In addition to an exquisite appreciation of anatomy, fundamental to an optimal utilization of ultrasound in medicine is an understanding of the physics of ultrasound. One must recognize the nature of acoustic energy, specifically its creation and interactions with its environs, to fully use it as a diagnostic and potentially therapeutic tool.
Sound is simply a type of energy that is transmitted through a medium as a mechanical wave (or vibration). Unlike electromagnetic waves (ie, light), which can travel through a vacuum, mechanical waves require a medium in order to transport their energy. Mechanical waves may have one of two forms, longitudinal or transverse (Figure 1-1). Acoustic energy is a longitudinal wave, where particle displacement is parallel to the direction of wave propagation. In contrast, in a transverse wave the particle displacement is perpendicular to the direction of wave propagation. This can easily be understood if one thinks of a Slinky, the metal spring toy. If the Slinky is stretched out and then given a little push along its axis, a longitudinal wave is created, where a zone of compression is propagated along the coil, accompanied by a zone of rarefaction. On the other hand, if the Slinky is swung side-to-side while holding one end fixed, a transverse wave is created (looking much like a sinusoid as the slinky bounces back and forth).
Physical depiction of the types of mechanical waves.
Although a physical depiction of an acoustic wave is more closely related to the Slinky analogy, characterization of the waves is facilitated by a sinusoidal representation. The zone of compression then corresponds to the positive maximum of a sinusoidal wave, and the zone of rarefaction corresponds to the negative minimum. Waves are described by their wavelength, frequency, propagation velocity, amplitude, and phase, each of which has very important considerations in the use of ultrasound in medicine.
The wavelength is defined as the distance a wave travels per one cycle, which is measured between any two corresponding points on the waveform, that is, from peak-to-peak, trough-to-trough, or zero-to-zero (measured from the same slope) (Figure 1-2). Frequency is inversely proportional to the wavelength and is defined as the number of cycles per second; one cycle/second is termed one Hertz. Frequency and wavelength are related by the speed of sound through a medium, which is a defined property of different media, by the following equation:
where λ is the wavelength in meters, v is the speed of sound through the medium in meters per second, and f is the frequency of the wave, in Hertz (sec−1). For example, the speed of sound through air is 330 m/s, through water, ...