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Terahertz spectroscopy probes cellular structure of skin

01 Dec 2020

University of Warwick and Chinese University of Hong Kong project could assist in skin cancer diagnosis.

Terahertz radiation, falling between the infrared and microwave regions of the spectrum, is attractive for in vivo applications due to its non-invasive and non-ionizing nature.

The limited penetration depths of THz radiation is thought to make it particularly suitable for analysis of the skin, diagnosing burns, scars and cancers.

However the complicated nature of living systems has to date presented a challenge, preventing current THz platforms from obtaining the accurate reflections from target tissues needed to build up images of the skin.

A project at the University of Warwick and the Chinese University of Hong Kong (CUHK) has now developed a THz platform intended to significantly enhance the characterization capabilities of THz spectroscopy, and published its study in Advanced Photonics Research.

The breakthrough involves a novel ellipsometry technique, providing mutliple complementary sets of spectral ratios and significantly boosting the performance of the technique.

A basic ellipsometry approach involves calculating the refractive index of target tissues measured in two directions at right angles to each other. The difference between these refractive indices is termed birefringence, and this is the first time that the THz birefringence of human skin has been measured in vivo according to the project. These properties can provide valuable information on how much water is in the skin and enable the skin thickness to be calculated.

"We wanted to show that we could do in vivo ellipsometry measurements in human skin and calculate the properties of skin accurately," said project leader Emma Pickwell-MacPherson of CUHK's terahertz research group.

"In ordinary terahertz reflection imaging, you have thickness and refractive index combined as one parameter. By taking measurements at multiple angles you can separate the two."

Tailored medicine from THz spectroscopy

The project's experimental platform employed a double-prism architecture mounted on a motorized stage, to provide two alternative optical paths and effectively allow four complementary sets of spectral ratios to be collected from the target.

After initial trials on a model of skin and its outer layer, or stratum corneum (SC), the project applied its platform to the forearms of five human volunteers, and found that the properties of the SC components and the epidermis could be computationally extracted from the spectral data using an algorithm.

The THz dispersion and birefringence sensitivity parameters are effectively probes for the level of hydration and the cellular inhomogeneity in the skin, according to the project, producing results in good agreement with microscope images and the observed biological processes taking place in the SC layer.

A THz platform capable of quantitatively assessing the condition of skin could be useful in clinical scenarios for the monitoring of skin cancer, or to assess the effectiveness of medications and moisturizers. The inherent sensitivity to water molecules could also allow the technique to detect areas of skin where the water circulation is different from the surrounding areas, potentially an early sign of problems.

"If this works well you could go into a clinic, put your arm on a scanner, your occlusion curve would be plotted and a suitable product for your skin could be recommended," commented Emma Pickwell-MacPherson. "We could get more tailored medicine and develop products for different skin responses. It could really fit in with the current focus on tailored medicine.”

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