Ultrasound imaging uses no radiation and is believed to be free from risks. Very high-frequency sound waves, inaudible to the human ear, are transmitted into the body by a probe in contact with the skin. Echoes are reflected back to a receiver in the probe from the junctions between tissues of different types. For instance, air and bone absorb most of the ultrasound, but fluid conducts it well. (The technique is very similar to sonar scanning or ‘echosounding’ used to detect underwater objects.) Ultrasound is fairly limited in its application. Its main use is for visualizing the foetus within the womb. By measuring the diameter of the foetal head the pregnancy’s duration can be assessed. The placenta can also be seen. In modern, ‘real-time’, ultrasound scanners the image is updated every fraction of a second, so that instead of a ‘still’ picture a ‘movie film’ is obtained and the operator and the parents can see the baby’s movements. Sometimes, even the baby’s heart can be seen in action. Ultrasound is also useful in distinguishing between solid tumours and fluid-filled cysts in the kidney, the thyroid gland and the liver.