Imaging interiors

IMAGING technology has come a long way since W Roentgen discovered x- rays in 1895. A variety of techniques, using a range of the electromagnetic spectrum, has been used to study biological tissues as well as materials. And imaging technologies such as Nuclear electromagnetic spectrum is Magnetic Resonance and Positron Emission Tomography are being routinely used in hospitals and laboratories.

One part of the electromagnetic spectrum which was hitherto unexplored for imaging was the terahertz - million million hertz (cycles per second) - region. Now, researchers at the AT&T Bell Labs at Holmdel have designed a setup that uses waves at a frequency of 0.1 to 3 terah@rtz to probe materials (Science, Vol @68, No 5218).

Terahertz 'waves are well-suited for imaging becapse different materials absorb and change the shape of the incoming wave ,in a characteristic way. These signatures can then be used to reconstruct an image and the chemical composition of the material under investigation.

Terahertz (or T-ray) imaging was not possible before because the technology available was limited by signal strength and detector efficiency, which meant that the recording of enough information to construct an image took several days. Now, by boosting the power of signal generators and improving signal processing methods, the new technique has reduced this to a few minutes.

The technique to generate the rays consists of firing infrared laser pulses into a dipole antenna which then reradiates the required T-rays. These are then focussed onto the sample by using lenses and the rays emerging from the sample are then collected to be processed by a computer. The information from these pulses is analysed by the computer (much like it is done in a computerised axial tomography or CAT scanner) and the image produced.

Though the use Of T-ray imaging for human scanning is not practical becauh only millimetre thickness samples can be imaged, it still has a variety of applications in studying materials. It can be used to analyse gases because many gases are strong absorbers Of X-rays and even trace quantities of these gases can be detected. This has potential applications in environmental monitoring.

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