T-ray vision

US researchers have developed an imaging system that can map the inside of an object in 3D.

A terahertz or T-ray imaging system that can map the inside of an object in 3D has been developed in the US.

The researchers hope to expand the small prototype set-up to a size feasible for security roles, such as airport luggage scanning.

As X-ray machines only create images from one perspective, some of the contents of a suitcase can remain hidden. Being able to scan in 3D could enable security staff to spot items concealed among other contents.

Having come to prominence in the mid-1990s, T-ray imaging is still in its infancy but even in two dimensions has significant advantages.

Unlike other techniques, such as X-rays or ultrasound, which give only 2D light-and-dark images, the equipment analyses the spectroscopic signature of a material produced by the T-rays. This information is used to produce a coloured image to indicate the presence of certain materials within an object. It will allow security staff to know exactly what an object is made of and could be useful for detecting bombs or bio-terrorism devices.

The system is also capable of extremely high resolution, while the rays are non-ionising and therefore safer than X-rays.

Researchers at the Rensselaer Polytechnic Institute in New York said they could already extend T-ray imaging into 3D by adopting similar principles to an X-ray CAT scanner.

One of the problems with this had been the long time taken to form the image, but professor of electrical, computer and systems engineering Xi-Cheng Zhang has resolved this difficulty. Conventional T-ray imagers work by bombarding materials with pulses of electromagnetic radiation of certain frequencies (around a trillion cycles per second), and recording the results on special detectors. The electromagnetic radiation is generated for this system by hitting a medium such as gallium arsenide with rapid pulses of near-infrared laser light.

However, whereas other devices use a refractive lens, the Rensselaer system has a Fresnel or binary lens. Fresnel lenses are more often associated with lighthouses or spotlights but their properties allowed the engineers to make their focal length proportional to the T-ray wavelengths they wanted to emit.

Exploiting the Fresnel lens, the scanning device can simultaneously emit several sets of T-rays at different frequencies. After the T-rays have bounced through the material they are collected again using a binary lens and an electro-optical detector crystal.

The multiple images of the inside of the object are then reconstructed via computer algorithms to create an image of the whole 3D profile, combining the results from each different frequency of T-ray. Zhang said that emitting and measuring the T-rays simultaneously ‘dramatically accelerates the imaging speed’.

He sees other likely applications in non-destructive mail contents inspection, semiconductor testing, manufacturing quality control and biomedicine.

The US government’s Defence Advanced Research Projects Agency is understood to be about to offer support.

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