The medical industry employs ultrasound to produce images of the human body. This section will equip you with the knowledge of the basics of ultrasound which will be needed when we learn about ultrasonography systems in module 2.

Ultrasound is, by definition, sound beyond the range of human hearing—that is, above 20 kHz in frequency. Ultrasound as used in medical diagnostic imaging operates at much higher frequencies, in the 2 MHz to 18 MHz range.

Ultrasound imaging is an echo-ranging technique that operates in a way similar to radar and sonar. A short pulse is transmitted at regular intervals. Between pulses the system listens for echoes. By measuring the time interval between the transmitted pulse and a returning echo, the distance to the source of the echo can be calculated, and the system can construct an image that represents the relative locations of all echoic structures. This same concept is used to map the locations of internal organs in our body.

Below you can find an explanatory video by Study Animated on how sound propagates through a medium.

Ultrasound is a mechanical longitudinal (compressional) wave. Propagation of the ultrasound wave is affected by a number of acoustic variables such as density and temperature. As the wave propagates through tissue, particle motion within the tissue oscillates back and forth, creating alternate regions of high pressure and low pressure. These wave half-cycles are compression (positive) and rarefaction (negative).

The figure below shows the propagation speed of ultrasound through various tissues. Since the propagation speed of ultrasound varies with different types of biological tissue densities, imaging systems use an average speed (c) of 1.54 mm/µs in order to calculate and display the relative positions of echoic structures. This can be calculated by using the range equation given in the figure. Naturally, when we have c (1.54mm/µs), we only need to know ‘t’ to calculate the position of the object that produced the echo.

Source: Nick Simons Institute, ‘Ultrasound’

Ultrasound, like any other wave, is described in terms of frequency, period, and wavelength. As with other wave types, the period and wavelength of ultrasound waves are inversely proportional to the frequency: the higher the frequency, the shorter the period and wavelength.