To become versed in the language of ultrasonography, it is necessary to review some of the basic principles of physics. The wave physics principles of ordinary (i.e., audible) sound apply to ultrasound (US) and its applications. Thus, to create a foundation for further discussions, a number of definitions and basic concepts are presented here.

Basic definitions and physics principles

Amplitude is the peak pressure of the wave (Figure 1.1). When applied to ordinary sound, this term correlates with the loudness of the sound wave. When applied to ultrasound images, this term correlates with the intensity of the returning echo.

Ultrasound machines can measure the intensity (amplitude) of the returning echo; analysis of this information affects the brightness of the echo displayed on the screen. Strong returning echoes translate into a bright or white dot on the screen (known as hyperechoic). Weak returning echoes translate into a black dot on the screen (known as hypoechoic or anechoic). The “gray scale” of diagnostic ultrasonography is the range of echo strength as it correlates to colors on a black–white continuum (Figure 1.2).

Velocity is defined as the speed of the wave. It is constant in a given medium and is calculated to be 1,540 m/s in soft tissue (i.e., the propagation speed of soft tissue is 1,540 m/s). Using this principle, an ultrasound machine can calculate the distance/depth of a structure by measuring the time it takes for an emitted ultrasound beam to be reflected back to the source (Figure 1.3).