Essentials of Ultrasound Physics

Ultrasound scanners use transducers to convert sound energy to electrical energy.

Ultrasound imaging uses the reflection of high-frequency sound waves to represent images from inside the body. Although pictures of internal body parts become visible, ultrasound does not use ionizing radiation to produce these images as X-rays do. Visualization of ultrasound images occurs in real time, which reveals the movement of internal organs and vascular blood flow. Four-dimensional or moving ultrasound scans produce real-time pictures of three-dimensional scans. Medical uses for ultrasound imaging include examining organ tissue following illness and monitoring the fetus during pregnancy.

Functions

According to Tufts University, sound waves in ultrasound reach frequencies of 2 megahertz and higher. Ultrasound requires the medium of organ tissue for imaging. As the frequency of the sound waves increases, the depth of the image becomes shallower. With higher frequencies, however, image resolution improves. Ultrasound resolution refers to the ability of the sound waves' echo to detect separate and distinct objects. Ultrasound scanners use transducers to convert electric energy into ultrasound energy. An oscillating transducer creates a mechanical probe, which makes possible the rapid and repeated sweeping of the ultrasound beam through tissue. Because ultrasound uses the medium of body tissue, attenuation of sound waves moving through the medium can result in artifacts throughout the image.

Features

The University of Washington website states that the term ultrasound describes frequencies above 20 kilohertz, which is the upper limit of human hearing. Medical images usually come from scans with frequencies 100 times higher than the audible human range. Specular echoes originate from large, smooth objects with regular shapes, while scattered echoes bounce off smaller, irregularly shaped objects. The scattering of echoes contributes to image attenuation. According to the University of Washington, however, most ultrasound machines also have potentiometers for adjusting the gain of the sound waves, which helps resolve subtle features in the images.

Doppler

Ultrasound works the same way that echolocation works for bats and sonar works for ships. Doppler ultrasound measures the motion of blood cells using the Doppler effect. According to the website Radiologyinfo.org, hospitals use three types of Doppler ultrasound: color, power and spectral Doppler. Color Doppler represents Doppler information in an array of colors from a specially designed computer program. Power Doppler works in more sensitive cases of minimal blood flow but lacks the ability to show the direction of blood flow. Spectral Doppler represents ultrasound information graphically instead of visually. Blood flowing through vessels shows up on a chart measuring distance traveled per unit of time.

References

• Radiology Info: Ultrasound--General
• Tufts Health Sciences: Ultrasound Physics and Terminology
• University of Washington: Ultrasound