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CHAPTER 2 recruited to minimise any random effects of fat. Furthermore, the statistical model incorporated each subject as a random factor which would accommodate for unique variations, including the effects of individual fat content. It was set out to determine if cadaveric tissues can serve as a good model for extrapolating penetration data to live tissues. In structures that were relatively low in muscle content, such as the distal upper limb, it was found that tissue thickness and light penetration in cadaver structures were comparable to that of live subjects (Figure 2.2). One possible confounding factor between live and cadaver subjects is the embalming medium, which could potentially alter the way light is scattered and absorbed. As there was no significant difference in thickness or penetration between cadaver and live subjects within the wrist, thumb and middle finger comparisons, it suggest that the embalming process had minimal impact on these structures. However, data from the upper arm revealed that body sites with larger proportions of muscle content (i.e. the biceps vs humerus sites) resulted in significant reductions in the overall structural thickness compared to live subjects (e.g. Figure 2.3a-b). The reduced muscle bulk is likely to occur from the removal of water content which is expected from the embalming process and/or the refrigeration storage of tissues, however age-related muscle size and water content differences may exist between the live and cadaver subjects. Reduced tissue thickness would improve penetration due to the reduced optical path length. However, with the exception of reduced water content, the overall matter within tissues of embalmed structures remains similar to live subjects, thus it is reasonable to expect similar levels of penetration in cadavers when accounting for the reduced optical path length in dehydrated tissues. Another difference between live and cadaveric tissues may be the displacement of blood in cadaveric tissues which may impact on the penetration. The greater variability of penetration observed in cadaver (Figure 2.1c) compared to live (Figure 2.1b) structures may therefore have resulted from the displacement of blood that is dependent on many variables during the embalming process. The purpose of making measurements in cadavers is to enable light readings that could otherwise not be performed in live subjects. In this study, the penetration through the cranium of cadavers was quantified. Given the minimal muscle content overlaying the cranium, tissue shrinkage is unlikely to have played a major impact on the optical path length. Assuming minimal fat content between the skin and the brain, this study would suggest that 100 mW/cm2 is sufficient to reach the external layers of cortex in live human subjects. 2.6 Conclusion In conclusion, the data demonstrates that at an intensity of 100 mW/cm2, 670 nm light can reach a depth of approximately 50 mm below the surface of structures comprising skin, bone muscle, tendons and ligaments, irrespective of skin tone. While increasing the light intensity increases penetration, it does not greatly increase the capacity to for light to penetrate thicker tissues over 51PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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