This chapter describes the fundamentals and advances in controlling tissue optical properties. As a major technology, the optical immersion method at usage of exogenous optical clearing agents (OCAs) is discussed. Water transport in a tissue, tissue swelling, and hydration at its interaction with an OCA are considered. Optical clearing properties of fibrous and cell-structured tissues are analyzed using spectrophotometry, frequency-domain, fluorescence, and polarization measurements; confocal microscopy and OCT; as well as nonlinear spectroscopy techniques such as two-photon fluorescence and SHG. In vitro, ex vivo, and in vivo studies of a variety of human and animal tissues such as eye sciera, skin, cerebral membrane (dura mater), gastric tissue, tendon, blood vessels, and blood are presented. OCA delivery, tissue permeation, and skin reservoir function are discussed. Cell and cell flow imaging at optical clearing are also discussed. Some important applications of the tissue immersion technique, such as glucose sensing, precision tissue laser photodisruption, as well as other techniques of tissue optical properties control, such as tissue compression and stretching, noncoagulating and coagulating temperature action, and tissue whitening, are described.
5.1 Fundamentals of tissue optical properties controlling and a brief review
Reflection, absorption, scattering, and fluorescence in living tissues and blood can be effectively controlled by various methods. Staining (sensitization) of biological materials is extensively used to study mechanisms of interaction between light and their constituent components, and also for diagnostic purposes, and selective photodestruction of individual components of living tissues. This approach underlies the diagnosis and photodynamic therapy (PDT) of malignant neoplasm, UV-A photochemotherapy of psoriasis and other proliferative disorders, angiography in ophthalmology, and many other applications in medicine.
In the visible and NIR regions, tissues and biological liquids are low absorbing, but highly scattering media (see Table 2.1). Scattering defines spectral and angular characteristics of light interacting with living objects as well as its penetration depth; thus, optical properties of tissues and blood may be effectively controlled by changes of scattering properties.