cific application for the brain. A noninvasive technique, like all those discussed here, ultrasound offers the additional advantage that the form of radiation it uses—acoustic waves—is widely agreed to pose virtually no threat of side effects. Ultrasound has become familiar to many women during pregnancy, because it is often used as a means to examine the fetus in the uterus, without adverse effects. The technique works by directing a pulse of acoustic waves at some body structure and then measuring the strength and speed of the waves that bounce off the body and return as echoes. Assuming an average speed for sound waves through body tissues (about 1,540 meters per second), the ultrasound device assembles the various echo times into an image. Organs and connective tissue tend to reflect sound waves quite differently, so that the outline of structures can be seen distinctly. Ultrasound can also produce useful images of blood flow and of structures that are moving, with a fairly high resolution of about 0.5 to 1 millimeter.
Sound waves penetrate very poorly through bone, and this fact might appear to make ultrasound imaging of the brain impossible. However, in one special case—the newborn baby, in whom the bones of the skull are quite thin and have not yet fused together—ultrasound can be invaluable, offering a chance to examine the brain without surgery.
Each imaging technique presented in this chapter has its own set of advantages and drawbacks that determine its best applications. For instance, the comparative approach described earlier for panic disorder and depression and their functional equivalents in everyday experience is a way to use the young technology of PET to full advantage. Even more powerful as an aid to research is the combination of PET with other forms of imaging. Although no technique currently available can do justice to the whole picture, each one offers a unique glimpse into the intricacies of the living brain.
Chapter 3 is based on presentations by Marcus Raichle.