Introduction to imaging: Non-ionising radiation
BMJ 2003; 327 doi: https://doi.org/10.1136/sbmj.0311402 (Published 01 November 2003) Cite this as: BMJ 2003;327:0311402
- John Frank, consultant in nuclear medicine and radiology1
- 1Charing Cross Hospital, London W6 8RF
Whereas imaging with ionising radiation uses x rays or gamma rays, non-ionising radiation uses either sound waves, in the case of ultrasound, or magnetisation of protons in magnetic resonance imaging (MRI).
Ultrasound
Ultrasound is high frequency sound waves in the megahertz range. These are produced by a special transducer, and pass into the body through what is called an acoustic coupler--a special jelly for abdominal and peripheral work, or a water bath when a structure such as the eye is being examined. The sound waves are reflected back in varying amounts from every anatomical interface and these reflected waves are detected to produce an image. Modern ultrasound machines contain a computer that generates the images and can send them either to film or a picture archiving communication system (fig 1).
A typical modern ultrasound machine. Relatively portable, it can be placed alongside the patient who lies on a couch or bed
Check it--and check it again
Interpreting an ultrasound
Bone and other structures containing calcium, such as gallstones, reflect all incident ultrasound waves and show up densely white with a very black area behind them, called an acoustic shadow, in which no detail can be seen. Ultrasound passes easily through liquid, such as urine in the bladder, and the liquid increases the sound waves passing through it. This acoustic enhancement increases the signal from the structures behind the liquid (fig 2).
An ultrasound image showing the more solid liver tissue with the fluid containing gall bladder. Reflection of ultrasound back from the interfaces in differing amounts enables good separation of the structures and so aids identification of the viscera
The investigation takes place in real time, making the interpretation much easier for the operator. The operator can also …
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