ABSTRACT
Ultrasonography is sound outside the range of normal hearing (over 20 kHz) and the ultrasonography used medically is of frequencies up to 100 times higher than this. Ultrasonography is used both for imaging of body structures and for measuring flow. When used for imaging, use is made of the fact that ultrasound will pass through body tissue until it meets an interface between structures of different density. Some of the sound will be reflected back whereas some will be refracted forwards. This principle, used in echocardiography, is shown in Figure 37.66. The proportions of sound reflected and refracted depend on the angle of incidence of the sound, and the difference in density of the two materials at the interface.
