Collaborating scientists from the University of Warwick and the Chinese University of Hong Kong (CUHK) have demonstrated a method that uses terahertz radiation to improve the diagnosis of skin conditions such as eczema, psoriasis, and skin cancer. Terahertz radiation (T-rays) falls between infrared and Wi-Fi on the electromagnetic spectrum. T-rays are able to pass through common materials such as plastics, ceramics, and clothes nondestructively, making them an attractive prospect for noninvasive inspections. Because the low-energy photons of T-rays are nonionizing, they are safe in biological settings. A demonstration of how the T-ray equipment can be used to scan an individual’s skin. Courtesy of University of Warwick. In terms of medical imaging, only the T-rays passing through the outer layers of skin (stratum corneum and epidermis) before being reflected back can be detected, as those traveling deeper are too attenuated for detection. T-rays, therefore, have the potential to be an effective route to monitoring those outer layers. To test this, a prism is used to focus terahertz light onto the skin aligned in a particular focal plane. Depending on the properties of the skin, the light is reflected back slightly different. Scientists can then compare the properties of the light before and after it enters the skin. To overcome certain limits of THz reflection spectroscopy, the researchers used ellipsometry, which involves focusing T-rays at multiple angles on the same area of skin. The researchers showed that by using ellipsometry they could accurately calculate the refractive index of skin measured in two directions at right angles to one another. The difference between the refractive indices is called birefringence, and it is able to provide information on how much water is in the skin, and enables the thickness of the skin 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 Emma Pickwell-MacPherson, a professor in the Department of Physics at the University of Warwick and the Department of Electronic Engineering at CUHK. “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.” Hydrated skin, she said, has a different refractive index than dehydrated skin. The new technology better quantifies the extent of skin’s hydration than do existing techniques. “If you’re trying to improve skin care products for people with conditions like eczema or psoriasis, we would be potentially able to make quantitative assessments of how the skin is improving with different products or to differentiate types of skin,” Pickwell-MacPherson added. To test their method, the researchers had volunteers place their arm on the imaging window of the T-ray equipment for 30 minutes, after first acclimating to the ambient temperature and dryness of the laboratory. By holding their skin against the surface of the imaging window, they blocked water from escaping from their skin as perspiration, a process known as occlusion. The researchers then made four measurements at right angles to each other every two minutes over half an hour, so they could monitor the effect of occlusion over time. Due to the sensitivity of T-rays to water, the researchers were able to observe a noticeable difference as water accumulated in the skin, suggesting that the method could be used to evaluate moisturizing skin care products. “We don’t have anything that’s really accurate for measuring skin that clinicians can use. Dermatologists need better quantitative tools to use, and use easily,” Pickwell-MacPherson said. “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. We could get more tailored medicine and develop products for different skin responses.” Future research will focus on improving instrumentation and how it might work as a practical device. The research was published in Advanced Photonics Research (www.doi.org/10.1002/adpr.202000024)