X-rays beyond the boundary

An imaging system with the potential to revolutionise the X-ray techniques used in medicine, biology, industry and security is to be built by researchers at University College, London.

They will create the equipment using a technique previously limited to highly precise synchrotron radiation facilities.

The technique could be used for the early diagnosis of breast tumours, detection of faint lung lesions in planar images instead of CT scans and imaging of blood vessels without contrast agents. It could also be used to detect microfractures and dilamination in composite materials, which is not always possible with X-ray imaging.

The five-year project, which has received more than £800,000 in funding from the EPSRC, will begin in October.

‘With a conventional X-ray, what you read on the detector are the photons that have not been absorbed by the material in their way,’ said Dr Robert Speller, professor of physics and head of the radiation group at UCL.

‘In an application such as mammography the difference between absorption by material that is normal and that which is not normal is very small — in the order of a few per cent.

‘Therefore, radiologists struggle to see small features in the breast. It is a very challenging area of work with well-documented limitations.’

Rather than measuring the degree of absorption, known as the attenuation coefficient, the XPCI (extremely high potential, X-ray phase contrast imaging) technique looks at the photons’ refractive index data; in other words, how much they are deviated by the material as they pass through.

In recent years, studies using a synchrotron radiation (SR) source have demonstrated that XPCI could substantially change the face of X-ray imaging.

However, as SR facilities use precise instruments it has proved hard to translate XPCI into something of use in a hospital setting.

‘When the photons pass through it changes the direction of the wavefront,’ said Dr Speller.

‘However, the amount by which the light is bent is very small. For instance, to measure a one millimetre change you would have to stand a kilometre away. This is why it has so far been hard to develop something that could be used in a clinical setting.’

The UCL team has devised a novel XPCI technique, based on the use of coded apertures, which solves most limitations of previous approaches.

Proof-of-concept experiments have shown it can provide results comparable with those obtained with SR using diverging, polychromatic beams generated by the off-the-shelf radiation sources commonly used in hospital X-ray departments.

‘The coded apertures can simply be described as sets of slits,’ said Dr Speller.

‘They are fixed in very precise positions. If the photons are deviated then the signal gets through and a cancer is present. If not, then they have not been deviated and there is no tumour.’

Work is now needed to optimise the position of the apertures to build a functioning system.

Project partners include Canon Europe, Barts and The London NHS Trust, E2V Technologies, X-Tek Systems and 3D X-ray, which will become involved once the prototype system is up and running.

Julia Pierce