A small device called an ultrasound probe is used, which gives off high-frequency sound waves.
You can't hear these sound waves, but when they bounce off different parts of the body, they create "echoes" that are picked up by the probe and turned into a moving image.
This image is displayed on a monitor while the scan is carried out. Most ultrasound scans last between 15 and 45 minutes. They usually take place in a hospital radiology department and are performed either by a radiologist or a sonographer.
They can also be carried out in community locations such as GP practices and may be performed by other healthcare professionals, such as midwives or physiotherapists who have been specially trained in ultrasound.
There are different kinds of ultrasound scans, depending on which part of the body is being scanned and why. The three main types are: