In choosing between various scanning techniques the factors to be considered include availability, cost, the type of equipment, the expertise of the medical and technical staff, and the inherent capabilities of the system. Although it is difficult to state dogmatically which scanning technique is best for each patient and condition, one or other technique is clearly preferable in some areas of medicine. Ultrasound, for example, should be used in obstetrics, while computerized tomography has revolutionised neuroradiological diagnosis. Nevertheless, there is still no substitute for good history taking and a thorough physical examination. The most important factor determining the choice of technique is the system's ability to answer the specific question required for the management of the patient.