Perturbed Resonance Frequency Analysis for Imaging Epidermal Thickness and Water Content Using Terahertz Spectroscopy

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In this study, we propose a novel perturbed resonance frequency analysis method where the modified, THz frequency, Fabry–P´ erot resonances of a tissue loaded quartz window are used to quantify superficial skin layer thickness and water content. The superficial layer is treated as an unknown matching layer between the quartz window and deep epidermis and resolution of the longitudinal modes provides a unique mapping from resonance frequency and width to layer water content and thickness. The approach is experimentally validated with a telecentric beam scanning system coupled to VNA extender operating in the 330– 500 GHz band. The system is used to observe the dynamic effects of stinging nettle exposure on in vivo human skin over a 52 x 52 mm2 field-of-view (FOV). Spectroscopic imaging revealed an immediate decrease in resonance frequency and width of up to 15 GHz and 20 GHz, respectively, compared to unperturbed skin, which maps to a 30μm increase in thickness and 25% decrease in water content. This immediate response was followed by a gradual recovery to pre-exposed values. These findings demonstrate additional dielectric window utility in terahertz biomedical spectroscopy in imaging beyond the standard field flattening and system calibration roles.