9.Doppler

Question 1

What is the Doppler Effect?

Answer 1

Answer: Doppler Effect is the change in the frequency of sound wave as a result of motion between the sound source, and the receiver. The frequency of reflected sound wave changes when the sound source and the receiver move closer together or move further apart.
In diagnostic ultrasound, the Doppler Effect occurs when red blood cells either move toward or away from the transducer.

Question 2

What is Doppler shift?

Answer 2

Doppler Shift is the difference between the transmitted frequency and the received frequency of the ultrasound waves returning from moving reflectors. Doppler shift is used to measure velocity of blood flow.doppler shift = reflected frequency - transmitted frequency

Question 3

What kind of information is obtained from Doppler shift?

Answer 3

Answer: In diagnostic imaging, Doppler shift provides information about velocity of moving red blood cells.
The Doppler equation describes the relationship between the frequency shift and the velocity of the red blood cells. The Doppler equation is programmed in ultrasound systems. When a Doppler shift is measured, the computer calculates the velocity of the red blood cells for interpretation. Clinically, the velocity of red blood cells is important, not the Doppler shift.

Question 4

What is the typical range of Doppler shift found in diagnostic imaging examinations?

Answer 4

The typical range of Doppler shift found in diagnostic imaging examinations ranges from -20,000Hz to 20,000Hz or -20 kHz to 20 khz.

Question 5

An ultrasound wave with a frequency of 5,000,000 Hz is transmitted in the body. An echo with a frequency of 5,005,000 Hz was received by the transducer. How much is the frequency shift?

Answer 5

5,000 Hz
frequency shift = received frequency - transmitted frequency
frequency shift = 5,005,000 - 5,000,000
frequency shift = 5,000 Hz

Question 6

Starting from the same point, the sound source is moving east at 12 miles/hour and the receiver is moving west at 10 miles/hour.
The Doppler shift will be———?
a) negative
b) positive
c) cannot determine

Answer 6

Answer: a. negative
The sound source and receiver are moving away from each other; therefore, Doppler shift will be negative.

Question 7

Starting from the same point, the receiver is moving west at 12 miles/hour and the source is moving west at 10 miles/hour. The Doppler shift will be ———–?
a) negative
b) positive
c) cannot determine

Answer 7

a. negative
The distance between sound source and receiver is increasing, therefore, Doppler shift will be negative.

Question 8

An & Miz transducer with a PRF of 5,000 Hz measures a Doppler shift of 7 kHz. The exam is repeated with a 4 MHz transducer.
What Doppler shift will be measured?
a) 4 MHZ

b) 3.5 Hz

c) 4 kHZ

d) 1.5 dB

e) 3,500 Hz

Answer 8

Answer: e. 3,500 Hz
Doppler Shift is directly related to the transducer frequency. The higher frequency transducers will yield higher frequency shifts than the lower frequency transducers.

Question 9

What is the Nyquist limit?

Answer 9

Answer: Nyquist Limit is the highest Doppler shift or velocity that can be displayed without the appearance of aliasing.

The Nyquist limit is equal to half of the pulse repetition frequency.
Nyquist Limit = PRF/2

Question 10

A pulsed ultrasound system has pulse repetition frequency of 5,000 Hz. What is the Nyquist limit?

Answer 10

The Nyquist limit is 2500 Hz.
The Nyquist limit is equal to one half of the pulse repetition frequency.
Nyquist Limit = PRF/2
Nyquist Limit = 5000/2
Nyquist Limit = 2500 Hz

Question 11

A 7.5 MHz transducer with a PRF of 5,000 Hz is imaging at a depth of 7 cm. What is the Nyquist frequency?
a) 4 MHZ

b) 3.5 HZ

c) 2.5 kHz

d) 2.5 dB

e) 5 kHz

Answer 11

Answer: C. 2.5 KHZ
Nyquist limit is the highest Doppler shift or velocity that can be displayed without the appearance of aliasing. The Nyquist limit is equal to half of the pulse repetition frequency.

Question 12

What is aliasing?

Answer 12

Aliasing is an artifact in which high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction. Aliasing is the wraparound of the Doppler signal where the higher positive Doppler shifts is displayed below the baseline. Aliasing occurs when the Doppler shift exceeds the Nyquist limit.

• The highest Doppler shift that can be measured without aliasing is equal to one half of pulse repetition frequency and known as Nyquist Limit.

Question 13

The higher the emitted frequency the more likely a pulsed wave signal is to alias.

Answer 13

Answer: True
Aliasing occurs when the Doppler shift frequency exceeds half of the pulse repetition frequency. Doppler Shift is directly related to the transducer frequency. The higher frequency transducers will yield higher frequency shifts than the lower frequency transducers. The higher the frequency of the transducer, more likely aliasing is to occur.

Question 14

The deeper the sample volume is the more likely a signal is to alias.

Answer 14

Answer: True
With deeper sample volumes, aliasing is more likely to occur.

Question 15

Which of the following will result in less aliasing?

Answer 15

Answer: d. all of the above
The sample volume placed at

1. shallower dept
2. lower blood flow velocity
3. lower transducer frequency

will result in less aliasing.

Question 16

Which of the following will result in more aliasing?

Answer 16

Answer: d. all of the above
The sample volume placed at greater depth, higher blood flow velocity, and higher transducer frequency will result in more aliasing.

Question 17

Which of the following techniques can be used to avoid aliasing?

Answer 17

Answer: e. all of the above
a) use continuous wave Doppler
b) use shallower sample volume
c) adjust the scale to maximum
d) use lower frequency transducer

Question 18

What is a pulsed wave Doppler?

Answer 18

Answer: In pulsed wave Doppler transducer one PZT crystal is used for both sending and receiving the ultrasound waves. It alternates between sending and receiving the sound waves. Pulsed wave Doppler can measure the flow from the selected area. It cannot measure high velocities and aliasing occurs.

Question 19

What is the advantage of pulsed wave Doppler?

Answer 19

Answer: The advantage of pulsed wave Doppler is that it can measure the flow from the selected area. This is called range resolution or range specificity. Echoes are analyzed and processed only from the sample volume area.

Question 20

What is the disadvantage of pulsed wave Doppler?

Answer 20

Answer: The disadvantage of pulsed wave Doppler is that it cannot measure high velocities and aliasing occurs. Inability to correctly measure high velocities is a major disadvantage of pulsed wave Doppler. Aliasing occurs with pulsed wave Doppler and the high velocities appear negative and below the baseline.

Question 21

Only pulsed wave Doppler exams use a sample volume.

Answer 21

Answer: True
Only pulsed wave Doppler exams use a sample volume. The advantage of pulsed wave Doppler is that it can measure the flow from the selected area. This is called range resolution or range specificity.

Question 22

Which of the following factors relate to the pulsed wave Doppler?
a) range resolution
b) sample volume
c) Nyquist limit
d) aliasing
e) all of the above

Answer 22

Answer: e. all of the above
The range resolution, sample volume, Nyquist limit, and aliasing all relate to the pulsed wave Doppler.

Question 23

Which of the following characteristics relate to the pulsed wave Doppler?

a) one crystal

b) damped PZT

c) lower sensitivity

d) low Q-factor

e) wide bandwidth

f) all of the above

Answer 23

Answer: f. all of the above

Question 24

What is a continuous wave Doppler?

Answer 24

Answer: In continuous wave Doppler transducer two PZT crystals are used. One crystal constantly transmits the ultrasound waves and the other crystal continuously receives the returning ultrasound waves. The continuous wave Doppler can accurately measure very high velocities but it cannot determine the exact location of the moving reflector.

Question 25

What is the advantage of continuous wave Doppler?

Answer 25

Answer: The **advantage of continuous wave Doppler is its ability to accurately measure very high velocities**. The velocities are measured along the entire length of the sound beam.

Question 26

What is the disadvantage of continuous wave Doppler?

Answer 26

Answer: The disadvantage of continuous wave Doppler is its inability to **determine the exact location** of the moving red blood cells. Velocities are measured along the entire length of the ultrasound beam. This is **called range ambiguity**. * ***The continuous wave Doppler does not have the ability to determine how far the reflector is from the transducer***.

Question 27

Which of the following factors relate to the continuous wave Doppler?

Answer 27

Answer: e. all of the above a) range ambiguity b) region of overlap c) unlimited maximum velocity d) no aliasing

Question 28

Which of the following characteristics relate to the continuous wave Doppler?

Answer 28

Answer: f. all of the above a) at least two crystals b) undamped PZT crystal c) high Q-factor d) narrow bandwidth e) higher sensitivity

Question 29

What is the significance of **gray scale variation** in the spectrum of a pulsed wave Doppler display?

Answer 29

Answer: The gray scale variation in the spectrum of a pulsed wave Doppler display **represents the number of red blood cells creating the reflection.**

Question 30

What is a packet or ensemble length?

Answer 30

Answer: **A group of ultrasound pulses is called a packet or ensemble length**. Multiple ultrasound pulses are needed to accurately determine red blood cell velocities by Doppler.

Question 31

What does the packet size in color Doppler refers to?

Answer 31

Answer: The packet size in color Doppler refers to the **number of ultrasound pulses per acoustic scan line.**

Question 32

What is color Doppler imaging?

Answer 32

Answer: In color Doppler imaging the **velocity information is coded into colors and superimposed onto a 2D grayscale image**. **Color flow imaging is based on pulsed Doppler principles and provides range resolution.**

Question 33

What is the commonly used process for determining direction in a Doppler instrument?

Answer 33

Answer: **Phase quadrate detection** is the commonly used process for determining direction in a Doppler instrument.

Question 34

What method is used to steer the color beam with a linear array transducer?

Answer 34

Answer: **Electronic time delays** are used to steer the color beam with a linear array transducer.

Question 35

What happens by increasing the wall filter during Doppler sampling?

Answer 35

Answer: Increasing the wall filter during Doppler sampling will reduce display of **low frequency Doppler shifts**.

Question 36

What would be the most likely result from **increasing the transmit frequency of the color Doppler**?

Answer 36

Answer: Increasing the transmit frequency of the color Doppler **will improve sensitivity to slow flow.**

Question 37

For Doppler, which incident angle results in no shift?

Answer 37

Answer: For Doppler, 90° incident angle results in no shift.

Question 38

What is a color power Doppler?

Answer 38

* Color **power Doppler is used to detect very low velocity or small volumes of blood flow.** * Color **Power Doppler works by detecting the presence of Doppler shifts**. * Color **power Doppler does not provide information about velocity or direction of blood flow**. * The **strength or amplitude** of the signal is processed which is **directly proportional to the number of moving red blood cells**.

Question 39

What are the advantages of power mode Doppler?

Answer 39

Answer: **The advantages** of power mode Doppler are as follows: * It has increased sensitivity to low velocities * It is unaffected by the Doppler angles * It has no aliasing

Question 40

What are the disadvantages of power mode Doppler?

Answer 40

Answer: The disadvantages of power mode Doppler are as follows: * it provides **no velocity or direction** of flow information * it has **low frame rate** which results in **poor temporal resolution** * it is **susceptible to motion** which creates **flash artifacts**

Question 41

What is duplex ultrasound scanning?

Answer 41

Answer: Duplex ultrasound scanning is the **simultaneous use of two-dimensional imaging and Doppler scanning**. The advantage of duplex ultrasound scanning is that it allows the sonographer to **accurately place the sample volume at the desired location.**

Question 42

What is a bidirectional Doppler?

Answer 42

Answer: A Bidirectional Doppler is a Doppler system that measures the **presence of blood flow and its direction**. **Direction of blood flow is determined by whether the Doppler shift is positive or negative.** A bidirectional Doppler system **can distinguish** that **blood flow is towards** the transducer, or **the blood flow is away from the transducer** by the measurement of **positive and negative Doppler shifts.**

Question 42

What is a non-directional Doppler?

Answer 42

Answer: **A non-directional Doppler is a Doppler system that measures the presence of blood flow without distinguishing its direction.** Non directional Doppler is used in clinical settings in which presence of blood flow is important, but the direction of flow is not important. For example non-directional Doppler is used to detect presence of blood flow in the arteries of foot.

Question 43

What is spectral analysis?

Answer 43

Answer: Spectral analysis is the study or interpretation of the **returning signals reflected from moving red blood cells**. * Spectral analysis is the technique that breaks up a complex signal into the individual velocities which make up the reflected Doppler signal. * **These returning signals are complex and contain many different frequencies. These reflected Doppler signals are converted into velocities**.

Question 45

What is the method of processing the Doppler signals?

Answer 45

Answer: **Fast Fourier Transform is the standard technique of spectral analysis used in pulsed and continuous wave ultrasound systems**. Fast Fourier Transform is a computerized, mathematical technique of digital **spectral analysis used to identify the different frequencies that combine to form the complex Doppler waveform.** The Fast Fourier Transform (FFT) Doppler analysis is accurate and displays all individual velocity components from the reflected signals.

Question 46

What is autocorrelation?

Answer 46

* Autocorrelation is a computerized, mathematical spectral analysis technique * used to identify different frequencies contained in the Doppler waveform. * This Doppler analysis tool is less accurate but faster than FFT. * Autocorrelation technique is used with color flow Doppler.

Question 47

What is the function of wall filter?

Answer 47

Answer: Wall filter excludes **low frequency Doppler shifts around the baseline**. It has **no effect on higher Doppler frequency shifts** created by moving red blood cells.