The concept of refractive index matching used for the enhancement of optical penetration depth of the whole blood is discussed on the basis on in vitro studies using NIR optical coherence tomography technique. It was found that blood optical clearing is defined not only by refractive index matching effect, but also by changes of the RBC size and their aggregation ability when chemicals are added. For example, for whole blood twice diluted by a saline adding 6.5% of glycerol reduces the total attenuation coefficient from 4.2 mm-1 to 2.0 mm mm-1, and correspondingly increases the optical penetration at 820 nm up to 117%. For other tested agents (glucose, dextrans, propylene glycol, and trazagraph) the enhancement of penetration was from about 20% up to 77%. In blood sedimentation study the regular or irregular oscillations or jumps of the RBC/plasma boundary were observed. The one- minute time period of regular oscillations well correlates with the kinetics of the aggregation process, described by the two subsequent stages of formation of the linear and 3-D aggregates.
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