Highly Accurate in Vivo Terahertz Spectroscopy of Healthy Skin: Variation of Refractive Index and Absorption Coefficient Along the Human Body

Abstract

A method to reconstruct the terahertz (THz) refractive index and absorption coefficient of in vivo tissue using THz pulsed spectroscopy (TPS) has been proposed. The method utilizes a reference THz window to fix the sample of interest during the TPS reflection mode measurements. Satellite pulses caused by multiple THz-wave reflections in the reference window are taken into account to accurately solve the inverse problem. The stability of the proposed method in the presence of various factors, including digital noise in the TPS waveforms and fluctuations of the reference THz window position, has been accurately analyzed. The method has been implemented to study in vivo the THz refractive index and absorption coefficient of the human skin. The skin from three persons has been measured, and the results agree with the well-known data on healthy skin spectroscopy in general, except for several regions of the skin. Thus, for the elbow, the hand, the knee, and the heel the THz refractive index and absorption coefficient considerably differ from the average values. The observed results are of principle importance for further development of novel approaches to skin diagnosis based on THz technologies.Index Terms-Absorption coefficient, inverse problem, material parameters, refractive index, spectroscopy of the skin, terahertz pulsed spectroscopy, THz technology.

Recent advances in terahertz technology for biomedical applications

“…In extracting these accurate values for different regions, Zaytsev et al proposed a highly accurate algorithm, considering errors from both imaging window aberrations and digital noise (25). Thus, any technique to measure skin hydration level such as the diabetic foot syndrome study presented earlier (22) needs to be carefully calibrated.…”

“…For example, normal gastrointestinal (GI) tract tissues from rats were recently measured using THz reflection imaging and spectroscopy ex vivo (25). The distinctively low water concentration in the stratified squamous epithelia (SSE) results in a different reflected THz signal and spectrum when compared to those of other GI tract tissues, again highlighting the importance of water concentration in THz contrast.…”

An empirical formula for temperature adjustment of complex permittivity of human skin in the terahertz frequencies

“…The common standard for live human skin studies is the use of the thin skin on the palmar aspect of the forearm, the “volar forearm” (e.g., Pennes [], Sivamani et al []). As there are only 3 in‐vivo data points available in THz studies derived unequivocally from the “volar forearm” [Pickwell et al, ; Echchgadda et al, ; Zaytsev et al, ], it is impractical to isolate the study to this area of the body. The low number of points necessitates the use of data from all sites reported as “thin skin.” This results in a greater variability of temperature.…”

Review of GaN-based devices for terahertz operation

“…15 THz-TDS systems are powerful tools for material spectroscopy, layer inspection, and transmission imaging of packaged objects. 16 These capabilities of THz-TDS systems are utilized in authentication, [17][18][19] nondestructive inspection of composite materials, [20][21][22][23][24][25][26][27][28] threedimensional imaging, [29][30][31][32] metrology and quality control of industrial products, [33][34][35][36][37] detection of concealed weapons, [38][39][40][41][42][43][44][45] art investigations, 46,47 tomography, [48][49][50][51][52] biomedical diagnosis, [53][54][55][56] material characterization, [57][58][59][60][61][62] thickness measurement, …”