ABSTRACT
Since the introduction of the three-dimensional imaging technique, significant improvements have been undertaken. With the development of computer technology, calculation times become shorter and the software becomes more sophisticated. In the initial years, three-dimensional ultrasound allowed the production of only still volumes. Thus, scans only represented one single moment within the total examination. Serial re-scanning is one of the newest developments in three-dimensional ultrasound. In the volume-rendering mode, still imaging now may be replaced by three-dimensional ultrasound ‘movies’. This technique may also be called four-dimensional ultrasound' or live three-dimensional ultrasound'. However, a frame rate of about 10 frames per second would be necessary to bring this technique to ‘real-time’ mode. Possible applications in prenatal diagnosis are examinations of the fetal extremities, fetal face and may be fetal behavior. Among the many advantages of four-dimensional ultrasound, improvement in imaging and the shorter training time for beginners to the three-dimensional technique are the most important.