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phys 314 > Doppler > Flashcards

Doppler Flashcards

1
Q

How does the frequency for Doppler differ from the frequency for 2D images?

A

Doppler frequency is lower to account for the attenuation from the RBC’s

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2
Q

How does the pulser improve accuracy when using doppler?

A

It generates longer 5-30 cycle pulses so that the entire cardiac cycle can be observed.

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3
Q

The Doppler shift detection is typically the result of what?

A

Quadrature phase detector

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4
Q

What is a quadrature phase detector? (4 steps)

A
  1. The transmitted signal is duplicated and stored (copied)
  2. The two signals are sent out 1/4 out of phase with eachother.
  3. The returning signal is compared to the transmitted signal to determine a pos or neg shift
  4. Signal sent to a loudspeaker or spectral analyzer for image processing
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5
Q

The Doppler shift signal contains what?

A

It is complex and contains all the shifts that occur in the sample gate

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6
Q

What does the spectrum analyzer do?

A

It is a means of separating the individual Doppler shifts from the complex beat frequency (like a prism separating white light into colours)

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7
Q

What is fast fourier transform?

A

The process by which doppler signals are separated into Doppler shifted frequencies

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8
Q

The Fast Fourier Transform analyzes the complex Doppler shift signal and then breaks it down by displaying what?

A

Magnitude and amplitude

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9
Q

What is magnitude?

A

The amount of positive or negative shift

Ex. Can be +4 kHz or -4kHz (same magnitude)

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10
Q

What does the amplitude of the Doppler shift represent?

A

Number of RBC’s that are contained within the sample volume

Ex. Two 4 kHz shifts have the same magnitude

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11
Q

What is the doppler shift equation?

A

Change in f = (2 X fo X vel X Costheta) / C

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12
Q

Sample volume is comprised of what? (3)

A

Beam width, gate length, and pulse length

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13
Q

What limits the amount of sampling?

A

Aliasing: when the Nyquist limit is exceeded (1/2 PRF is exceeded)

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14
Q

What are the 5 ways to correct for aliasing?

A
  1. Change the baseline
  2. Increase the PRF/ scale (sample faster)
  3. Lower the frequency
  4. Increase the Doppler angle
  5. Switch to CW (no image)
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15
Q

What is high PRF mode?

A

When the velocities exceed the Nyquist limit some machines are able to fire the next pulse before waiting for all of the echoes to return

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16
Q

What are fast sweeps used for? Why?

A

Intracycle measurements. Easier caliper placement and increased accuracy for systolic events

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17
Q

What are slow sweeps used for?

A

Intercycle measurements like rate and rhythm

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18
Q

CW has an oscillator that produces what?

A

A continuous AC voltage for motion detection

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19
Q

Since Doppler is sensitive to any motion, what is used to reject low frequencies from motion?

A

Wall filter

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20
Q

What is a consequence of setting the wall filter too high or too low?

A

Too high = It can reject real blood velocities in the lower frequency range (near baseline)

Too low = clutter and distortion

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21
Q

What are synonyms for Colour Doppler?

A

Colour Doppler Imaging (CDI), Colour Flow Imaging (CFI), Colour Flow Mapping (CFM)

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22
Q

Colour Doppler is processed using a technique called what?

A

Autocorrelation

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23
Q

Ensemble length is another way of saying what?

A

Packet size (Number of pulses used for each colour scan line)

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24
Q

What is another name for sign, magnitude and power?

A

Sign = Hue

Magnitude = Saturation

Power = Luminance

25
Q

How does the ensemble length affect frame rate?

A

Longer ensemble length = slower frame rate

26
Q

Better luminance =

A

A better signal

27
Q

Define ‘hue’:

A

Color that you see, represents sign (direction of flow)

28
Q

Define ‘saturation’:

A

Represents the magnitude of the Doppler shift or how fast the blood is travelling (More white = less saturation, faster flow)

29
Q

Define ‘luminance’:

A

Brightness of the hue and saturation. Represents the power of the flow, tells us the RBC density

30
Q

Define ‘variance’:

A

Displays the amount of velocities that are different than the mean (gives more detail than averaged velocities seen in a hue or saturation map).

31
Q

Define ‘priority’ and low/high:

A

Determines whether or not the colour overlay will overwrite the low level echoes

High priority = precedence to colour (may overwrite pathology)
Low priority = precedence to 2D

32
Q

Energy mode is also known as what?

A

Power Doppler

33
Q

In Power Doppler, the colour displayed represents what?

A

Strength of the Doppler shift (instead of velocity)

34
Q

Name the pros (3) and cons (4) of power Doppler:

A

Pros:

  • Free of aliasing
  • Less dependent on angle
  • Very sensitive to low flow states

Cons:

  • No direction
  • No flow speed
  • No character info
  • Susceptible to blooming artifact
35
Q

Does the transducer differ between 2D and Doppler applications?

A

No

36
Q

How do you determine if flow is in phase or out of phase?

A

Quadrature Phase Detector

37
Q

What is another name for the transmitted signal?

A

Reference signal

38
Q

What is considered “good” doppler and what range can FFT produce?

A

FFT = 100-1000 spectra per second

Good Doppler = 1000 spectra per second

39
Q

How are the doppler shifts plotted?

A

On the y-axis (over time)

40
Q

What are the factors affecting magnitude?

A

All elements of the doppler shift equation and:

  • RBC velocity
  • Angle of insonation
  • Operating frequency

Know relationships

41
Q

What are the factors affecting amplitude (brightness)?

A
  • RBC density (increase)
  • Attenuation (decrease)
  • Power and gain (increase)
42
Q

How does the PW Doppler sample volume change?

A

It is a 3 dimensional volume that changes in size relative to depth, when placed in the far field it will be very large.

43
Q

What is responsible for picking up the waveforms of two neighbouring vessels simultaneously?

A

Sample volume

44
Q

Where does FFT happen?

A

Spectrum analyzer

45
Q

What is bin?

A

The density of RBC’s at a certain time (brightness) similar to a pixel

46
Q

Why can the amplitude be inaccurate?

A

Due to scatter (constructive and deconstructive interference changes the amplitudes artificially)

47
Q

What artifact are small gates associated with?

A

Spectral Broadening

48
Q

A low saturation will have what velocity?

A

High velocity = low saturation (white)

49
Q

What is the consequence of High PRF mode?

A

Range ambiguity where flow may be picked up from more than one depth resulting in a spectral tracing of multiple vessels due to multiple gates

50
Q

What is a good solution for aliasing in echo?

A

Switch to CW

51
Q

What can be done with a CW echo signal?

A
  • Sent to speaker

- Processed into an image

52
Q

Why does doppler affect frame rate?

A

Several pulses per scan line are needed for 2D with colour overlay (reduces frame rate)

53
Q

What is the minimum amount of pulses needed for CD?

A

3

54
Q

What is autocorrelation? (5)

A

A mathematical means of quickly determining the sign, mean, magnitude, power and variance of the returning signal.

55
Q

How many doppler samples are typically displayed per scan line and what does this affect?

A

100-400

Reduces frame rate

56
Q

What frame rate does colour doppler have?

A

5-50 Hz

57
Q

What does the colour data in an image represent?

A

The mean or average of the velocities present

58
Q

How many pulses are required to calculate a mean velocity?

A

Min = 3 pulses

Common = 10-20 pulses (more accurate)

59
Q

If variance is on, what may be seen?

A

Aliasing (more velocities).

phys 314 (5 decks)

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