Recent technology advancements in photonics have spurred a prodigious progress toward the development of innovative methods and systems for clinical functional optical imaging, laser surgery, and phototherapy.^1,2 Development of optical biomedical methods and techniques has stimulated a great interest in the study of optical properties of human tissues, which define the efficacy of tissue optical probing and light action on tissue and when are known (measured) give an opportunity to predict precise photon propagation trajectories and fluence rate distribution within irradiated tissues.

This study is focused on the measurements of the refractive index of hemoglobin solutions in the visible/near-infrared (NIR) spectral range at room temperature for characteristic laser wavelengths: 480, 486, 546, 589, 644, 656, 680, 930, 1100, 1300, and 1550 nm. Measurements were performed using the multiwavelength Abbe refractometer. Aqua hemoglobin solutions of different concentrations obtained from human whole blood were investigated. The specific increment of refractive index on hemoglobin concentration and the Sellmeier coefficients were calculated.

A distinctive feature of this review is a critical analysis of methods and results of measurements of the optical properties of tissues in a wide spectral range from deep UV to terahertz waves. Much attention is paid to measurements of the refractive index of biological tissues and liquids, the knowledge of which is necessary for the effective application of many methods of optical imaging and diagnostics. The optical parameters of healthy and pathological tissues are presented, and the reasons for their differences are discussed, which is important for the discrimination of pathologies and the demarcation of their boundaries. When considering the interaction of terahertz radiation with tissues, the concept of an effective medium is discussed, and relaxation models of the effective optical properties of tissues are presented. Attention is drawn to the manifestation of the scattering properties of tissues in the THz range and the problems of measuring the optical properties of tissues in this range are discussed. In conclusion, a method for the dynamic analysis of the optical properties of tissues under optical clearing using an application of immersion agents is presented. The main mechanisms and technologies of optical clearing, as well as examples of the successful application for differentiation of healthy and pathological tissues, are analyzed.

This study presents refractive index measurements of human and porcine adipose tissues and lipid droplet content in the visible and near-infrared. The coefficients of the Sellmeier formula were calculated for approximation of tissue dispersion. For the first time, to the best of our knowledge, the phase transition temperatures and temperature increments $dn/dT$ of adipose tissue were quantified for a wide wavelength range from 480 to 1550 nm and from room temperature up to 50°C. For human abdominal adipose tissue, the refractive index increment averaged across all wavelengths is $dn/dT=-(3.54\pm 0.15)\times {10}^{-4}°{\mathrm{C}}^{-1}$, for porcine tissue $dn/dT=-7.92(0.74)\times {10}^{-4}°{\mathrm{C}}^{-1}$, and for porcine lipid droplet $dn/dT=-6.01(0.29)\times {10}^{-4}°{\mathrm{C}}^{-1}$. Data available in literature for refractive indices of adipose tissues measured by different techniques are summarized and compared with the received data.

Blood refractive index modelling in the visible and near infrared spectral regions

Photoacoustic (PA) imaging (PAI) is an emerging powerful tool for noninvasive real-time mapping of blood and lymphatic vessels and lymph nodes in vivo to diagnose cancer, lymphedema and other diseases. Among different PAI instruments, commercially available raster-scanning optoacoustic mesoscopy (RSOM) (iThera Medical GmbH., Germany) is useful for high-resolution imaging of different tissues with high potential of clinical translation. However, skin light scattering prevents mapping vessels and nodes deeper than 1–2 mm, that limits diagnostic values of PAI including RSOM. Here we demonstrate that glycerol-based tissue optical clearing (TOC) overcomes this challenge by reducing light scattering that improves RSOM depth penetration. In preclinical model of mouse limb in vivo, the replacement of conventional acoustic coupling agents such as water on the mixture of 70 % glycerol and 30 % ultrasound (US) gel resulted in the increase of tissue imaging depth in 1.5–2 times with 3D visualization of vessels with diameter down to 20 μm. To distinguish blood and lymphatic networks, we integrated label-free PA angiography (i.e., imaging of blood vessels), which uses hemoglobin as endogenous contrast agent, with PA lymphography based on labeling of lymphatic vessels with exogenous PA contrast agents. Similar to well-established clinical lymphography, contrast agents were injected in tissue and taken up by lymphatic vessels within a few minutes that provided quick RSOM lymphography. Furthermore, co-injection of PA contrast dye and multilayer nanocomposites as potential low-toxic drug-cargo showed selective prolonged accumulation of nanocomposites in sentinel lymph nodes. Overall, our findings open perspectives for deep and high resolution 3D PA angio- and lymphography, and for PA-guided lymphatic drug delivery using new RSOM & TOC approach.