What Is Doppler Ultrasound?

Meg Higa

Doppler ultrasound is the technology of emitting a high frequency tone to measure its “bounce” from an object of varying densities, as well as the movement and velocity of anything within the object. It has applications in a variety of fields, including military and industrial, but it is best known as a means for medical imaging. A pregnant woman’s pelvic area has semi-solid bone, dense muscle tissue, and watery fluid. Ultrasound can distinguish these. The additional capability of measuring the “Doppler shift” in the reflected sound wave can furthermore determine, for example, whether the blood pumping out of the heart of an unborn baby is developmentally sufficient and healthy.

Doppler ultrasound works on the same principle as the echolocation ability of bats.
Doppler ultrasound works on the same principle as the echolocation ability of bats.

The basic principle of ultrasound is sonar — the echolocation ability of bats and dolphins to “see” not by sight, but by emitting a high pitched click or scream and then evaluating the characteristics of its reflection off the surfaces and things in their living space. An example of the Doppler effect is a car driving past a stationary pedestrian. As the car approaches, the sound of its engine is heard to increasingly rise to a noticeably higher pitch; and as the car passes and recedes, the sound correspondingly decreases in pitch. Its speed and sound is unchangingly constant; but the sound waves generated by the engine are actually being compressed or stretched by its movement. A blind pedestrian can evaluate the characteristics of this shifting pitch and make a good determination of the car’s movement direction and speed.

Certain heart conditions can be detected with an ultrasound.
Certain heart conditions can be detected with an ultrasound.

The Doppler effect was theoretically articulated by a namesake Austrian physicist in 1842, but it was not for another hundred years that sonography, visually graphing or displaying sound, became a vigorous scientific field. Doppler sonography, requiring continuous measurement of minute changes in reflected sound frequencies over time, required correspondingly more precise and fast electrical and electronic systems. Improvements in medical devices using Doppler ultrasound continue to be developed, especially in their contact probe and their data display.

The color Doppler ultrasound utilizes the Doppler effect to create moving color images of the inside of the body.
The color Doppler ultrasound utilizes the Doppler effect to create moving color images of the inside of the body.

The tethered probe of ultrasounds are electro-acoustic transducers, converting electrical energy into sound energy and vice versa. The sound generated by them can’t be heard or felt by humans — from 1 to 18 megahertz in frequency, variable to penetrate deeper into human tissues. A Doppler ultrasound might emit a continuous tone, but most models transmit the tone and receive its echoes as a succession of very rapid pulses. The advantage of the latter is that a single pulse can also be analyzed, such as translating the time delay of the echo into distance and creating more accurate three-dimensional images.

A blocked artery in the heart may be detected using a medical Doppler ultrasound device.
A blocked artery in the heart may be detected using a medical Doppler ultrasound device.

Most Doppler sonogram displays are digital computations of the electronically encoded sound data into a best recreation of true body anatomy. One area of ongoing sonography research is to refine and exhaust exactly how every type of human tissue absorbs some and reflects some of all the frequencies within the range of these instruments. The computer programs for display translation are updated accordingly with new, truer, information.

Bats can navigate areas by producing sounds and then listening to how they reflect off of surfaces.
Bats can navigate areas by producing sounds and then listening to how they reflect off of surfaces.

A medical Doppler ultrasound device measures the direction and speed of things in the human body with a high level of precision. The most common application is evaluating blood movement, such as the diminished flow of a heart’s blocked artery or the reversed backflow of one of its weakened valves. It is also a valuable added tool for monitoring the development of a fetus in the womb by both measuring its own blood circulation as well as the healthy rate of fluid exchange with its mother.

Doppler ultrasound might be used to see a developing fetus.
Doppler ultrasound might be used to see a developing fetus.

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