The picture becomes clear for magnetic resonance imaging
Glossary
atom. The fundamental unit of all matter consisting of a nucleus of protons and neutrons surrounded by orbiting electrons (or in the case of hydrogen, just one electron). For more information see Back to Basics: Atoms and molecules (Australian Academy of Science).
decibel (dB). A logarithmic scale used to denote the intensity, or pressure level, of a sound relative to the threshold of human hearing. A step of 10 dB is a ten-fold increase in intensity or sound energy and actually sounds a little more than twice as loud.
The quietest sound we can hear is 0 dB; a soft whisper has about 100 times more sound energy and so is about 20 dB. A power lawn-mower has a factor of 109 more sound energy and is about 90 dB. A rock band may be as high as 110 dB. Above 120 dB the sound produces discomfort and even pain. The scale is often adjusted to take account of the reduced sensitivity of human hearing to high and low frequencies and is then specified as dB(A). On this adjusted scale (the A-weighted scale), the range of human hearing is about 3 to 140 dB(A).
For more information see What is a decibel? (How Stuff Works, USA); What is a decibel? (University of New South Wales, Australia); and Intensity and the decibel scale (The Physics Classroom, USA).
element. A substance made up of only one type of atom. For more information see our Back to basics topic, Atoms and molecules.
protons and neutrons. Small particles that form the nucleus of an atom. Protons have a small positive charge; neutrons have no charge.
radio waves. Low frequency electromagnetic radiation. Radio waves have wavelengths ranging from less than a centimetre to as long as 100 kilometres. The hertz (Hz) is the unit of frequency and means one complete oscillation per second. Many frequencies are much higher than this so other units are used (eg, 1 megahertz (1MHz) = 1,000,000Hz).
We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM radio (amplitude modulation), FM radio (frequency modulation) and CB radio (citizens' band), television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency – at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource.
The frequency of radio waves used in magnetic resonance imaging range from 1-100 megahertz, depending on the strength of the magnetic field in the scanner. This is close to the range of frequencies used for FM radio (88-108 megahertz). For more information see How the radio spectrum works (How Stuff Works, USA).
superconducting magnet. A type of electromagnet (a temporary magnet formed when an electric current is conducted through a coil of wire). In superconducting magnets, the wire is cooled to a temperature close to absolute zero. At this temperature, there is virtually no resistance to the flow of electricity through the wire. For more information see How electromagnets work (How Stuff Works, USA).
surface coil. A device that closely fits against the surface of the part of the body being imaged. It acts like a radio antenna, receiving radio signals from the patient's body.
Page updated January 2007.