Ultrasound is based on the fact that high-frequency sound wave bounce off tissues of different density at different rates. For example, bone reflects back nearly all sound waves that hit it, whereas fluids allow the waves to pass through. Sound waves are bounced off the organ being investigated, and the reflected waves are translated into a picture so the doctor can see what they mean.
Ultrasound scanners operate at the high frequency of 20 kilohertz (kHz) or 20,000 vibrations a second (Hertz). The maximum vibration a human can hear is about 6000 Hertz.
Ultrasound machines produce a moving image from which selected still photographs are taken. Part of the patient’s body is coated with oil, and a small pen-like probe that contains the sound recorder and microphone is placed on the skin. Once the area has been scanned, the instrument is moved a few centimetres and another scan is taken, and so on, until the entire area under investigation has been covered.
Almost any part of the body can be examined by ultrasound, with the exception of the head because the sound waves cannot penetrate bone. Because of the ribs, it is also difficult to see into the chest.
The most useful aspect of ultrasound is its ability to examine the foetus of a pregnant woman without the risks associated with X-rays. The size, position and sometimes sex of the baby can all be seen, and some of the internal organs of the baby, particularly the heart, can be checked. Abnormalities such as spina bifida, hydrocephalus and certain other congenital disorders can be identified. A routine scan may be performed between the sixteenth and eighteenth week of pregnancy when the foetus can easily be seen and transformed into an image. Another scan is sometimes performed later in the pregnancy, after about 32 weeks.
The breasts can be carefully checked for cysts, fibrous lumps or cancer by ultrasound, as the cancer cells reflect sound in a different way from normal cells. The gall bladder and liver can be checked for damage and stones, the kidneys and pancreas for cysts and stones, the thyroid gland and spleen for enlargement, tumours and cysts, among many other uses.
Ultrasound can also be used to study the flow of the blood. Among other interesting pieces of information to emerge is that the blood flow in the carotid artery (the main artery carrying blood to the brain) is strongly influenced by external stimuli, such as the phone ringing or someone coming into the room.
Ultrasound is frequently used to guide a needle towards its destination in a biopsy.
Unlike X-rays, sound waves have no effect on the tissues exposed to them, so ultrasound is completely safe. It can be repeated as often as required without concern.