Innovation: Gas scans could end lung X-rays

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AN ADVANCE in medical imaging should allow diagnosis of lung tumours without the need for an X-ray scan.

The safe and simple technique provides images of the gas space in lungs, and of the lung tissue itself. The image shows any variations in the nature of the tissue, such as that caused by a tumour.

Lung cancer causes nearly 40,000 deaths each year in the UK. At present, most cancerous lung tumours are identified from X-ray images but this carries its own hazard because the high energy rays can be harmful - particularly when heavy doses are needed to discriminate between healthy and tumorous lung tissue.

When the new technique is commercialised it may offer the prospect of mass screening for lung cancer. The new technique developed by Dr Arnold Wishnia and colleagues at the Stony Brook campus of the State University of New York is a variant of magnetic resonance imaging (MRI), a safer method than X- rays for detecting tumours which has been in use since the early 1980s. But the effectiveness of MRI varies according to the organs being scanned - the lungs being particularly hard to scan.

In standard MRI, images are generated by placing the body in a magnetic field and stimulating its water molecules with radio waves. The nuclei of the two hydrogen atoms in a water molecule consist of lone subatomic particles called protons that can be regarded as tiny magnets. Each of these nuclear magnets acts like a compass needle, lining up its magnetic poles with those of the field. When exposed to radio waves, the protons absorb some of the energy of the waves, which shows up as a drop in the intensity of the radio signal passing through the tissue. The variation provides a map of the water content of the tissue.

In the lungs, however, these variations are not great enough to provide a contrast between tissue types. Instead of using protons, Dr Wishnia's method uses atoms of the gas xenon. The nucleus of the xenon atom has similar magnetic properties to protons and can also absorb radio waves in a magnetic field. But whereas the body is replete with water, the xenon must be introduced artificially. This is safe and easy, because the gas can be inhaled in a mixture with air; xenon is an extremely non- reactive gas and so has no harmful effects. Indeed, it is already used as an anaesthetic.

When inhaled into the lungs, the xenon provides a magnetic resonance image of the gas lung space, which may be useful for studying respiration. The xenon finds its way into the lung lining, allowing the tissues to be imaged. It is then gradually distributed through the body, allowing other parts to be scanned.

Dr Wishnia says xenon nuclei respond to the environment in different ways from protons, so the contrast between tissue types may in some cases be better than in proton MRI.