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CHAPTER 1 in the range of 0.5-4 J/cm2 at the cellular level in order to provide clinical benefits. This energy density (in ‘J/cm2’ or equivalent) is determined by the irradiance at the cellular site (in ‘W/m2’ or equivalent) and the duration of irradiation (in ‘s’ or equivalent). While the time of irradiation can be easily managed, the irradiance can be quite variable. The irradiance at the cellular site depends not only on the radiosity of the treatment unit (in ‘W/m2’ or equivalent) but also the degree to which light can penetrate thorough obscuring layers to the underlying tissues of interest undergoing treatment. In fact, the amount of penetration is the key challenge in optimising treatment strategies. Another contributing factor to the poor acceptance of PBM as a treatment is the poor understanding of its mechanism(s) of action. Despite many investigations since the 1980s, the cellular pathways leading to purported benefits remain to be fully elucidated. While some suggestions about the mechanism were made in the 1990s (Friedmann et al., 1991), it was not until more recently that studies have explained possible mechanisms in more detail (reviewed in de Freitas and Hamblin, 2016). The current section will first discuss the factors affecting light penetration (Section 1.2.1), focusing mainly on the photobiological properties of human tissues. A review of PBM mechanisms (Section 1.2.2), including both well-evidenced conclusions and other hypotheses will follow, and finally, the effects of PBM in different injuries/diseases will be discussed (Section 1.2.3). This section includes both clinical and preclinical studies. 1.2.1 Factors affecting penetration The amount of energy or the number of photons delivered to the targeted cellular level is dependent on many factors. When light hits any biological tissue, it can be either scattered or absorbed. The amount scattered will eventually be either absorbed or escape. The quantity of ‘escaped’ photons is commonly referred to as penetration. Penetration, therefore, reflects the amount of attenuation by both absorption and scattering, given the energy input. This section will discuss the optical properties of common human tissues, in terms of both scattering (Section 1.2.1.1) and absorption (Section 1.2.1.2). A clear understanding of the fate of photons that are delivered to tissues is essential to facilitate decisions on light treatment parameters necessary to estimate the amount of light reaching the targeted cellular site. 1.2.1.1 Scattering Scattering is a physical process where any form of radiation deviates from the original trajectory when passing through a medium. The scattering coefficient (μs), expressed as cm-1, is often used to describe the attenuation caused by scattering in a particular medium. The attenuation is dependent on a number of factors such as the wavelength of incident light and the medium 17PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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