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Flashcards in Chapter 19 Deck (100):
1

What is the change in frequency called?

Doppler shift or Doppler freq

2

When does the freq of sound changes?

when the sound source and receiver move closer together or farther apart

3

When does the freq of sound doesn't change?

when the distance between the sound source and receiver remains constant

4

The process of extracting the low Doppler freq from the transducers carrier freq is called what?

demodulation

5

When is the Doppler shift positive?

when the reflected sound has a higher freq than the transmitted sound

6

What is the Doppler shift formula? (2)

Hz=reflected freq- transmitted freq
or
Doppler shift= 2vf (cos)/c

7

When is the Doppler shift negative?

when the reflected freq is lower than the transmitted freq

8

When are Doppler shifts created?

when transmitted sound waves strike moving red blood cells

9

Doppler frequencies indicate___ not speed

velocity

10

What is the speed of Doppler defined as?

speed which a purely a magnitude, indicates the distance that a red blood cell moves in 1 second

11

What is velocity defined as?

defined by magnitude and a direction

12

What is the relationship between velocity of blood and Doppler shift measured in hertz?

directly

13

What does the x-axis of a Doppler spectrum represent?

time

14

what does the y-axis of a Doppler spectrum represent?

Doppler shift or velocity

15

What is the frequency difference?

difference between the received and transmitted sound waves

16

What is the relationship between the transmitted freq and the Doppler shift measured in hertz?

directly

17

When is the entire velocity measured?

when the blood cells are moving parallel to the sound beam

18

If the sound beam and flow direction are not parallel, what determines how much of the velocity is measured?

the percentage of the true velocity that is measured depends on the cosine of the angle between the sound beam and direction of motion.

19

What is the formula for the measured velocity for cosine?

=true velocity x cosine angle

20

What is the relationship between the cosine angle and Doppler shift?

directly

21

What angles are vessels parallel?

zero or 180 degrees

22

What is the cosine of zero?

1

23

What is the cosine of 180?

-1

24

What happens to the measured velocity at angle other than zero or 180?

only a portion of the true velocity is measured

25

What are the values for Doppler shift ranges?

20 hz-20,000 hz

26

What are typical transducer frequencies?

2MHz-10 MHz

27

What is the most common clinical doppler device?

they are bidirectional, which distinguish the direction of low toward or away from the transducer

28

What is commonly used for signal processing technique for bidirectional Doppler?

phase quadrature, or quadrature detection

29

True or false: arterial velocities are higher than venous velocities

true

30

how many crystals are required for a continuous wave Doppler?

two
one to transmit and another to receive

31

what is the advantage of a continuous wave Doppler?

its ability to accurately measure very high velocities.

32

What are some disadvantages of a continuous wave Doppler?

primary disadvantage is that the exact location of moving blood cells cannot be determined.
lack of TGC

33

What is range ambiguity?

signal arise from all blood cells in the region of overlap between the transmit and receive beams. this limitation is called range ambiguity

34

What is duplex imaging?

anatomic imaging and Doppler

35

Do continuous wave transducers have a backing material?

no

36

Since continuous wave transducers do not have a backing material what are the results?

undampened transmitted signal
narrow bandwidth
high quality factor
higher sensitivity

37

What does the matching layer do in transducers?

increase the efficiency of sound transmitted in and out of the body

38

What is the most important thing to remember about dedicated continuous wave transducers?

they have increased sensitivity and they are able to detect low amplitude reflections and small Doppler shifts

39

in a pulsed wave Doppler, how many PZT crystals are necessary?

one
they alternate between sending and receiving sound pulses

40

what are advantages of pulsed wave doppler?

being able to select the exact location where velocities are measured

41

What is it called when you are able to select the exact location where velocities are measured?

range resolution, range specificity, or freedom from range ambiguity artifact

42

What is a small marker called in pulsed doppler?

a gate or sample volume

43

What is a disadvantage of pulsed Doppler?

inaccurate measurement of high velocity signals.

44

What is aliasing?

high velocity flow in one direction is incorrectly display as traveling in the opposite direction. this inaccuracy is called aliasing, a false identity

45

What are the characteristics of pulsed wave Doppler transducer?

low quality factor
lower sensitivity
wide bandwidth pulses

46

The very top of the display in an aliasing spectrum is what?

Nyquist limit

47

What conditions create aliasing?

aliasing occurs only with pulsed Doppler, not with CW doppler
aliasing occurs when the Doppler sampling rate is too low in comparison to the measure blood velocities

48

What is the formula for nyquist limit?

Hz= PRF (Hz)/2

49

What is the nyquist limit?

the highest Doppler freq or velocity that can be measured without the appearance of aliasing

50

What are two ways to avoid aliasing?

raise the Nyquist limit
reduce the Doppler shift

51

do shallow or deep sample volume have more chance of creating aliasing?

deep
b/c PRF is low and nyquist limit is low

52

What does shallow gate mean?

high PRF

53

What does a deep gate mean?

low PRF

54

What is seen with less aliasing?

slower blood velocity
lower freq transducer
shallow gate (high PRF)

55

What is seen with more aliasing?

faster blood velocity
higher freq transducer
deep gate (low PRF)

56

What are five techniques to avoid aliasing?

1. adjust the sale to its max
2. select a new ultrasonic view with a shallower sample volume
3. select a lower freq transducer
4. use baseline shift
5. use a CW Doppler

57

What is the advantage of increasing the PRF?

raises the nyquist limit and reduces aliasing

58

What is the disadvantage of high PRF?

reduced sensitivity to low velocities

59

What advantage of selecting a new ultrasonic view with a shallower sample volume do?

Increasing PRF, increases nyquist limit= reduces aliasing

60

What is a disadvantage of selecting a new ultrasonic view with a shallower sample volume?

none

61

What never occurs with CW Doppler?

aliasing

62

What occurs as a result of no aliasing on CW Doppler?

range ambiguity

63

What does eliminating aliasing improve?

improves the ability to measure the maximum velocity with Doppler

64

What is the strategy for adjusting the baseline?

aliasing remains but the display is more appealing to the eye

65

What are gray shades on a Doppler spectrum related to?

amplitude of the reflected signal
or
number of blood cells creating the reflection

66

What does color doppler provide?

is a pulsed US technique
has range resolution
is subject to aliasing

67

What information does color Doppler provide?

info regarding direction of flow and is considered semi-quantitative

68

What does color doppler report?

mean or average velocities

69

What does spectral (pulsed and CW) doppler report?

peak velocities

70

What are color maps for?

to convert measured velocites into color that appear on the image

71

What are two type of dictionaries for color maps?

velocity mode
variance mode

72

What do the colors in a velocity mode map mean?

the color provide info on flow direction and velocity

73

What does the black region on a color map mean?

no Doppler shift

74

What info do variance mode provide?

in addition to direction and speed info, variance mode distinguishes laminar flow from turbulent flow

75

What does colors on the left side represent on a variance map?

laminar flow

76

What does colors on the right side represent on a variance map?

turbulent flow, which is associated with pathology

77

With variance maps, how do the colors change on the map?

side to side as well as up and down

78

With color Doppler, multiply pulses are used to accurately determine blood velocities. These multiply pulses are called what?

packet or ensemble

79

What are the advantages for packets composed of a larger number of pulses?

more accurate velocity measurement
increased sensitivity to slow flow

80

What are disadvantages of packets with more pulses?

more time needed to acquire data
reduced frame rate
decreased temporal resolution

81

What does power mode identify?

presence of Doppler shift
it doesn't evaluate speed or direction

82

What is power mode?

nondirectional color Doppler

83

What are other names for power Doppler?

energy mode or color angio

84

What is processed in power mode?

the strength (amplitude) of the reflected signal is processed without regard to the bloods direction or speed

85

What are the three advantages of power mode?

1.increased sensitivity to slow flow or velocity, such as venous flow or flow in small vessels
2. unaffected by Doppler angles, unless the angle is exactly 90 degrees
3. no aliasing, since the velocity info is ignored

86

What are three disadvantages of power mode?

1. no measurement of velocity or direction
2. lower frame rates than conventional color flow Doppler
3. susceptible to motion of the transducer, patient, or soft tissues, which may result in a burst of color or flash artifact

87

What are low frequency Doppler shift artifacts called?

clutter

88

What are color doppler artifact called?

ghosting artifact

89

How is clutter or ghosting artifact eliminated?

a wall filter set to high

90

What is another word for wall filter?

high pass filters

91

Wall filter affects what?

low freq Doppler shifts but doesnt affect high freq Doppler shifts

92

What is a special form of mirror image artifact that arises only with spectral Doppler?

crosstalk

93

What does crosstalk results from?

Doppler gain is set too high or
the incident angle is near 90 degrees between the sound beam and the flow direction, when flow is at the beams focus

94

What is spectral analysis?

a tool that breaks the compelx signal into its basic building blocks and identifies the individual velocities that make up the reflected Doppler signal

95

What are two methods of spectral analysis that are commonly used?

Fast Fourier Transform (FFT)
autocorrelation

96

What is a digital technique that is used to process both pulsed and CW Doppler signals?

FFT

97

What are the two advantages of FFT?

exceedingly accurate
displays all individual velocity components that make up the complex reflected signal

98

The ___ display of the FFT distinguishes laminar flow from turbulent flow?

spectral

99

What is the wider range of velocities and Doppler shift within the sample volume?

spectral broadening

100

What is the digital technique used to analyze color flow Doppler?

autocorrelation or correlation function