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Ultrasound scan

NHS Direct Online Health Encyclopaedia



Ultrasound scanning has become increasingly important as a way of scanning without using any radiation such as X-rays. Because it does not use radiation, it is thought to be completely safe.

It uses very high frequency sound waves to create an image of part of the body.  The frequency of the sound waves is around ten million cycles per second (10MHz) which means that the human ear is not able to hear them.

Ultrasound scanning uses similar principles to the sonar systems used to detect submarines. When the sound waves are directed at part of the body, different density tissue reflects it in different ways.  These reflected waves are translated into an image by a computer.

Ultrasound of even higher intensity can cause tissue warming and is sometimes used for treatment purposes by physiotherapists.

How is it performed?

In preparation for the scan, the patient may be asked to refrain from eating or to drink some water, depending on which part of the body is to be scanned.

Gel is put on the skin around the area that is going to be scanned. This helps the scanner to move smoothly over the area and to remain in continuous contact with the skin.

As the scanner is moved, waves of ultrasound pass through the body, and a picture is formed on a computer screen. This should be a painless procedure.

Sometimes in early pregnancy, doctors use a small ultrasound scanner that is inserted into the vagina, to carry out a scan. This gives them a good picture of the womb.

What is it used for?

Ultrasound scanning is used widely for checking on pregnant mothers and their unborn babies.  Most pregnant women are screened by ultrasound at around the 16th to 20th week of pregnancy. 

The screening process can:

  • Detect twins or other multiple pregnancies;
  • Check that the baby is the right size for the stage of pregnancy;
  • Check that there are no problems with the baby's development like anencephaly (poor brain development), Spina bifida (lack of bone covering parts of the spinal chord), cleft lip / palate;
  • Measure the rates of blood flow through the baby's heart;
  • Detect some types of congenital heart disease;
  • Display the position of the afterbirth (placenta) so that any problems arising from its incorrect positioning can be safely managed;
  • Enable staff to carry out other types of tests such as amniocentesis, foetal blood sampling and chronic villus sampling;
  • In conjunction with small tubular instruments called foetal endoscopes or foetoscopes, enable surgeons to carry out procedures while the foetus is in the womb (for example, correcting a blocked urinary tract that is causing a swollen bladder);
  • Enable parent(s) to 'see' their unborn child.

Ultrasound scanning is also useful for examining soft tissue and fluid filled organs in the body such as the bladder and gallbladder, which do not show up clearly on X-rays. It can detect abnormalities such as tumours.

Ultrasound waves cannot easily pass through bone or gas, so it is of less use for some parts of the body.  For example, those parts of the body surrounded by bone like the brain and spinal cord. The lungs and the intestines are also not suitable for ultrasound examination.

Pulses of ultrasound are also sometimes used as a treatment for conditions such as rheumatoid arthritis and to heal fractures and wounds.

© Queen's Printer and Controller of HMSO, 2005

Crown copyright material is reproduced with the permission of the controller of HMSO and the Queens Printer for Scotland




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