Doppler Principles

Question 1

What is the Doppler Effect?

Answer 1

The Doppler Effect occurs when there is a change in the frequency of sound caused by the motion of the source or the observer or both. In sonography, it is the change in frequency of sound caused by the motion of red blood cells.

Question 2

What happens to the echo frequency when red blood cells move toward the transducer?

Answer 2

The echo frequency will be larger than the transducer frequency, indicating antegrade flow relative to the probe.

Question 3

What happens to the echo frequency when red blood cells move away from the transducer?

Answer 3

The echo frequency will be smaller than the transducer frequency, indicating retrograde flow.

Question 4

What methods do sonographers use to detect and analyze Doppler shifts? 3

Answer 4

Sonographers use
1. spectral waveforms
2. Colour flow imaging
3. Audible sounds.

Question 5

What information can be determined from Doppler analysis? 6

Answer 5

1. Presence of flow
2. Direction of flow
3. Quality of flow
4. Velocity of RBCs
5. Resistance/pulsatility
6. Timing.

Question 6

How is the Doppler shift expressed mathematically?

Answer 6

∆𝒇𝒇 = 𝒇𝒓𝒓 − 𝒇𝒕𝒕, where fr is the returned frequency and ft is the transmitted frequency.

Question 7

What factors influence the Doppler shift? 3

Answer 7

1. Transmitted frequency
2. Velocity of the moving blood
3. Angle between the moving blood and the sound beam.

Question 8

What is the significance of the angle of insonation?

Answer 8

It is the most important factor influencing the calculation of the velocity of the RBCs.

Question 9

What is spectral analysis?

Answer 9

Spectral analysis separates received Doppler shift frequencies into individual frequency components using fast Fourier transform (FFT).

Question 10

What is color flow imaging?

Answer 10

Color flow imaging assesses Doppler shifts using color to represent the mean frequency shift calculated by multiple pulses sent along a scan line.

Question 11

What does color Doppler provide?

Answer 11

Mean velocities and direction of flow, but it is considered qualitative information.

Question 12

What is power Doppler?

Answer 12

Power Doppler measures the intensity of the signal rather than Doppler shifts, making it more sensitive to flow.

Question 13

What are some common Doppler artifacts? 4

Answer 13

1. Noise or blooming
2. Flash or clutter
3. Aliasing
4. Mirror image.

Question 14

What is aliasing in Doppler ultrasound?

Answer 14

Aliasing occurs when the Nyquist limit is exceeded, resulting in inaccurate display of information.

Question 15

How can aliasing be compensated for? 5

Answer 15

1. Moving the baseline
2. Increasing the PRF
3. Increasing the Doppler angle
4. Lowering the operating frequency
5. Changing to continuous wave.

Question 16

What is the purpose of the wall filter?

Answer 16

To eliminate low frequency noise from the spectral tracing.

Question 17

What is the role of gain in Doppler optimization?

Answer 17

Gain controls the amplitude of the returned Doppler shift information, affecting the visibility of the spectral signal.

Question 18

What is the ideal gate size for spectral Doppler?

Answer 18

Approximately one third of the vessel diameter to capture the highest velocities from the center of the vessel.

Question 19

What does the baseline adjustment do in Doppler?

Answer 19

It allows the entire spectral signal to be seen and prevents aliasing.

Question 20

What is the effect of adjusting the color box size?

Answer 20

The width impacts the frame rate and must be balanced with temporal resolution.

Question 21

How can aliasing be compensated in a lab setting?

Answer 21

Increase the Doppler angle (heel or toe probe) or lower the operating frequency.

Change to continuous wave.

Question 22

What is a mirror image artifact?

Answer 22

An artifact that can affect both colour and spectral tracings, demonstrating Doppler flow on both sides of the baseline due to over gaining or an angle of incidence too close to 90°.

Question 23

What happens if the gain is set too high in Doppler imaging?

Answer 23

The strength of the voltages can bleed into the adjacent channel, resulting in flow on both sides of the baseline.

Question 24

What occurs with colour Doppler when a strong specular reflector is present?

Answer 24

A false vessel will appear deep to the real one.

Question 25

What is the main difference between Pulsed Wave Doppler (PW) and Continuous Wave Doppler (CW)?

Answer 25

PW produces sound pulses at regular intervals, allowing isolation of signals from a desired depth, while CW continuously transmits sound without depth selection.

Question 26

What is the limitation of Pulsed Wave Doppler (PW)?

Answer 26

Sampling too little can result in aliasing, and the machine cannot send out the next pulse until the first is returned.

Question 27

What is one advantage of Continuous Wave Doppler (CW)?

Answer 27

It can measure very high velocities with no aliasing since there is no Nyquist limit.

Question 28

What is the Zone of Sensitivity in Continuous Wave Doppler?

Answer 28

The overlap between the transmitted sound and the returning echoes where sampling occurs.

Question 29

What is required for a Continuous Wave Doppler (CW) scanner to function?

Answer 29

Two piezoelectric elements are required to send and receive sound at the same time.

Question 30

What type of image is produced with Continuous Wave Doppler?

Answer 30

Only a spectral tracing is acquired; no 2D image is produced.