Near Infrared Spectroscopy (NIRS) is commonly used for functional brain studies. With this method, brain signals can be easily obtained, but the interpretation of these signals still remains unclear. This chapter provides a simple model to interpret the NIRS signal, which is based on the following assumptions: 1. The NIRS signal may reflect Hb levels only in the capillaries and not in large vessels; 2. The brain has a lighter color than the other tissues, indicating that the Hb concentration in brain tissue is very low and intensity level of the NIRS signal is very high; 3. A photon that hits a large vessel is too weak to be detected in the surrounding high signal environment; 4. Cerebral blood flow (CBF) can be separated into cross-sections (the number of capillary beds) that are multiplied by the velocity. This model can explain the typical signal pattern observed during task performance, where oxy-Hb levels increase and deoxy-Hb levels slightly decrease.
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Bear and Lambert’s law (1) defines the absorption of material as follows:
(1) where
Io, I, μa and
L stand for intensity of input light, the detected intensity of light, the absorption coefficient and the optical path length, respectively. The absorption coefficient
μa is defined as
(2) where
ε and
c stand for the extinction coefficient and the concentration of the material, respectively. The extinction coefficient depends on the material, such as oxy-hemoglobin (oxy-Hb) or deoxy-hemoglobin (deoxy-Hb) concentrations. Figure 1 shows a spectrum of extinction coefficients of oxy-Hb and deoxy-Hb.
Figure 1. Wavelength dependency of the extinction coefficients of oxy-Hb and deoxy-Hb
The two curves cross at approximately 800 nm, which is called an isosbestic point. Many instruments use the wavelengths from both sides of this point to obtain the hemoglobin concentrations. We can easily obtain the μa by measuring the light intensities I and Io in non-scattering materials because L can be definitely determined. In highly scattering material, such as brain tissue, the skull or other biological tissues, a photon emitted from a source will hit the numerous particles in the tissue, change its direction every time it hits a particle and, therefore, have a long path to be detected, which elongates optical path length L. A ratio between L and the real source detector distance d (optode distance) is defined as the differential path length factor (DPF), which has a value near 5.3 ±0.3 depending on the head position (Delpy et al., 1988;Wray et al., 1988).
DPF = L/d(3)