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NMR Electromagnet
From: Science Museum
| By:
Anthony Wilson |
EDITOR'S INTRODUCTION |
The methods of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are taken for granted in laboratories and hospitals today. They offer a non-invasive way of understanding the structure of molecules or the condition of human tissue. This feature looks at the development of the techniques, from a small lab experiment to extensive commercial exploitation. |
 s its cumbersome name suggests, nuclear magnetic resonance (NMR) is a complex technique. Its principle is straightforward however: it allows scientists to investigate the atoms inside a spectrum. The process involves 'interrogating' the atoms by broadcasting radio waves of a precisely defined frequency from a transmitter close to the specimen, and detecting and decoding the returned transmissions -- the 'answers' -- that these atoms give. Different groups of atoms in the specimen respond to particular frequencies in this radio transmission in a manner similar to the well-known example of the wine glass responding to a particular note sung by a high-powered soprano. The response that the atoms give comes from their nuclei. These themselves can become radio transmitters, emitting a signal that can be picked up by a radio aerial mounted alongside. Precise measurement of the frequency and behaviour of this signal tells the experimenter what sort of atoms are present in the specimen, and in what numbers. More important, it can yield information about how those atoms are joined to others in a chemical compound, what other atoms are nearby, and even about the chemical reactions that are going on inside the specimen when the experiment is done. |
For the technique to work, the atomic nuclei in the specimen must themselves be tiny magnets, as many nuclei are. These magnetic nuclei must also be lined up to point in particular directions, rather than lying in random orientations as they normally do. Alignment of the nuclei is done by exposing them to the field of a powerful electromagnet. Within the magnetic field produced the magnetic nuclei line up rather like compass needles aligning themselves north-south under the influence of the Earth's magnetism. |
The NMR apparatus in the Science Museum (see image above) from which this magnet comes was built by Rex (later Sir Rex) Richards (b. 1922) and colleagues in an Oxford chemistry laboratory in the early 1950s. More than thirty kilometres of wire went into the hand-wound coils of the electromagnet. With its massive cast-iron yoke, it was the first magnet in Britain constructed specially for this new technique. |
To investigate the shape and structure of molecules, a liquid sample in a glass phial was placed in the narrow gap between the magnet's mirror-flat poles, together with the coils for transmitting and detecting radio waves. Modern Laboratory NMR machines look very different: smart anonymous boxes which can be bought off the shelf. Computer aided to give instant results, and yielding much more detailed information than in the early days, they are among the most powerful tools available to researchers in chemistry and biochemistry. |
Twenty years after Richards' pioneering work, Peter Mansfield (b. 1933) and his partners at Nottingham University were building an electromagnet of different design. With water-cooled coils more than half a metre across (but no iron core), it was designed to apply the techniques of NMR to a very much larger specimen: the living human body. |
Because water contains hydrogen atoms and hydrogen nuclei are magnetic, NMR can be used to detect water. And since the human body is two-thirds water, NMR can provide a non-invasive method of finding out about conditions inside the body. Different types of tissue contain different amounts of water, and diseased tissue differs from tissue that is healthy; NMR equipment can 'focus on' different areas within the body and give information about the type and condition of the tissue there. |
Mansfield's first apparatus was the prototype for a novel technique of medical diagnosis, magnetic resonance imaging (MRI). By 1980 his laboratory had developed an MRI system which could 'look at' different areas within the human skull, building up an image which showed a cross-sectional slice through the patient's head. Ten years later, whole-body MRI machines had been installed in many hospitals around the world, often giving doctors a sharper and more informative picture of what was going on inside their patients than the X-ray CT scanner can, and seemly without the health risks that come with X-rays. |
But the story has a less happy ending for the country where Richards and Mansfield did their work. In 1988 it was said that Britain had fewer MRI machines in its hospitals than any other country in the developed world. Not for the first time commercial exploitation of high-quality research seemed to have been left to overseas companies. British industry -- and British patients -- were losing out. |
Increasingly for health managers world-wide, the arrival on the market of the MRI machine revived a familiar dilemma. When funds are short, is it right to spend a large sum on a box of advanced technology wizardry -- highly beneficial, but only to comparatively few patients -- or is it better to invest those resources in more down-to-earth forms of health care? |
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