The aim of this chapter is to discuss the key properties of sound and explain how sound can be used to produce a diagnostic image. The Doppler principle, by which the velocity of moving tissues can be determined, is introduced. The clinical applications and advantages of ultrasound are described.

We tend to be more familiar with sound than with the other radiations used in radiography. From our own experience, we may have noticed that the light from a lightning bolt arrives more quickly than the following loud clap of thunder. This tells us something important about sound – it travels at about 340 metres per second in air, while light, like other electromagnetic radiations (including X-rays), travels at a staggering 300 000 kilometres per second in a vacuum. Sound, unlike electromagnetic radiations, needs a physical medium to travel through, since it consists of a travelling series of vibrations passing through atoms and molecules. It cannot pass through the vacuum of space and hence (perhaps fortunately!) we cannot hear the sun’s activity, although we are bathed in its light and heat radiations. Sound, unlike light and X-rays, cannot be considered as discrete packets or quanta of energy.

The speed and amplitude of sound is greatly affected by the medium through which it passes, much more so than in the case of electromagnetic radiations such as light. You may have noticed the very different pitches of sound that you hear when your head is under water, compared with when your head breaks back above the surface. Sound travels at slightly different speeds in different body tissues. The speed of sound is affected by the compressibility of the medium. It travels faster in more rigid materials which resist being compressed, and more slowly in materials such as fluids and gases which can be compressed easily. As an analogy, think of how much easier you can push an object using a firm rod made of wood, compared with using a bendy rod made of rubber. Table 42.1 lists the speed of sound in air and biological tissues and it can be seen that the more rigid substances allow sound to travel faster. Reflection and refraction of sound can occur at boundaries between materials or tissues.

Table 42.1 Approximate speeds of sound in different materials and tissues

Reflection is a very important property of sound waves, as it provides echoes at tissue boundaries, and these are used to depict structures in diagnostic ultrasound. It also gives some information about the nature of tissues. A cyst, which mostly contains fluid, will return few or no echoes (the signal is termed hypoechoic or anechoic) from within itself, as shown in Figure 42.1, while a haemorrhage will return echoes from within itself (the signal is echoic or hyperechoic), as shown in Figure 42.2. This is because a haemorrhage also contains blood cells and proteins (which return echoes) in addition to just fluid. The walls of a cyst may return strong echoes at the capsule–fluid boundary.

Figure 42.1 The cysts within this polycystic ovary contain watery fluid and appear anechoic (echo free).

Figure 42.2 This haemorrhage into a joint returns many small bright echoes from the substances within it, such as blood cells and proteins. It is echogenic (echo producing).

Whenever an ultrasound beam reaches a boundary between two materials with different acoustic impedances, some of the beam will be reflected, and the remainder transmitted. The acoustic impedance, symbol Z, refers to the amount of opposition that a medium presents to sound waves trying to pass through it and is affected by the compressibility and density of the medium. The greater the acoustic impedance between two tissues, the greater is the amount of sound reflection at the boundary between them.

The acoustic impedances of body tissues can be seen in Table 42.2.

Table 42.2 Acoustic impedance (Z) values for air and body tissues

Large Z differences return good strong echoes but

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