Ultrasound consists of high frequency sound waves too high for the human ear to detect, rather like the noise used by bats and dolphins to determine where they are. These waves are emitted by an ultrasound probe and travel harmlessly through the body bouncing off various layers of tissue. The probe then hears these echoes which are relayed onto a screen allowing the pictures to be interpreted. Ultrasound is now the method of choice for monitoring the foetus during pregnancy and in diagnosis of numerous conditions involving organs such as the liver, kidney, heart, blood vessels and joints. There has been no known hazard from ultrasound during its use in medicine for over 40 years, nor is there discomfort with this method of examination.