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The Doppler Effect is commonly applied in sonography to provide medical imaging to help diagnose patients where speed of blood flow has to be measured. Doppler ultrasonography is useful in cardiology, obstetrics, and neurology. Rather than using true doppler ultrasound, which would measure change in sound frequency, what we call Doppler actually compares the phase of the sound energy, and measures the shift in that phase. Phase shifts are more drastic than frequency shifts, so phase-shift-compared ultrasound, still commonly called doppler ultrasound, magnifies the likelihood of detecting motion.
What is Doppler?
Austrian physicist Christian Doppler proposed the doppler effect in 1842 as the change in colored light from stars, and the change in the sound of a passing object like a train. In 1845, he and Buys Ballot proved the Doppler Effect on sound waves: A sound’s pitch would change if its source or recipient is in motion.
When a sound is emitted as a constant, but it seems to the recipient of the sound like it changes because there is a change in frequency of a sound wave at different distances, that’s the doppler effect. You can see how this can be applied to the human body with doppler ultrasound.
The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
Doppler Ultrasound
While regular ultrasound can show images from inside a body, the movement of blood and its speed cannot be measured with plain ultrasound.
Doppler ultrasound can detect and measure the movement of red blood cells as they cause a change in pitch of the reflected sound waves (called the Doppler effect). There would be no phase shift (rate of change in pitch) to measure if the blood wasn’t moving.
How Doppler Ultrasound is Performed
Ultrasound technicians trained in ultrasound imaging (sonographers) press a wand (transducer) against skin over the area of the body being diagnosed.
Types of Doppler Ultrasound
Commonly, two types of Doppler transducers used are Continuous Wave Doppler and Pulsed Wave Doppler.
Continuous Wave Doppler Ultrasound units use two crystals, one to send and one to receive sound echoes. The transmitter inputs a continuous sinusoidal wave so a receiver can detect the phase shift. An audible sound is recorded and analyzed. CW Doppler provides high sensitivity to low velocities and detection of high velocities without aliasing. However, CW Doppler cannot distinguish between the sending and receiving signals or extraneous echoes. CW Doppler ultrasound does not produce a precise image like Pulsed Wave Doppler does.
Pulsed-Wave Doppler Ultrasound is combined with Duplex ultrasound to produce images of a certain structure while eliminating unwanted structures. PW uses a pulsed echo system. The PW transducer both sends and receives the signal. It sends in short bursts, on and off, so it can receive when it is not sending.
Why Doppler Ultrasound?
Ultrasonography is less invasive. Sonographers can be called to perform this as an alternative to arteriography and venography, which involve injecting dye into the blood vessels so that they can be detected as X-ray images.
Duplex Ultrasound
A duplex ultrasound is a combination of tests to see how blood moves through your arteries and veins.
Duplex means that two types of ultrasonography are employed: regular ultrasounds, and Doppler ultrasounds. The conventional ultrasound shows the structure of your blood vessels and the Doppler shows the movement of your red blood cells through the vessels.
Duplex ultrasound produces images that can be color coded to show physicians where your blood flow is severely blocked as well as the speed and direction of blood flow.
Other Uses of the Doppler Effect
In 1848, Hippolyte Fizeau proved the same effect on electromagnetic waves (Doppler-Fizeau Effect).
Today, common uses of Doppler are in radar detection of weather to calculate distance and show direction of movement of weather conditions to diagnose or forecast them, and police use doppler radar to detect speeding motorists.
Smartsound Ultrasound is an exclusive dealer for the major player in sonography – Biosound Esaote, makers of MyLab technology such as MyLab50, which can perform conventional and Doppler sonograms. Visit DrSmart.com to read more about doing Doppler sonograms with MyLab50, or with the powerful MyLab40, and even with the portable MyLab5.
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