Measurement of tissue optical properties in the context of tissue optical clearing.

Nowadays, dynamically developing optical (photonic) technologies play an ever-increasing role in medicine. Their adequate and effective implementation in diagnostics, surgery, and therapy needs reliable data on optical properties of human tissues, including skin. This paper presents an overview of recent results on the measurements and control of tissue optical properties. The issues reported comprise a brief review of optical properties of biological tissues and efficacy of optical clearing (OC) method in application to monitoring of diabetic complications and visualization of blood vessels and microcirculation using a number of optical imaging technologies, including spectroscopic, optical coherence tomography, and polarization- and speckle-based ones. Molecular modeling of immersion OC of skin and specific technique of OC of adipose tissue by its heating and photodynamic treatment are also discussed.

Blood optical clearing studied by optical coherence tomography.

The main limitation of optical imaging techniques for studying biological tissues is light scattering leading to decreasing of transmittance, which lowers the imaging quality. In this case, an immersion method for optical clearing of biological tissues can provide a possible solution to this problem, because the application of biocompatible clearing agents can reduce light scattering. Optical clearing represents a promising approach to increasing the imaging depth for various techniques, for example, various spectroscopy and fluorescent methods, and optical coherence tomography (OCT). We investigate the improvement of light penetration depth in blood after application of polyethylene glycol, polypropylene glycol, propylene glycol, and hemoglobin solutions using an OCT system. Influence of clearing agents on light transport in tissues and blood was also investigated in the mouse tail vein.

Refractive index of solutions of human hemoglobin from the near-infrared to the ultraviolet range: Kramers-Kronig analysis.

Because direct measurements of the refractive index of hemoglobin over a large wavelength range are challenging, indirect methods deserve particular attention. Among them, the Kramers-Kronig relations are a powerful tool often used to derive the real part of a refractive index from its imaginary part. However, previous attempts to apply the relations to solutions of human hemoglobin have been somewhat controversial, resulting in disagreement between several studies. We show that this controversy can be resolved when careful attention is paid not only to the absorption of hemoglobin but also to the dispersion of the refractive index of the nonabsorbing solvent. We present a Kramers-Kroning analysis taking both contributions into account and compare the results with the data from several studies. Good agreement with experiments is found across the visible and parts of near-infrared and ultraviolet regions. These results reinstate the use of the Kramers-Kronig relations for hemoglobin solutions and provide an additional source of information about their refractive index.