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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
What occurs with a shallow sample volume?
PRF and therefore Nyquist frequency is higher
26
What is the relationship between sample volume size and doppler spectrum?
Smaller sample volumes (gates) create Doppler spectra with cleaner spectral window
27
What are the gray shades of a spectrum related to?
Amplitude of the reflected signal | Number of RBCs creating the reflection
28
What is color flow Doppler?
Form of 2-D multigated Doppler where the shifts are coded into colors and superimposed on a 2-D image
29
What is color Doppler based on?
Pulsed ultrasound
30
What is color Doppler subjected to?
Range resolution or specificity | Aliasing
31
What is velocity vs variance mode?
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
What do the different colors tell us on velocity mode?
The colors are present information on flow direction only
33
What do the different colors tell us on variance mode?
The colors provide information on flow direction and the presence or absence of turbulence
34
What does the appearance of the colors located on the either side of the color bar mean in variance mode?
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
What are Doppler packets?
Multiple ultrasound pulses, or ensemble length
36
What is associated with a small packet?
Less accurate Doppler Less sensitive to low velocity flow Higher frame rate, improved temporal resolution
37
What is associated with a large packet?
More accurate Doppler More sensitive to low velocity flow Lower frame rate, reduced temporal resolution
38
What are advantages to more pulses in a packet?
Greater accuracy of the velocity measurement | Sensitivity to low flows is also increased with many pulses in the packet
39
What are disadvantages to more pulses in a packet?
More time is required to acquire the information | Frame rates and temporal resolution are reduced
40
What is color power Doppler?
Dim-witted color Doppler where the Doppler information regarding direction and actual velocity is not calculated
41
What are advantages of color power Doppler?
Increased sensitivity to low flows Not affected by Doppler angles unless the angle equals 90° No aliasing
42
What are limitations to color power Doppler?
No measurement of velocity or direction Lower frame rates Susceptible to motion of transducer, patient or soft tissues = flash artifact
43
What is the role of continuous wave Doppler?
Identifies highest velocity jets anywhere along the length of the ultrasound beam; range ambiguity; no aliasing
44
What is the role of pulsed wave Doppler?
Accurately identifies the location of flow; has good temporal resolution; range resolution, aliasing
45
What is the role of color flow Doppler?
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
What is the role of power mode Doppler?
Allows the use of color with low velocities or small volumes of blood flow; greatest sensitivity
47
What is spectral analysis?
Extraction or identification of the individual frequencies making up the complex signal of an echo returning after striking mass of moving blood cells
48
What are current methods of spectral analysis?
Pulsed or continuous wave Doppler - Fast Fourier Transform | Color flow Doppler – autocorrelation or correlation function
49
How does anemia affect Doppler?
Hemodynamics may differ from normals | A Doppler exam can always be successfully performed regardless of the patient's hematocrit
50
At what angle will a transducer measure the greatest Doppler shift?
0° or parallel
51
Where must a transducer be located to measure a reflection with the greatest amplitude?
Blood cells that are located closest to the transducer, which also undergo less attenuation
52
What is ghosting?
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
What is a high pass filter?
Eliminates low magnitude Doppler shifts that are created from moving anatomy rather than red blood cells
54
What do wall filters serve?
A "reject" for Doppler | They exclude low-level Doppler shifts around the baseline while having no effect on large Doppler frequency shifts
55
In what type of flow do wall filters eliminate color?
Slow flows | They do not change fast flows
56
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
Color Doppler gain too high – decrease color gain | Color assigned to low velocities – increase wall filter
57
How is the following color Doppler image optimized? | Right color, color in lumen only – rainbow, multiple colors in lumen
Color aliasing – increase color scale
58
How are the following color Doppler images optimized? | Too little or no color
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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