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Ultrasound Physics > 17 Doppler > Flashcards

17 Doppler Flashcards

1
Q

What is Doppler shift or frequency?

A

Change or variation in frequency of sound as a result of motion between the sound source and receiver

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2
Q

What is a positive change or shift?

A

When source and receiver are approaching each other

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3
Q

What is a negative change or shift?

A

When source and receiver are moving apart

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4
Q

How is Doppler created?

A

When sound reflects off of RBCs

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5
Q

What is the range of Doppler shift?

A

20Hz - 20 kHz

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6
Q

What is demodulation?

A

Extraction of the Doppler frequency from the transducer frequency

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7
Q

How is bidirectional Doppler analyzed?

A

With phase quadrature processing

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8
Q

What is the equation for Doppler shift?

A

Doppler shift = received frequency - transmitted frequency

FD = FR - FE, FE = emitted freq, FR = reflected freq

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9
Q

What is THE Doppler equation?

A

Doppler shift = (2 x reflector speed x incident frequency x cos(angle)) / propagation speed

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10
Q

What is Doppler shift directly related to?

A

Blood cell speed
Frequency of the transducer
Cosine of the angle between flow and the sound beam

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11
Q

What is Doppler inversely related to?

A

Speed of sound in the medium

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12
Q

What does Doppler measure and not measure?

A

Measures velocity, not speed

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13
Q

What is the equation for measured velocity?

A

Velocity (measured) = true velocity x cos(angle)

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14
Q

What is the # of crystals in a continuous wave doppler transducer?

A

2 crystals

1 is continuously transmitting and the other is continuously receiving

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15
Q

What is range ambiguity?

A

Echoes that arise from entire length of overlap between the transmit and receive beams

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16
Q

What is the advantage of continuous wave doppler?

A

High velocities are accurately measured

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17
Q

How many crystals are in a pulsed wave doppler?

A

1 crystal that alternates between sending and receiving

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18
Q

What is the advantage of a pulsed wave doppler?

A

Echoes come only from the area of interrogation, the sample volume or gate

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19
Q

What is the disadvantage to pulsed wave doppler?

A

Aliasing, errors in measuring high velocities

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20
Q

What is the horizontal and vertical axes of a Doppler spectrum?

A

Horizontal - time

Vertical - frequency or velocity

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21
Q

What is aliasing?

A

When high velocities appear negative with pulsed Doppler

High velocity measurements are inaccurate if PRF is too low in comparison to the measured Doppler shift

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22
Q

What is Nyquist frequency?

A

Doppler frequency at which aliasing occurs

= to 1/2 the PRF

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23
Q

How is aliasing eliminated?

A

Use continuous wave Doppler
Select a transducer with a lower frequency
Select new view with a shallower sample volume
Increase the scale, same view
Baseline shift

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24
Q

What will reduce Doppler shift and shrink the spectrum?

A

Lower frequency transducers

25
Q

What occurs with a shallow sample volume?

A

PRF and therefore Nyquist frequency is higher

26
Q

What is the relationship between sample volume size and doppler spectrum?

A

Smaller sample volumes (gates) create Doppler spectra with cleaner spectral window

27
Q

What are the gray shades of a spectrum related to?

A

Amplitude of the reflected signal

Number of RBCs creating the reflection

28
Q

What is color flow Doppler?

A

Form of 2-D multigated Doppler where the shifts are coded into colors and superimposed on a 2-D image

29
Q

What is color Doppler based on?

A

Pulsed ultrasound

30
Q

What is color Doppler subjected to?

A

Range resolution or specificity

Aliasing

31
Q

What is velocity vs variance mode?

A

Look-up tables that color doppler uses to convert measured velocities into colors
Velocity - colors on map do not vary side-to-side
Variance - colors on map vary side-to-side

32
Q

What do the different colors tell us on velocity mode?

A

The colors are present information on flow direction only

33
Q

What do the different colors tell us on variance mode?

A

The colors provide information on flow direction and the presence or absence of turbulence

34
Q

What does the appearance of the colors located on the either side of the color bar mean in variance mode?

A

The right side shows turbulent or disturbed flow that is random and chaotic
The left side shows laminar or parabolic flow which is uniform and smooth

35
Q

What are Doppler packets?

A

Multiple ultrasound pulses, or ensemble length

36
Q

What is associated with a small packet?

A

Less accurate Doppler
Less sensitive to low velocity flow
Higher frame rate, improved temporal resolution

37
Q

What is associated with a large packet?

A

More accurate Doppler
More sensitive to low velocity flow
Lower frame rate, reduced temporal resolution

38
Q

What are advantages to more pulses in a packet?

A

Greater accuracy of the velocity measurement

Sensitivity to low flows is also increased with many pulses in the packet

39
Q

What are disadvantages to more pulses in a packet?

A

More time is required to acquire the information

Frame rates and temporal resolution are reduced

40
Q

What is color power Doppler?

A

Dim-witted color Doppler where the Doppler information regarding direction and actual velocity is not calculated

41
Q

What are advantages of color power Doppler?

A

Increased sensitivity to low flows
Not affected by Doppler angles unless the angle equals 90°
No aliasing

42
Q

What are limitations to color power Doppler?

A

No measurement of velocity or direction
Lower frame rates
Susceptible to motion of transducer, patient or soft tissues = flash artifact

43
Q

What is the role of continuous wave Doppler?

A

Identifies highest velocity jets anywhere along the length of the ultrasound beam; range ambiguity; no aliasing

44
Q

What is the role of pulsed wave Doppler?

A

Accurately identifies the location of flow; has good temporal resolution; range resolution, aliasing

45
Q

What is the role of color flow Doppler?

A

Provides 2-D flow information directly on anatomic image

Poor temporal resolution because of multiple packets; pulsed ultrasound, subject to range resolution and aliasing

46
Q

What is the role of power mode Doppler?

A

Allows the use of color with low velocities or small volumes of blood flow; greatest sensitivity

47
Q

What is spectral analysis?

A

Extraction or identification of the individual frequencies making up the complex signal of an echo returning after striking mass of moving blood cells

48
Q

What are current methods of spectral analysis?

A

Pulsed or continuous wave Doppler - Fast Fourier Transform

Color flow Doppler – autocorrelation or correlation function

49
Q

How does anemia affect Doppler?

A

Hemodynamics may differ from normals

A Doppler exam can always be successfully performed regardless of the patient’s hematocrit

50
Q

At what angle will a transducer measure the greatest Doppler shift?

A

0° or parallel

51
Q

Where must a transducer be located to measure a reflection with the greatest amplitude?

A

Blood cells that are located closest to the transducer, which also undergo less attenuation

52
Q

What is ghosting?

A

Doppler artifact that results from low velocity motion from pulsating vessel walls that can also produce small Doppler shifts that “bleed” into the surrounding anatomy

53
Q

What is a high pass filter?

A

Eliminates low magnitude Doppler shifts that are created from moving anatomy rather than red blood cells

54
Q

What do wall filters serve?

A

A “reject” for Doppler

They exclude low-level Doppler shifts around the baseline while having no effect on large Doppler frequency shifts

55
Q

In what type of flow do wall filters eliminate color?

A

Slow flows

They do not change fast flows

56
Q

How do you optimize the following color Doppler images?
Too much color – color throughout image, confetti
Too much color – color extending outside of vessel lumen

A

Color Doppler gain too high – decrease color gain

Color assigned to low velocities – increase wall filter

57
Q

How is the following color Doppler image optimized?

Right color, color in lumen only – rainbow, multiple colors in lumen

A

Color aliasing – increase color scale

58
Q

How are the following color Doppler images optimized?

Too little or no color

A

90° angle between flow and sound – steer color box or reposition transducer
Color Doppler gain too low – increase color gain
Scale too high, making system insensitive to slow flows – decrease scale
Wall filter too high when actual velocities are slow – decrease wall filter

Ultrasound Physics (20 decks)

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