Prospective mothers often come away from the clinic clutching a fuzzy grey picture showing some part of their fetus. In this article we explain how ultrasound imaging works and describe our work on extending it into three dimensions. Ultrasound images are simply a map of the reflected echoes when a pulse of high frequency sound is transmitted into the body - Figure 1 shows a typical example. Echoes which come back first relate to material close to the skin, showing up as bright patches near the top of Figure 1.
Later echoes come from deeper in the body and are used to shade more distant parts of the image.
There are other ways of producing images of the inside of the body, but they are generally either more expensive than ultrasound, or they involve exposure to X-rays or other radiation. They are considered safe, even for scanning the fetus in utero. They are quick and efficient to use, and not unpleasant for the person being scanned. Their main drawback is the speckled nature of the image, and the fact that only a two-dimensional slice is produced at any given time. A typical ultrasound image.