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Chapter 9:
Other Methods of Tissue Optical Properties Control
As it was already mentioned in the Introduction, squeezing (compressing) or stretching of a soft tissue produces a significant increase in its optical transmission. The major reasons for this are the following: (i) increased optical tissue homogeneity due to removal of blood and interstitial fluid from the compressed site [see Eq. (6)]; (ii) closer packing of tissue components causes less scattering due to cooperative (interference) effects; and (iii) less tissue thickness. Mechanisms underlying the effects of optical clearing and changing of light reflection by soft tissues at compression and stretching were proposed in a number of theoretical and experimental studies. It should be emphasized, however, that squeezing-induced effects in tissues containing little blood, such as sclera, are characterized by a marked inertia (a few minutes) because of the relatively slow diffusion of water from the compressed region. It was suggested that compression of sclera may displace water from the interspace of collagen fibrils, increasing the protein and mucopolysaccharide concentrations. Since these proteins and sugars have refractive indexes closer to that of the collagen fibrils, a more index-matched environment can be created.
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