More than one out of every four medical diagnostic imaging studies in the world is now estimated to be an ultrasound study and the proportion continues to increase (WFUMB 1997). This situation has come about because of the remarkable advances that have taken place in the physics and engineering of ultrasonic imaging since the medical applications of ultrasonics were last reviewed, some 30 years ago, inReports on Progress in Physics (Wells 1970). In1970, the emphasis was on the biological effects, surgical and therapeutic applications of ultrasonics; little more than 20% of that review was concerned with diagnosis. That was a fair balance then, because medical imaging was dominated by x-radiography, with a small contribution from radionuclide studies; neither x-ray computed tomography nor nuclear magnetic resonance imaging had yet been invented and, except perhaps for applications in obstetrics, gynaecology and cardiology, ultrasonic imaging was generally regarded only as a laboratory curiosity.

The choice of the best imaging technique in any given clinical situation is based on considerations such as the resolution, contrast mechanism, speed, convenience, acceptability and safety. Ultrasound scores highly in all of these: spatial resolution of a millimetre can be obtained in abdominal scanning, tissue contrast is good (and can be enhanced by using contrast agents), it is a real-time method, convenient to use, very acceptable to patients, and apparently quite safe. It can also be a relatively inexpensive technology. Its main disadvantages are that its images are spoilt by the presence of bone or gas, and the operator needs a high level of skill, both in image acquisition and interpretation.

Without diagnostic imaging tools, the doctor is blind. Imaging is one of the cornerstones of diagnosis in modern medical practice: the others are clinical examination and the various branches of pathology. Moreover, the applications of imaging are not limited to diagnosis. Increasingly, open access surgery is being replaced by minimally-invasive procedures, and image-guided intervention is becoming more common. For many of these purposes, ultrasonic imaging is the best method.