Abstract

In the near-infrared (NIR) wavelength range (600–1000 nm), light absorption by the main chromophores such as water and hemoglobin in tissue is relatively low. For brain tissue, the highly transmitting and scattering characteristics of photons are often used in NIR spectroscopy (NIRS) and optical topography to gain information about the physiological processes (Kato et al. 1993; Maki et al. 1995; Villringer et al. 1993). Thus, the optical properties of human brain tissue for NIR light were well investigated in many studies (Bevilacqua et al. 1999; Choi et al. 2004; Stolik et al. 2007), which demonstrated that light can easily penetrate a few centimeters into the tissue even by transcutaneous irradiation. Figure 54.1 shows the structure of the brain. Lychagov et al. (2006) investigated the transmittance of 810 nm laser irradiation through ex vivo human skull and scalp. The value of transmittance varies from 0.5% to 5% in the case of a sample with scalp and from 1% to 16% in the case of a skull alone. Although the degree of photon absorption needed for low-level laser therapy (LLLT) is quite little as compared to that by the primal photoacceptors such as hemoglobin, melanin, and water, the NIR light in a brain tissue must be absorbed for photobiomodulation to occur. The laser fluence from 1 to 20 J/cm² is commonly used in LLLT for brain diseases, while the intensity generally varies from 5 to 50 mW/cm² depending on the actual light parameters and the therapeutic objects. On the assumption of the expected therapeutic power density at the dura mater, NIR laser irradiation with a few hundreds of milliwatts per square centimeter would be necessary for transcutaneous application of LLLT in a clinical situation. Meanwhile, the intensity should be controlled so that the irradiance on skin is well below the American National Standards Institute (ANSI) maximum permissible exposure (MPE) level. According to ANSI Z136.1, the skin exposure MPE for wavelengths ranging from 600 to 1000 nm is 200–800 mW/cm² (exposure duration, 10 to 3 × 10⁴ s). Figure 54.2 shows various brain diseases that have been treated with LLLT.