Airport security staff can spend their entire day staring at two-dimensional, static X-ray scans. Soon, however, they may be able to interact with these images, rotating a scanned object on the screen and even analysing its chemical composition.
It can be difficult to identify objects from the 2D images generated by X-ray scanners, says Paul Evans, head of the Imaging Science Group at Nottingham Trent University, UK. And while the latest X-ray scanners can glean information about the atomic or molecular weight of a substance, and so help distinguish between materials, the results are crude. The best they can manage is to show metal objects in one colour, organic materials in another and everything else in a third colour.
On an X-ray image, a lump of gorgonzola cheese inside a suitcase "looks identical to TNT," says Keith Rogers of Cranfield University in Shrivenham, Wiltshire, UK. With funding from the US Department of Homeland Security and the British Home Office, he, Evans and Anthony Dicken, also at Cranfield, have been developing ways to get around these limitations.
One established approach is to capture X-ray scans that give a sense of depth. 3DX-Ray, a company based in Loughborough, UK, has been selling stereoscopic X-ray machines for the past decade. To use them, security staff wear polarised spectacles, similar to those used in 3D movie screenings, which help the brain interpret two scans captured from slightly different viewpoints as a single 3D image. Tests by the US Transport Security Administration (TSA) have shown that even the limited depth information available from these scans significantly increases the probability of identifying a suspect object.
Evans says he can extract much more depth information as an object passes through a security scanner. His technique - called kinetic depth effect X-ray imaging, or KDEX - builds up a 3D image of the object which can be rotated and viewed from a wide range of angles.
In a regular airport security scanner, the X-ray source sits beneath the conveyor belt, with a line of detectors above. KDEX uses six or seven sets of these detectors. "We take snapshots of the object at different relative angles," says Evans, each shot contributing towards a 3D image. Importantly, the technique still requires only one X-ray source - which helps to keep its cost down.
Nick Fox, chief technical officer of 3DX-Ray, says KDEX offers some clear advantages. "It's a very powerful way of getting 3D information," he says. A gun or knife might be identifiable from any angle, he adds, but improvised explosive devices are trickier to recognise, so any extra visual information is welcome.
Explosive devices can be harder to recognise in a scan, so any extra depth information helps.
But while KDEX may help in recognising objects, it does not ease the task of detecting or distinguishing between materials. Yet, as Rogers points out, X-rays are routinely used for this kind of analysis. The standard technique is X-ray crystallography, which relies on the diffraction pattern produced when an X-ray beam scatters off a crystalline substance. Comparing the pattern against reference images for known materials allows a range of substances to be identified.
At first sight the technique appears to have little in common with security scanners, which build up an image by measuring how X-rays are absorbed by an object. But Rogers and Dicken have used KDEX to do both tasks simultaneously. Because only one set of detectors is measuring absorption data at any one time, the other detectors can be used to detect diffraction patterns instead.
Presenting their findings at the annual Denver X-ray Conference in Colorado last week, the team showed that they could detect aluminium and aluminium oxide and tell which was which. That's just a start, says Rogers. Because each material has a unique diffraction signature, the technique could be used by customs inspectors to detect consignments of fake pharmaceuticals.
Fox says that people have long been trying to deploy X-ray diffraction in security tasks, but their efforts have been stymied by the costs involved, the slowness of the machines and by the fact that the diffraction signals are weak and hard to capture. At the Denver conference Rogers and his team presented a new way to tackle this last problem without increasing the intensity of the X-rays or resorting to the large, expensive X-ray sources commonly used in diffraction analysis.
Dubbed focal construct geometry, their technique involves sending the source beam through an opaque mask with holes in a ring pattern, so generating hundreds of narrow X-ray beams. Each beam will produce a conical pattern when it scatters off the material of interest. With hundreds of these beams hitting the target, the scattered cones will intersect. Arranging the beams so that these crossover points coincide with the detectors will effectively boost the signal (Journal of Applied Crystallography, vol.43, p.264).
Evans is hopeful that the new techniques, though still in development, are the breakthrough that the security industry has been waiting for. They require no complicated moving beams or detectors and involve doing nothing to a bag beyond putting it on a regular conveyor belt, he says.
It remains to be seen whether airports will be enthusiastic. Despite the favourable TSA tests, no airport has yet adopted the 10-year-old 3DX-Ray technology. Its only use has been in situations where an exceptional level of security is required, such as checks on people entering VIP areas at the Beijing Olympics. "Security in airports is a very price-sensitive issue," says Fox. To be successful, new technology must be available at a price that the airports are willing to pay.
Evans is optimistic. Two US companies have started building KDEX prototypes, which could help drive down the cost of next-generation X-ray security equipment.
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