculated by measuring the change in absorption at two or more wavelengths.
Principle of Brain Function Measurement by Near Infrared Light
Though near infrared light can penetrate relatively deeply into living organisms, it does not penetrate easily into the human head. For this reason, the inside of the brain is irradiated from the surface of the head by near infrared light via optical fiber, and light, after being absorbed and scattered at the cerebral cortex, is condensed by optical fiber again at a distance of about 30 mm from the surface of the head in the case of an adult. At this time, light reaches an area about 20 mm deep from the surface of the head, indicating that it is absorbed by hemoglobin at the cerebral cortex. Because living organisms significantly scatter light, near infrared light introduced by optical fiber is scattered by various types of tissue. Some of this scattered light reaches the optical fiber in the light receiving probe, and is then guided to a photomultiplier where it is converted to electrical signals.