Doppler is commonly used in Ultrasound to diagnose discrepancies in the hemodynamics of blood flow. The Doppler Effect is applied in sonography to provide medical imaging of vessels and the flow within them. Both color Doppler and Power Doppler are used to image blood flow. Pulsed wave and continuous wave Doppler help diagnose patients where the speed of blood flow has to be measured. Doppler ultrasonography is useful in most applications that diagnostic Ultrasound is used in. Examples where Doppler is used are cardiology, obstetrics, abdominal, small parts and neurology. When imaging body parts a high frequency wave is sent into the body. If a moving target is encountered the original frequency is shifted by the wave that is produced by the moving target. The difference between the original frequency and the moving target is demodulated and manipulated to show either an image or a graph.
What is Doppler?
Austrian physicist Christian Doppler in 1842 proposed that the Doppler effect is seen in the change in colored light from stars, and the change in sound of a passing object as in a passing train. In 1845, he and Buys Ballot proved the Doppler Effect of sound waves: A sound's pitch would change if its source or recipient is in motion.
When sound is emitted as a constant by an object and a recipient is moving towards or away from the source a change in frequency will occur, this is the Doppler effect. As you can imagine this Doppler effect can give useful information when applied to the human body during Doppler ultrasound examination.
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 B-mode ultrasound can show images from within the body, the movement of blood and its speed cannot be measured with it alone.
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) if the blood wasn't moving.
How Doppler Ultrasound is Performed
Ultrasound technicians trained in ultrasound imaging (sonographers) place a probe (transducer) over the area of the body being diagnosed. First an image of the region of interest is acquired. Then the vessel is located and the transducer is angled so that the best Doppler shift can be obtained. The best location would be to be able to look directly into the vessel (much like looking into a tunnel). Because of how the vessels are located in the body this is of course often not possible. Thus maneuvering the probe or steering the ultrasound/Doppler beam will make this plausible.
Acquired Doppler information can be heard, shown on a chart (much like and EKG) or in colored blood streaming through the vessel.
Types of Doppler Ultrasound
Commonly, two types of Doppler transducers are used Continuous Wave Doppler and Pulsed Wave Doppler.
Continuous Wave Doppler ultrasound transducers use two crystals, one to send and one to receive echoes. The transmitter emits 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 images like Pulsed Wave Doppler does.
Pulsed-Wave Doppler Ultrasound is combined B-mode to produce images of a certain structure. 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 a less invasive and less expensive modality which is easy to schedule and perform in an emergency situation. Sonographers can be called to perform this as an alternative to arteriography and venography, which involves 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 B-mode ultrasounds, and Doppler ultrasounds. The conventional ultrasound shows the structures of your blood vessels and Doppler shows the movement of your red blood cells through the vessels and the sound of the frequency shifts.
Duplex ultrasound produces images that can be color coded to show physicians where the blood vessel is narrow or blocked. In some situation blood flow is reversed and an abnormal color pattern is created. With Color Doppler speed and direction of blood flow information is obtained.
Doppler Ultrasound Semantics
It is interesting to note that in sonography, the word Doppler is used similar to the way the word Kleenex is. People use tissues and refer to them as Kleenex when they're not. In sonography, the word Doppler is used when true scientific Doppler is not what is really being done. It's similar, but different.
The technical point I would like to make is that Doppler is technically concerned with sound frequency, whereas in ultrasound applications we compare the PHASE of the sound. In what we do, Phase Shifts are more detectable and are what we measure, not frequency, but it's similar so it's referred to as Doppler sonography. Semantics!
Rather than using true Doppler ultra sound, 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.
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: radar detection of weather (to calculate distance and show direction of movement of weather pattern) to diagnose or forecast them, and police use doppler radar to detect speeding motorists.
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