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What are MRI scans?
A magnetic resonance imaging (MRI) scanner can build up detailed 3D pictures of organs inside the body, without using either radioactivity or X-rays. Instead, the patient lies within a huge magnet - as strong as the ones used to pick up cars in scrapyards. MRI cannot be used on people who have anything metal inside their bodies, such as a pacemaker.
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An MRI scanner.
How does MRI work?
MRI relies on the way that hydrogen atoms, which make up nearly two-thirds of the human body, absorb and then give off magnetic energy at radio frequencies. The nuclei of these hydrogen atoms are like tiny spinning magnets, and so they respond to changes in magnetic fields. Computerised images are calculated from variations in how this energy is absorbed and emitted across the body. As very little energy is involved, the normal biochemistry of the body is completely unaffected.
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MRI scan of the brain.
How is MRI used?
In medicine, MRI is mainly used for looking at damage to soft parts of the body - muscles, tendons and ligaments, as well as the brain. At the moment, MRI scans can only be done on patients who lie still. This looks set to change with the next generation of MRI scanners. These will have a much shorter exposure time, so they can be used to look at moving subjects - like a baby in a mother's womb.
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MRI scan of a CJD brain, showing its spongy appearance.
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