9.1 Tissue Compression and Stretching
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. On the other hand, compression reduces specimen thickness d, which might increase the effective scatterer concentration inside the tissue. Therefore, compression may also give rise to an increase in tissue scattering coefficient Î¼s. However, the total effect on the change in optical properties, which is proportional to the product of Î¼sd, is characterized by less scattering.
Sometimes the increase in scatterer concentration is likely to be more dominant than the reduction in index mismatch. In addition, reduction of tissue thickness causes an increase in local chromophore concentration (for bloodless tissue, or tissue specimens having aggregated andâor coagulated blood), i.e., the absorption coefficient increases. The authors of Ref. 57 observed that compression caused leaking around the specimen. Some of the extracellular fluids along the edge of the tissue sample were forced out upon compression. Unless sufficient pressure was applied to rupture the cell walls, the intracellular fluids would be retained by the cells in the bulk of the sample. When compressed, tissue thickness was reduced so that the volumetric water concentration was increased. This may explain the increase of the absorption coefficient at the wavelengths of water bands with compression.