# Doppler Flashcards

1
Q

What is a doppler shift?

A

A change in sound frequency caused by motion between the transmitted and received frequency
Difference between the transmitted and received frequency
Doppler shift = received - transmitted

2
Q

What is another word to describe doppler shift?

A

Doppler frequency

3
Q

What does a positive doppler shift indicate?

A

The source is moving to the transducer

The reflected frequency is higher than transmitted frequency

4
Q

What does a negative doppler shift indicate?

A

The source is moving away from the transducer

The reflected frequency is lower than transmitted frequency

5
Q

What are typical values for Doppler shift?

A

20-20,000 Hz

Audible

6
Q

What is demodulation?

A

It extracts the lower frequency doppler shift from the higher frequency transducer

7
Q

What is the doppler equation?

A

Doppler shift = (2 x velocity of blood x transducer freq. x cos0)/prop. speed

8
Q

How is doppler shift related to velocity?

A

Doppler shift directly related to velocity

Faster the velocity, the greater the doppler shift

9
Q

How is doppler shift related to frequency?

A

Doppler shift directly related to transmitted sound frequency
Transducer frequency doubled, doppler shift doubled

10
Q

What happens if doppler angle is not parallel (0 or 180) to the direction of flow?

A

The doppler shift is not 100%

Velocity is determined by the cosine of the beam angle compared to motion

11
Q

What is the equation for measured velocity?

A

Measured velocity = true velocity x cos0

12
Q

What is the cosine of 0?

A

1

13
Q

What percent of doppler shift is measured at 0 degrees?

A

100%

14
Q

What is the cosine of 60?

A

0.5

15
Q

What is the percentage of doppler shift at 60 degrees?

A

50%

16
Q

What is the cosine of 90?

A

0

17
Q

What is the percentage of doppler shift at 90 degrees?

A

0%

18
Q

What type of signal processing is used for bidirectional doppler?

A

Flow to transducer is above line, flow away is below line

19
Q

What does speed measure?

A

Only magnitude

20
Q

What does velocity measure?

A

Magnitude and direction

21
Q

How many crystals does CW use?

A

Two
One always transmitting
One always receiving

22
Q

What are the advantages of CW doppler?

A

Accurately measure high velocities w/ no aliasing

Higher sensitivity for detecting smaller shifts

23
Q

What is the disadvantage of CW doppler?

A

Cannot determine the exact location or depth of the moving blood cells (range ambiguity)

24
Q

How many crystals does PW doppler use?

A

Only one crystal must be used

Alternates between sending and receiving sound pulses

25
Q

What are the advantages of PW doppler?

A

Select the exact location where velocities are measured

26
Q

What is the disadvantage of PW doppler?

A

Aliasing occurs and we cannot accurately measure high velocities

27
Q

What term is used to describe systems that perform imaging and PW doppler at the same time?

A

Duplex imaging

28
Q

When does aliasing occur?

A

When high velocities of PW doppler are incorrectly displayed as being negative
If PRF is too low relative to doppler shift or when doppler shift reaches nyquist limit

29
Q

What is the Nyquist limit?

A
```The doppler frequency at which aliasing occurs
Nyquist limit (kHz) = PRF/2```
30
Q

What are the two main ways to avoid aliasing?

A

Reduce doppler shift

Raise Nyquist limit

31
Q

What can you do to reduce doppler shift?

A

Change to a lower frequency transducer

32
Q

What can you do to raise the Nyquist limit?

A

Take a volume from a shallower location because PRF is increased when depth is decreased
Increase the scale, which increases PRF
PRF increase raises Nyquist limit

33
Q

How might adjusting the scale eliminate aliasing?

A

It maximizes the PRF, which increases Nyquist limit

34
Q

How might changing depths eliminate aliasing?

A

When the sample volume is taken from shallower depth, PRF is increased, raising Nyquist limit and reducing aliasing

35
Q

How might a lower frequency transducer affect aliasing?

A

Lower frequency transducers cause lower doppler shifts, which are less likely to exceed Nyquist limit or to alias

36
Q

How might shifting the baseline affect aliasing?

A

It slides the baseline down so the entire velocity is in the same direction, eliminating the appearance of aliasing
Only changes scale appearance

37
Q

What is the advantage of using CW doppler in aliasing situation?

A

Aliasing does not occur in CW

38
Q

What two factors affect the shades of gray on the doppler spectrum?

A

Amplitude of the reflected signal

Number of blood cells causing reflection

39
Q

Does color doppler use pulsed ultrasound?

A

Yes

40
Q

Is color doppler subject to range resolution and aliasing?

A

Yes

41
Q

What information does color doppler provide?

A

Direction or location of flow

Average velocities

42
Q

What velocities does color doppler measure?

A

Average or mean velocities

43
Q

What velocities does spectral doppler measure?

A

Peak velocities

44
Q

What information does the color map provide?

A

Direction and velocity of flow

45
Q

How can you know if flow has a positive or negative doppler shift based on color map?

A

Positive is the color above the black stripe

Negative is the color below the black stripe

46
Q

What is a doppler packet?

A

A group of multiple pulses that are used to accurately determine RBC velocities using doppler

47
Q

What are advantages to a longer doppler packet?

A

More accurate for measurement

More sensitive to low flow

48
Q

What are disadvantages to a longer doppler packet?

A

More time is needed to get data

Reduced frame rate and temporal resolution

49
Q

What are other terms used to describe a doppler packet?

A

Packet

Ensemble length

50
Q

What is power doppler?

A

Non-directional color doppler that displays doppler shift without considering speed or direction

51
Q

What are other terms for power doppler?

A

Energy mode

Color angio

52
Q

What are advantages of power doppler?

A

Increased sensitivity to low flow
Not angle dependent (unless 90)
No aliasing

53
Q

What are limitations of power doppler?

A

No direction or velocity info
Lower frame rate and temporal resolution
Soft tissue, transducer or patient movement may be displayed

54
Q

What is clutter?

A

Low frequency shift artifacts on spectral doppler displays created by slow moving anatomy

55
Q

What is ghosting?

A

Low frequency shift artifacts on color doppler displays created by slow moving anatomy

56
Q

What does wall filter do?

A

Eliminates the low frequency doppler shifts from moving anatomy, but leaves those high frequency from blood cells

57
Q

What is another name for wall filter?

A

High pass filters

58
Q

What is cross talk?

A

Mirror artifact image that arises from spectral doppler

Shows spectral doppler above and below baseline

59
Q

When does cross talk occur?

A

When doppler gain is too high or incident angle is near 90 degrees

60
Q

What is spectral analysis?

A

Tool that breaks the complex signal into the basic building blocks, identifying individual velocities that make reflected doppler signal

61
Q

What are the two methods of spectral analysis?

A

Fast Fourier Transform

Autocorrelation

62
Q

What is FFT and what type of signals is it used for?

A

A digital technique used in PW and CW doppler

It is highly accurate and displays all the individual velocities that comprise the reflected doppler signal

63
Q

What is laminar flow?

A

Flow in which most cells move at similar velocities, causing a clear spectral window and a spectral trace as a narrow line

64
Q

A

When many different directions and speeds (wider range) are found in the sample volume

65
Q

What is autocorrelation and what type of Doppler is it sued for?

A

It is the digital technique that is used to analyze color flow doppler
It is faster than FFT but less accurate

66
Q

What will happen to color Doppler if the angle of incidence is 90 degrees?

A

There is no doppler shift with 90-degree incidence, either color or spectral
No color appears when angle is at 90 degrees

67
Q

What is the first thing you should do if there is no color flow in a vessel?

A

The first to do if no color shows in a vessel is to check the angle between flow and sound, making sure it is not 90 degrees

68
Q

What causes “Color confetti, and what does this look like?

A

Color confetti is when the color gain is set at a level that is much too high
It looks like color filling the entire color box instead of the appropriate amount of vessel

69
Q

If you are using a linear probe and have already corrected for angle and there is still no color flow in a vessel what is the next setting you should adjust?

A

For a linear probe, after adjusting the angle, you should then adjust the color gain to try to get more color in the vessel

70
Q

In a sector shaped image what is the first setting you should adjust if color flow is not seen?

A

In a sector image, the first thing you should adjust is the color gain

71
Q

When using spectral Doppler what will cause gray scale noise to appear throughout the spectrum?

A

In spectral doppler, noise will be seen throughout the spectrum when the pulsed doppler gain is too high

72
Q

How will aliasing appear on color Doppler?

A

It will appear as one color around the vessel wall, but a different color in the center of the vessel that is incorrect

73
Q

What is the best way to eliminate aliasing on color Doppler?

A

To eliminate aliasing in color doppler, it is best to increase the velocity scale

74
Q

For color Doppler how does increasing the scale affect sensitivity?

A

Increasing the scale decreases the sensitivity to slow or low flows
Low velocities may be lost

75
Q

What affect does wall filter have on color Doppler?

A

Wall filter removes the color from slowly moving reflectors (blood cells, vibrating tissues) by eliminating low frequency doppler shifts

76
Q

What effect does wall filter have on spectral Doppler?

A

Wall filters eliminate the low velocity flows near the baseline of a doppler spectrum