Ultrasound is the basis of the navigation system used by bats. It was first developed by man as a means of measuring the depth of water beneath a ship and detecting shoals of fish - or submarines. A device called a transducer uses a piezoelectric crystal to convert electricity into high-pitched sound waves. These waves pass easily through liquids and soft tissues, but are reflected back to the transducer by denser substances. If the scanner is moved backwards and forwards and the reflected sound waves are displayed on a screen, a two-dimensional picture appears of a slice through the body. Bones are still best visualised using X-rays.
Echocardiography is a variant of ultrasound used to examine the heart. It gives very clear pictures of the heart valves and chambers - and, indeed, ultrasound may be used before birth to diagnose congenital heart disease in the unborn infant. Doppler ultrasound is another variant which uses the change in sound with motion to measure blood flow (the Doppler effect is heard, for example, when a police siren changes pitch as the car approaches and then goes away). Doppler studies allow recognition of blocked sections of artery.
Ultrasound examination of the kidneys and liver gives similar information to X-rays, and is useful in guiding the needle when a biopsy is needed to remove a small piece of a suspicious looking part of the organ for examination.
Ultrasound causes no discomfort as the lubricated transducer slides over the skin, but its great advantage over X-ray imaging is that the sound waves emitted by the transducer are absolutely safe. Even when low dosages of X-rays are used, the person being examined runs a slight risk of later developing a cancer, especially leukaemia. This risk is extremely small; one extra death from cancer occurs for every two to three million X-ray examinations of the chest. It was the knowledge of the small risk to the developing baby that led obstetricians to pioneer the use of ultrasound. Their experience led to the more general use of the technique. Follow-up of the millions of babies, now adults, exposed to ultrasound before birth 20-30 years ago has made it possible to give the reassurance that this procedure is as close to being totally safe as any medical examination.Reuse content