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Application of many optical methods, such as photoacoustics, optical coherent tomography, and terahertz spectroscopy, for tissue imaging and disease diagnostics is limited due to small probing depth caused by strong scattering and absorption in tissues [1, 2]. It is possible to increase the efficiency of optical techniques by using tissue optical clearing (TOC), which allows one to control tissue optical properties by exposing tissue to biocompatible liquids - optical clearing agents (OCAs). When exposed to an OCA with hyperosmotic properties, such as glycerol or highly concentrated glucose solution, it is possible to achieve a considerable decrease of tissue scattering within the entire spectral range and absorption on the water bands due to refractive index matching phenomenon and dehydration of the tissue, and therefore to get a significant increase of the optical and THz wave transmittance [1-3]. In vitro studies were performed on laboratory rats of the Vistar strain (sexually mature females weighing 300–400 g were used). The animals were inoculated with the tumor cells by subcutaneous injection into the area of the scapulae. Studies were conducted in 28 days after injection. The weight and thickness of the samples were measured before and after optical clearing by using a micrometer with an accuracy of ±0.001 mm and an analytical balance (Scientech, SA210, USA) with an accuracy of ±1 mg. The samples of the area 1.5*1.5 cm2 were placed in a closed sealed container, to which was added 2 ml of OCA. 70% glycerol solution, 40% glucose solution and 50% fructose in a 30% alcohol solution were used with the addition of saline as an OCA. The transmission spectrum of glycerol in the frequency range of 0.05–1.0 THz was measured by using the terahertz (THz) time-domain spectrometer. Measurements of the refractive index of OCA were carried out before the start of the experiment and every 5 minutes after placing the skin sample in the solution with OCA for 15 minutes, then every 10 minutes for another 45 minutes. It was found that the effectiveness of optical clearing of the skin over the tumor is higher compared to healthy skin. A good correlation was shown between the kinetics of the sample mass and THz absorption and refraction. An assessment of skin dehydration also showed that the concentration of water extracted from the skin above the tumor is greater compared to healthy. It is especially important that enhancing the image contrast due to dehydration and TOC in the pathology area is extremely significant in diagnosing precancerous conditions and treating early stages of tumor, due to the possibility of selective increase of the scattering contrast of cell nuclei relative to surrounding cytoplasm and interstitial fluid and thus following their size and internal rearrangements [5].
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