T-ray imaging gets under the skin

How do we know what’s happening just beneath our skin? It’s only a few millimetres thick, made up of several layers and peppered with glands, blood vessels, specialised cells and nerve endings. The very outermost layer – the stratum corneum – is made up of about 20 layers of cells and is the brick wall that protects your body. You can see if it’s dry and wrinkly or plump and hydrated, and doctors mostly rely on a trained eye, sometimes followed by biopsies, to diagnose skin diseases.

Scientists from the UK and Hong Kong have devised a new method for ‘looking’ at the make-up of our skin, which can reveal its structure and health with greater accuracy. Eventually, they believe, it could usher in new non-invasive methods for doctors to monitor skin conditions and tumours, the full extent of which are often hidden beneath the skin.

This requires the use of terahertz (THz) radiation or T-rays, which form a little-exploited part of the electromagnetic spectrum. With frequencies of 0.3-3.0THz and wavelengths of 100 micrometres to one millimetre, they sit between microwaves and infrared light, but are less well known than their neighbours.

“For the last century we’ve been diagnosing skin disease by looking – with years of training, intense magnification and bright lights,” says Dr Joseph Hardwicke, a plastic surgeon who focuses on skin cancers and who’s collaborated with physicists from the University of Warwick and the Chinese University of Hong Kong. “Terahertz radiation has the potential to give us objective measures of the skin. And this could move us towards more personalised treatments.”

This part of the electromagnetic spectrum had been very little researched or exploited until late last century, leading physicists to speak of the “terahertz gap”. While infrared and X-rays have already helped us see in the dark or deep into the human body, T-rays, which can penetrate outer layers of non-​conducting materials such as wood and plastics – and human skin, open up new possibilities.

Researchers have devised a method that fires T-rays from a range of different angles to create a richer picture of skin than current methods allow – a feat of engineering and physics. “It’s very fiddly,” says Professor Emma MacPherson in the Department of Physics at Warwick. “Mechanically clunky at the moment, but with elegant physics behind it.” For years, T-ray research has been hampered by difficulties in generating the radiation, although recent systems have become cheaper and neater.

It’s the first time, says MacPherson, that terahertz radiation has been used with this method and this level of detail to measure properties of skin on a living person. T-rays are sensitive to water and the chemical composition of an object, and respond differently to moisture levels in skin. Importantly they are low-energy and non-ionising, unlike the more powerful X-rays, which are more able to harm living tissue and which penetrate materials more deeply. “There’s also evidence that THz can distinguish clinically important differences between healthy and diseased soft tissue that X-ray can struggle with in terms of achieving good contrast in images,” says Dr Don Arnone, a physicist who pioneered terahertz imaging and spectroscopy before founding the terahertz analysis instrumentation company TeraView.

Using two prisms, the team perfected a technique that allowed them to focus terahertz pulses at a variety of angles onto the same area of skin. Depending on moisture levels and other skin properties, the rays will be reflected back slightly differently – allowing scientists to compare light properties before and after it enters the skin. To gain more information than standard reflection methods used in spectroscopy, the team deployed ellipsometry – focusing T-rays at right angles to each other in four separate positions. “Ellipsometry is used to study thin films, where spectroscopy is used to study bulk or larger substances,” says Arnone.

They were able to use these methods to calculate accurately the refractive index of skin in two directions at right angles to each other. This difference between the two refractive indices is called birefringence and this information, says MacPherson, reveals via calculation the moisture content and thickness of skin. “In ordinary T-ray reflection imaging, the skin’s thickness and refractive index combine as one parameter. By measuring at different angles, you can separate the two.”

During experiments, MacPherson and her colleagues had to overcome practical problems – skin accumulates moisture if it’s blocked against a surface – as the forearm was against the glass in her experiment. “That changes the refractive index of the skin, so we had to account for that.”

Part of the research success depended upon volunteers’ ability to keep their arms completely still during the half hour or so that the process required, and the team got around this with a glass platform and moveable prisms. “We needed perfect measurements,” says MacPherson. “We’ve proved that human skin does have birefringence in the terahertz zone – not all materials do – and that’s important.”

Medical imaging is just one application of this part of the electromagnetic spectrum.Terahertz light has been used in astrophysics to identify gas clouds between stars. It’s been used – controversially – in security systems to ‘see’ beneath clothing for metal objects, and by the pharmaceutical sector to measure the crystalline state of drugs in tablets. It’s used too in quality assurance in the automotive sector to assess paint coatings, and on jet engine turbines, and to analyse semiconductor chip failure and quality, says Arnone.  

In the art world, terahertz light has helped reveal the brushstrokes and hidden details of delicate artefacts and precious paintings. Recently researchers used it to determine the freshness of pork.  

Next steps for MacPherson and her colleagues are clear – and there’s some groundwork to cover. She’s applied for funding to develop a medical-grade robot that could map skin with great precision.

Clinical trials could begin in the near future, says Hardwicke. “The tech is there; it’s now a matter of design – and of course regulation. It works nicely in the lab, but will it work within the NHS?”

This technology could be helpful in detecting the progress of skin cancers, by revealing the extent of a tumour beneath the skin. This could ultimately speed up surgery and allow surgeons to prepare in advance for procedures such as skin grafts. “It would really save time,” says MacPherson.

It would be at least five years, says Hardwicke, until this could find its way into medical settings.

It could also allow medics to detect and develop the best treatment possible for skin conditions such as eczema and psoriasis, which at present is more of a trial and error approach. It could also cut development times for new treatments. “This technology is really good at noticing hydration levels of the skin,” says MacPherson. “It would really help with trying to treat people efficiently as well as monitor them.”