6.1 Blood Flow and Microvessel Network Imaging
Small blood microvessels can be clearly identified visually by the naked eye in in vivo study of hamster and rat skin, when a transparent window in the skin was created by glycerol drops applied to the subdermal side of a native hamster dorsal skin flap window preparation, or by intradermal injection of glycerol or 40% glucose (see Fig. 34).
In in vitro study of fresh human fat tissue with a topical application of a propylene glycol (PG) solution (50â80% with pure water), blood vessels are also seen. In order to have a quantitative sense of fat tissue optical clearing, the corresponding OCT images were registered (see Fig. 58). Two images of a fat sample without and with topical application of PG solution were captured. It is well seen that OCT techniques are very capable of visualizing the fat cells because of their sizes, which are within the limits of the OCT system spatial resolution. By comparison of images of Figs. 58(a) and 58(b), the light penetration enhancement is evident. Particularly, the blood vessel wall is clearly visualized after the topical application of PG solution, whereas without application of chemical agents, the light is almost blocked by the blood vessel. This is due to the fact that the blood inside the vessel highly scatters the incoming light, which degrades the imaging performance.
Besides more precise visualization of the vessel network, immersion agents may have influence on blood microvessels' functioning that gives the possibility to control functioning of tissue within the area of agent action. Functioning of microvessels of rat mesentery under the topical action of glycerol and glucose was described. A topical application of 75% glycerol during the initial period of 1â3 s led to a slowing down of blood flow in all microvessels (arterioles, venules, and capillaries). After 20â25 s, the stasis appeared and vessels were dilated by 30% on average, intravascular hemolysis took place; to 1 min after agent application, diameters of vessels were increased still more, to 40%. To the 6th min, stasis was maintained in all vessels, but the diameters of vessels were slightly decreased. Such changes of microcirculation were only local within the area of glycerol application. The topical application of glucose also decreased blood flow velocity in microvessels. For example, at the introduction of 40% glucose on a venule with a diameter of 11 Î¼âm and with initial an flow rate of 1075 Î¼mâs, the flow rate decreased to 510 Î¼mâs at 3 s after the glucose application, and to 202 Î¼mâs at 5 s.