Ultrasound uses high-frequency sound waves to create images of organs, structures and systems within many areas of the body. A machine sends out the sound waves, which reflect off body structures and are received by a computer, which uses them to create images (called a sonogram). A clear, water-based conducting gel is applied to the skin over the area being examined to help with the transmission of the sound waves. A handheld probe called a transducer is moved over the area being examined. You may be asked to change position so that other areas can be examined. Ultrasound imaging, referred to as ultrasonography, allows an examination of soft tissues and body cavities, without using invasive techniques. Ultrasound is often used to examine a foetus during pregnancy. (Unlike with an X-ray or CT scan, there is no ionising radiation exposure with this procedure.)