How Does Ultrasound Work?

Have you ever wondered what kind of magic is taking place in your ultrasound to produce the image you see? Personally, I love witnessing that lightbulb moment when an ultrasound image transforms from a mess of grey static to a diagnostic visual in a student’s eyes!

Diagnostic ultrasound imaging can be related to many things that use sound waves to “map out” an area, the way sonar on a ship is employed to read the contours of the sea floor, a fish finder locates schools under the water’s surface, or a bat uses echolocation to avoid colliding with trees as it navigates through the dark forest. In these cases, sounds are emitted and the “echoes” then listened for as the signal bounces off of surfaces, creating a sort of topographical map of what lies ahead or below.

With medical ultrasound, we use a transducer (or “probe”) to create sound waves from electrical energy. The shape, size, and frequency range of that probe dictate how powerfully and deeply the sound waves are transmitted through the tissue (and the resolution of the resulting image—read more about probe selection HERE). Luckily, unlike the bat but much like mapping sonar, our ultrasound machines do the work of displaying the returning signals as a visual image for us to then interpret.

See, as sound waves travel through living tissue, they can be slowed or absorbed (attenuated) by the density of the tissue, reflected back to the transducer, or even diverted into adjacent tissues. While these behaviors can occasionally result in artifacts, the ultrasound does a pretty good job of displaying what it “hears” as an image representing the organs and tissues being examined.

If sound waves encounter a tissue interface that is reflective enough to bounce 100% of the beam back to the transducer, that very strong echo is displayed as a bright white signal, which corresponds to a dense or highly echogenic surface such as bone. At the other end of the spectrum, sound waves travel easily through fluid; so with little to no echo returning to the transducer, fluid-filled structures such as the urinary bladder appear black in the ultrasound image. As you can imagine, a more cellular fluid such as blood or pus may be a darker grey, with lighter, more echogenic particles appearing to move or swirl through it. Organ tissue can range in shades of grey, with whiter tissues being more dense or reflective and darker tissues being higher in fluid content or less dense.

This is useful information when encountering a disease state, as it allows us to consider what might be causing a tissue to look abnormal. For example, darkening of organ tissue may occur with edema, cysts, or congestion whereas whitening of those same tissues could indicate infiltration, mineralization, or fibrosis.

Want to learn more? Watch our Ultrasound Basics video...

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