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CHAPTER 1 microstructure. A large scattering coefficient indicates a medium that is ‘prone’ to scattering, and therefore more attenuation. With PBM, the light must pass through different tissue layers before reaching the target site. These layers may include the skin, adipose tissue, muscle, bone, connective tissue, and blood/lymphatic tissue. While the scattering coefficient of connective and blood/lymphatic tissue is intricate to estimate, that of skin, adipose tissue, muscle, and bone have been well studied. By integrating decades of research, Jacques (2013) modelled the scattering coefficient for the major tissues in the human body ranging from UV to NIR light (roughly 250-2000 nm). A decreased scattering coefficient is observed with a longer wavelength, regardless of tissue type. All scattering coefficients are between 1 and 200 cm-1. Skin generally has the largest scattering coefficient (4-200 cm-1); that of muscle (1-60 cm-1), bone (4-50 cm-1), and adipose tissue (4-30 cm-1) are very similar across the spectrum. Since scattering coefficients of the above-mentioned tissue types are within the same order of magnitude, these tissues could therefore be considered as a homogenous medium in terms of scattering. 1.2.1.2 Absorption When irradiation enters a medium, chromophores present in the medium absorb photons of specific wavelengths, and thereby attenuate the incident light. The absorption coefficient (μa) is, therefore, derived to describe the extent of attenuation in cm-1. The absorption coefficient is dependent on a number of factors such as the presence of chromophore, the wavelength-selectivity of chromophore and the abundance of chromophore. The larger the absorption coefficient, the more light is attenuated. There are a variety of endogenous chromophores in the human tissue, and the major ones are melanin, water, oxygenated and deoxygenated haemoglobins (HbO2 and HbR respectively), oxygenated and deoxygenated myoglobins (MbO2 and MbR respectively), and lipids. Each of these chromophores has distinctive patterns of absorption across the range of visible to NIR light (400-1800 nm; Figure 1.3). Melanin generally has the highest absorption coefficient across the range, starting from 103 cm-1 and gradually decreasing to the 101 cm-1 range around 1800 nm. Water, on the other hand, starts in the 10-4 cm-1 range and gradually converges with melanin from around 1400 nm. Lipids, displaying a pattern similar to water, start from around 10-2 cm-1 and gradually increase to 10-1 cm-1 with two local peaks at around 900 nm and 1200 nm. Both HbO2 and HbR start with a high absorption coefficient in the 103 cm-1 range at 400 nm and dramatically decrease to the 100 cm-1 range at 1000 nm. The two chromophores have almost identical absorption coefficients from 400 nm to almost 600 nm, with a global peak around 440 nm, a local valley at 480 nm, and a local peak around 550 nm. HbO2 has a distinctive drop in its absorption coefficient just before 700 nm and then plateaus from around 800 nm. The two myoglobins, on 18PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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