Application of terahertz (THz) spectroscopy for biological tissues is strongly limited by the extremely low penetration depth due to THz absorption by tissue water. One of the possible solution of such problem is the usage of THz wave penetration-enhancing agents (PEA) for optical clearing of tissues. In the present paper, the transmission-mode THz spectroscopy of a set of PEAs (polyethylene glycol with different molecular weight, propylene glycol, ethylene glycol, and dimethyl sulfoxide) was performed in order to reconstruct their dielectric properties and compare them with that of water. The obtained results emphasize the feasibility of using PEG to enhance the depth of THz wave penetration into tissues.
Terahertz (THz) pulsed spectroscopy is a convenient instrument for studying the THz dielectric response of healthy and abnormal tissue in a wide spectral range. One of the most promising applications of THz pulsed spectroscopy is associated with non-invasive, least-invasive and intraoperative medical diagnostics of malignancies in various localizations, including the skin, the breast, the colon, and the brain .
In our research, we developed a method for reconstructing the THz dielectric response of biotissues in vitro and in vivo using the THz pulsed spectroscopy [2–5]. We applied this method for studying healthy and pathological tissues of the skin and the brain.
(i) We observed statistical differences between THz dielectric properties of ordinary and dysplastic nevi of the skin in vivo. This highlights an ability for non-invasive early diagnosis of dysplastic nevi and melanomas of the skin using the THz spectroscopy and imaging [3–5].
(ii) By studying the THz dielectric permittivity of non-melanoma skin cancers in vitro (i.e. basal cell carcinoma and squamous cell carcinoma), we justify an ability for discriminating malignant tissues from surrounding normal skin using preoperative and intraoperative THz imaging [6,7].
(iii) Finally, the results of measuring the THz dielectric response of gelatin-fixed malignancies of the brain in vitro allow us to analyze an ability for discriminating brain gliomas from surrounding normal tissues during the neurosurgery using the THz technologies.
The observed results of THz measurements agrees well with the data of biotissues studying using other modern modalities of optical imaging, such as intraoperative exogenous fluorescence imaging and optical coherence tomography, as well as with the data of biotissue histology. These results highlight the prospective of THz spectroscopy, imaging and endoscopy use for non-invasive, least-invasive and intraoperative medical diagnosis of malignancies.
 O.A. Smolyanskaya,·M.M. Nazarov,·O.P. Cherkasova,·J.-P. Guillet,·J.-L. Coutaz, A.A. Konovko, Y.V.Kistenev,·P. Mounaix, I.A. Ozheredov, V.L. Vaks, A. Yaroslavsky,·N.V. Chernomyrdin, K.I. Zaytsev, S.A. Kozlov,·J.-H. Son, V. Wallace,·A.P. Shkurinov, ·V.V. Tuchin, “Terahertz biophotonics as a tool for studies of dielectricand spectral properties of tissues and bioliquids relatedto water content,” Progress in Quantum Electronics (2017, Submitted).
 IEEE Transactions on Terahertz Science and Technology 5(5), 817 (2015).
 Applied Physics Letters 106(5), 053702 (2015)
 European Journal of Cancer 51, S167 (2015).
 Optics and Spectroscopy 119(3), 404 (2015).
 Journal of Physics: Conference Series 486(1), 012014 (2014).
 Journal of Physics: Conference Series 584(1), 012023 (2015).
We have developed a method of terahertz (THz) solid immersion microscopy for imaging of biological objects and tissues. It relies on the solid immersion lens (SIL) employing the THz beam focusing into the evanescent-field volume and allowing strong reduction in the dimensions of the THz beam caustic. By solving the problems of the sample handling at the focal plane and raster scanning of its surface with the focused THz beam, the THz SIL microscopy has been adapted for imaging of soft tissues. We have assembled an experimental setup based on a backward-wave oscillator, as a continuous-wave source operating at the wavelength of λ = 500 μm, and a Golay cell, as a detector of the THz wave intensity. By imaging of the razor blade, we have demonstrated advanced 0.2λ-resolution of the proposed THz SIL configuration. Using the experimental setup, we have performed THz imaging of a mint leaf revealing its sub-wavelength features. The observed results highlight a potential of the THz SIL microscopy in biomedical applications of THz science and technology.
We have performed the <i>in vitro</i> terahertz (THz) spectroscopy of human brain tumors. In order to fix tissues for the THz measurements, we have applied the gelatin embedding. It allows for preserving tissues from hydration/dehydration and sustaining their THz response similar to that of the freshly-excised tissues for a long time after resection. We have assembled an experimental setup for the reflection-mode measurements of human brain tissues based on the THz pulsed spectrometer. We have used this setup to study <i>in vitro</i> the refractive index and the amplitude absorption coefficient of 2 samples of malignant glioma (grade IV), 1 sample of meningioma (grade I), and samples of intact tissues. We have observed significant differences between the THz responses of normal and pathological tissues of the brain. The results of this paper highlight the potential of the THz technology in the intraoperative neurodiagnosis of tumors relying on the endogenous labels of tumorous tissues.