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Inc > US Physics 19 > Flashcards

US Physics 19 Flashcards

1
Q

what is demodulation?

A

Extracting doppler waveform from the carrier frequency

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2
Q

What is the 2 in the doppler equation

A

represents 2 doppler shifts. One from moving red blood cells and another shift when sound makes it back to transducer.

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3
Q

What is doppler equation

A

Doppler shift = (2 x velocity of blood x transducer frequency x cos Θ)/propagation speed

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4
Q

What is the cosign of 0

A

1

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5
Q

what is the cosign of 90

A

0

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6
Q

What is the cosign of 60

A

0.5

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7
Q

What is greatest advantage of continuous wave doppler

A

ability to measure accurately even at high velocities

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8
Q

Primary disadvantage of continuous wave doppler and secondary disadvantage

A

range ambiguity - CW doppler signals arise form all blood cells in region overlap beams makes difficulty to get exact location of moving blood cells. Lack of time gain compensation. Lower amplitude reflections of deeper blood cells

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9
Q

What don’t cw dopplers use in device

A

backing material

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10
Q

What does lack of backing material do

A

improved quality factor, lower bandwidth and greater sensistivity

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11
Q

What is disadvantage of pw dopplers

A

they use backing material like imaging transducers

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12
Q

What’s the downside to using backing material

A

worse quality factor, greater bandwidth loss of sensitivity

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13
Q

what’s the nyquist limit

A

amount over which aliasing occurs. Velocity above that of display causes aliasing

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14
Q

Under what circumstances does aliasing occur?

A

Pulsed wave only (not continuous), when the sampling rate is too low compared with the measured velocity

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15
Q

What is equation of nyquist limit

A

PRF/2

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16
Q

How do you avoid aliasing (3 ways)

A

change the doppler shift or increase the PRF, thus increasing the nyquist limit, lower frequency probe

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17
Q

At what depth are you more likely to get aliasing?

A

When insonating deep because PRF is lower

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18
Q

Why does lower frequency probe cause less aliasing

A

Lower amount of doppler shift (think about doppler shift equation)

19
Q

What is downside to adjusting scale to eliminate aliasing?

A

When you adjust scale to max you are also adjusting PRF to maximum. Lowers sensitivity to lower velocity.

20
Q

What does scale adjustment do?

A

When you adjust scale to max you are also adjusting PRF to maximum.

21
Q

Downsides to repositioning probe to eliminate aliasing by finding area where area of clinical interest is more shallow

A

there are no downsides to it

22
Q

Downsides to selecting lower frequency transducer to eliminate aliasing

A

with duplex imaging it can produce lower quality image

23
Q

Downside to using baseline shift to eliminate aliasing?

A

If signal completely wraps around itself it will be ineffective. Otherwise there are none.

24
Q

Can you get aliasing with continuous wave doppler? What is downside to CW?

A

Never. You have range ambiguity.

25
Q

What is range ambiguity and what do you see it with?

A

You don’t know the depth you are working with because doppler information continuously reflects back to source before and after the region of interest.

26
Q

Summarize the strategy employed of the different ways to eliminate aliasing?

  1. Shift baseline
  2. Move to shallower region
  3. Increase scale
  4. Use continuous wave
  5. Reduce probe frequency
A
  1. Display more visually appealing only
  2. Increased PRF
  3. Increased PRF
  4. Never aliases, but range ambiguity
  5. Decrease the doppler shift
27
Q

Compare pulsed doppler to CW doppler transducer (5) qualities

A 
Pulsed/CW
at least 1 crystal/always 2
dampened PZT/No dampening
low Q factor/high q factor
wide bandwidth/low bandwidth
lower sensitivity/highwe sensitivity
28
Q

Effect of frequency on imaging probe vs a doppler probe

A

high frequency - improved resolution with imaging

Lower frequency - avoids aliasing

29
Q

What is difference in what velocity is measured with color vs spectral (pulsed and CW) doppler

A

Pulsed and CW - peak velocity

Color doppler - mean velocity

30
Q

Are color doppler subject to aliasing?

A

yes - it uses pulsed wave doppler

31
Q

What are two types of color mode doppler

A

Variance mode (indicates laminar and turbulent flow in addition to velocity) and velocity mode (indicates just velocity).

32
Q

What is clue to which side is laminar vs turbulent with variance maps

A

laminar = L or left side

33
Q

How are color doppler images created?

A

With packets containing multiple pulses

34
Q

What are upsides/downsides to creating color doppler with packets?

A

Packets composed of large # pulses = more accurate velocity measurement, inc sensitivity to low flow. More time needed to acquire data, lower frame rate and poorer temporal resolution

35
Q

What is power color flow doppler?

A

It displays only power and not speed or direction.

36
Q

3 upsides and downsides to power color flow doppler?

A

3 + inc sensitivity to low flow, unaffected by angles (unless exactly 90 degrees), no aliasing
3- no measurement of velocity or direction, lower frame rates than color flow, susceptible to motion of transducer leading to sudden burst of color

37
Q

what is clutter/ghosting artifact with doppler and what caused by?

A

low frequency doppler shifts due to slow moving stuff like heart

38
Q

When do you see clutter vs ghosting?

A

Clutter - seen with spectral displays

Ghosting - Seen with color flow

39
Q

How do you get rid of clutter/ghosting

A

Wall filter to reject low frequency shifts

40
Q

Another name for wall filter

A

high pass filter

41
Q

What does crosstalk look like and what is cause?

A

Wave above and below baseline appears the same. It reflects either gain set too high or incident angle near 90 degrees.

42
Q

Two ways to perform spectral analysis of doppler waveform

A

autocorrelation and FFT

43
Q

Benefits of FFT over autocorrelation

A

FFT very accurate, but not as fast as autocorrelation and so given large amounts of data autocorrelation used with color flow doppler

44
Q

Typical doppler shift in diagnostic imaging applications

A

between 20Hz AND 20kHz (audible)

Inc (10 decks)

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