An ultrasound scanner works by sending high frequency sound waves from a probe into a patient’s body and measuring reflections of those waves from tissue boundaries.

Sound waves  travel through different body tissues at different speeds The speed of sound in “soft tissue” (such as skin, fat, and muscle) is nearly constant at about 1500 meters (m) or ~5000 feet (ft) per second (s). The speed of sound in bone (~4000 m/s, or ~13,100 ft/s) and air (~340 m/s, or ~1120 ft/s) differ greatly from soft tissue.

When sound waves encounter a tissue boundary, such as the surface of an  organ, some waves reflect (bounce back from the boundary) while some waves refract (continue forward through the new tissue but at a different angle). Such a boundary is called an acoustic interface. The densities of the tissues on each side of the interface determine the amount of reflected and refracted energy.

The scanner calculates the distance from the probe to the acoustic interfaces based on the time it takes for echoes to bounce back to the probe (similar to the way speed-checking radar guns work, except those use faster radio waves). The distances and intensities of the reflected sound waves are then displayed on a screen to form a two-dimensional (2D) image. Modern ultrasound scanners can produce three dimensional (3D) images which are very useful for planning surgical procedures.

Because ultrasound scanners use sound waves rather than ionizing radiation, they are considered safe

Ultrasound imaging is used to investigate the cause of pain, swelling, and infection in the body’s internal organs; to examine fetuses in pregnant women; and to examine the brain in infants. They are also used to help guide biopsies, diagnose heart conditions, and assess tissue damage after a heart attack.

Examples of clinical ultrasound images.
Abdominal Aorta
Images courtesy of Siemens Healthcare GmbH. 

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