Dec 2002 Journal

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The nucleus in perspective

The nucleus has had a bad press recently. In speculations as to whether Iraq again possesses weapons of mass destruction, the most feared have been nuclear devices. Closer to home, when British Energy recently ran into financial difficulties, we were reminded that this company generates its electricity in nuclear power stations using uranium as fuel. When the company was privatised in 1996 it was deliberately not called British Nuclear Power for fear that nobody would want to buy shares in an organisation labelled with what The Times termed ‘the dreaded N-word’.

By themselves, of course, nuclei are neither good nor bad. Every atom has a nucleus at its centre, and the whole world (including ourselves) is made up of atoms. Nuclei consist of a mixture of protons (positively charged) and neutrons (uncharged). Almost all nuclei are completely stable. Very large ones, however, are unstable and give rise to natural radioactivity, as in the radium discovered by Marie and Pierre Curie. Some other nuclei can be made unstable and may produce radioactivity by bombarding them with free neutrons. In some of these processes huge amounts of energy can be generated. This forms the basis of the nuclear power industry (and of the nuclear bomb dropped on Hiroshima). Nowadays radioactive materials have many beneficial applications in sterilisation processes and in radiation therapy for cancer patients.

A modern non-invasive medical technique depends on the smallest nucleus of all, that of the hydrogen atom which consists of just one proton. Hydrogen exists throughout our body, most commonly in the form of water (H2O). There are thus many H atoms in fluids like blood and urine, but fewer in muscle and still fewer in bone. Lipids such as fat also contain many hydrogen atoms. These can act as tiny magnets. In the hospital the patient is asked to lie on a bed surrounded by a large electro-magnet, and the hydrogen nuclei inside the body tissues then align themselves with the magnetic field. A radiofrequency transmitter is then switched on as well, to generate a second magnetic field which makes the hydrogen nuclei flip, resonating in harmony with the radio waves. The signals resulting from this nuclear magnetic resonance technique provide information on the concentration and environment of hydrogen nuclei in different parts of the patient’s body. This can enable healthy and diseased tissue to be distinguished. When the direction of the magnetic field is varied, a three-dimensional image is obtained and computerised. Medical people term this useful diagnostic tool MRI or magnetic resonance imaging (so omitting the word ‘nuclear’ which might upset patients). Radiologists, however, maintain that NMR stands for No More Radiologists while MRI means More Radiologists Immediately!
Michael Spiro

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