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CHAPTER 2 parameter results in: 1) a change in the optical path length and 2) alterations in absorbance and scattering that is dependent on the additional tissue layers and/or layer thicknesses. Tissue thickness readings could have been affected by varying force applied when using callipers, however the measuring error is estimated to be within (1-3 mm), and therefore unlikely to greatly affect the findings. While tissue thickness is the main determinant of penetration, improved penetration through the humerus compared to the biceps in live subjects was observed, despite that the humerus site was thicker. Greater penetration was also observed in the middle finger compared to the thumb, despite these structures showed no significant difference in tissue thickness. The main difference in the optical path between the structures compared above is the proportion of bone and muscle content. Compared to bone, muscle has a relatively greater capacity to absorb 670 nm due to the higher content of both deoxygenated and oxygenated haemoglobins (Bashkatov et al., 2011; Maud and Foster, 2006), myoglobins (Barham et al., 2010). Based on combined absorbance and scattering coefficients (Bashkatov et al., 2011; Tuchin et al., 2006), the following order of penetration between 600-700 nm (ordered least to most) would be expected for the following tissues: 1) subcutaneous fat, 2) skin, 3) muscle, and 4) bone. The absorbance coefficient of bone is 20%-60% lower than muscle, while the scattering coefficient for both these tissues are similar (Bashkatov et al., 2011; Jacques, 2013; Tuchin et al., 2006), thus the additional proportion of bone content in the humerus and middle finger is likely to have improved the level of penetration compared to the biceps and thumb respectively. No effect of skin tone on red light penetration was found. Melanin is a key chromophore in the skin and is the primary determinant of skin tone thus subjects with darker skin tone have more melanin present. Melanin has an absorption spectrum that increases with shorter wavelengths; at 670 nm, melanin absorption is reduced to approximately 55% compared to at 500 nm, and 35% compared to at 400 nm (Anderson and Parrish, 1981; Meglinski and Matcher, 2002). Furthermore, the absorption coefficients of skin at 670 nm for different Fitzpatrick skin types (II-VI) are between 0.5-2 cm-1 (Saager et al., 2015). As the melanin layer is relatively thin compared to other tissue layers, its impact on overall penetration is negligible compared to layers with thicknesses that are several orders of magnitude greater. The findings that 670 nm penetration was largely unaffected by skin tone is therefore consistent with these studies characterising melanin absorption at this wavelength. Compared to skin, the absorption and scattering coefficients of fat are ~50% and 200% greater respectively, over the 600-700 nm range (Bashkatov et al., 2011). Fat content would therefore greatly affect penetration because: 1) the interaction of absorbance and scattering is not linear and increasing both is likely to greatly reduce penetration (Meglinski and Matcher, 2002; van Gemert et al., 1989), and 2) skin layers are generally thinner, and can be up to an order of magnitude thinner, than fat layers. While fat content was not measured in this study, lean subjects were 50PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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