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Making waves in cancer detection

July 23, 2002

We`re all familiar with X-rays being used to look inside our bodies. But according to physicists, medical imaging in the future is likely to be based on an as yet unused type of radiation known as terahertz radiation.

At the 26th International Conference on the Physics of Semiconductors in Edinburgh on Thursday 1 August, Dr David Whittaker from the University of Sheffield will describe how he, Dr Michael Johnston and co-workers at the University of Cambridge, produce short pulses of this terahertz radiation from a piece of semiconducting material. Their pulses are more powerful than any generated by the same method so far, and increase the chances of a commercially viable imaging system - such as a detection system for skin cancer - being developed.

Terahertz radiation - also known as T-rays - is part of the broad spectrum of electromagnetic radiation that also includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All electromagnetic radiation travels in waves, and radio waves have the longest wavelengths (the distance between two crests of these waves) in the spectrum, while gamma rays have the shortest. T-rays lie within the infrared region of the spectrum, close to the microwave region, and have a wavelength of about one tenth of a millimetre. This makes them ideal candidates for certain types of medical imaging as they are too large to scatter randomly off cell walls, but small enough to provide images with useful resolution.

"One company, TeraView Ltd in the UK, is using terahertz radiation to look at the various layers in skin. It allows you to see the boundaries between the layers, and the thickness of the layers" says Dr Whittaker. This is because water absorbs terahertz radiation, and since each layer of our skin contains a different amount of moisture, the different layers will absorb a different percentage of the T-rays shone at them.

The boundaries between the layers also reflect some T-rays. So by measuring the time taken for a T-ray pulse to be reflected back from a boundary beneath a layer of skin, the thickness of that layer can be determined. "The hope is that you could produce images of skin tumours and distinguish the benign ones from the malignant ones by differences in layer thickness" explains Dr Whittaker.

The Cambridge/Sheffield team produce their T-ray pulses by shining extremely short pulses of laser light at the surface of a semiconductor. The energy in each laser pulse is enough to move electrons (sub-atomic particles) from their normal positions inside the semiconductor. The spaces the displaced electrons leave behind are known as `holes`, and in semiconductors the steady movement of either electrons or holes - which both carry electric charge - causes an electric current to flow. Accelerating electric charges emit electromagnetic radiation, and in this case the electrons and holes begin accelerating, and create a small electric current and a pulse of terahertz radiation before they start slowing down again.

By placing a specially designed prism onto the semiconductor surface, the researchers have managed to produce the strongest pulses yet via such a technique. This method lines up the electrons and holes in such a way that around 20 times more of the terahertz radiation they emit leaves the surface of the semiconductor than in other systems, where almost all of it remains trapped inside.

Less powerful T-ray sources are already being used in trials to test the viability of a skin cancer detection system. "Hopefully the sources we`re producing will be more effective" says Dr Whittaker. "For any imaging application, the time taken to acquire the image decreases as you increase the power of your source, and if you can get a good signal more quickly, you can scan more quickly" he adds.

Not only could such a method of medical imaging be quick, it should also be much safer than X-rays. This is because unlike X-rays, T-rays don`t contain enough energy to remove electrons from their parent atoms in cells. This is important, as it is this removal of electrons that makes cells more likely to mutate. The relative safety of terahertz radiation means similar systems could be used for scanning baggage at airports, or by the cosmetics industry for measuring the effectiveness of moisturisers. And by using terahertz radiation in the same way that radio waves are used to carry signals to and from our mobile phones, a new method of wireless communication could even be created.

"The idea is to use it for free space networks within buildings for faster communication between computers" explains Dr Whittaker. Each computer in a network would have a transmitter and a receiver, and files and e-mails would be beamed around the room via T-rays, removing the need for a tangle of wires connecting the machines. "Because it is absorbed by walls and by water in the atmosphere, terahertz radiation gives you security and also provides isolation between different buildings" adds Whittaker.

Institute of Physics