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US physics > Doppler - Ch 18 > Flashcards

Doppler - Ch 18 Flashcards

1
Q

what is doppler shift/frequency? what is positive change? negative?

A

difference bw received and transmitted frequencies; source + receiver approach each other, reflected frequency is higher than transmitted; source + receiver move apart, reflected frequency is less than transmitted.

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2
Q

what does doppler shift measure? units? typical values, when is this created?

A

frequency shift not amplitude; Hertz, cycles per sec; 20 Hz - 20kHz when sound reflects off red blood cells.

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3
Q

what does demodulation do? what is it performed by?

A

extracts the doppler frequency from the transducer frequency, performed by a demodulator

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4
Q

to get a GENERAL IDEA of whether doppler shift is pos or neg what can you calculate?

A

doppler shift = received frequency - transmitted frequency

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5
Q

what is the actual doppler shift equation?

A

doppler shift = (2 x reflector speed x incident frequ x cos(angle)) / propagation speed

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6
Q

what is doppler shift related to? (what is used as numerator in its equation)

A

blood cell speed, frequency of the transducer, cosine of angle bw flow and sound beam

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7
Q

what is doppler shift inversely related to? (what is used as denominator in its equation)

A

speed of sound in the medium

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8
Q

what does the 2 represent in the doppler equation? what creates this?

A

double doppler shift; 1st occurs when sound strikes rbc, 2nd occurs from sound wave reflecting off rbc and returning to transducer

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9
Q

does doppler measure speed or velocity? why? what is the equation?

A

velocity; bc velocity= magnitude and direction, speed= magnitude only; velocity= true velocity x cos(angle)

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10
Q

why does doppler frequency depend on direction? when the beam is parallel and the angle is 0 in what direction is the flow? 180?

A

the magnitude of shift depends on the cosine angle bw sound beam and direction of blood flow; toward transducer; away from transducer

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11
Q

at what angle is there max (most accurate) doppler shift? no doppler shift? what does cosine = at 60 degrees?

A

0 or 180 degrees (cosine = 1); 90 degrees (cosine = 0) (no flow); 0.5

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12
Q

the angle should never exceed what value?

A

60 degrees

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13
Q

how many crystals are in a CW doppler? how do they work? what is an advantage and disadvantage? what is this doppler aka?

A

2 crystals: one continuously transmits the other continuously receives; high velocities are accurately measured; echoes arise from overlap bw transmit and receive beams (range ambiguity); jet sniffer

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14
Q

how many crystals are in a pulsed wave doppler? how do they work? what is an advantage and disadvantage? what is this doppler aka?

A

one crystal, alternates sending and receiving; range specificity: echoes arise only from sample gate (center of lumen); errors in measuring high velocities; duplex US

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15
Q

what are the axes of a doppler spectrum?

A

x-axis = time y-axis = doppler shift or velocity

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16
Q

what is aliasing? in which doppler does it occur? where is it seen in the spectrum? what is wrap-around aliasing?

A

errors where high velocities appear negative; if the pulsed doppler sampling rate (PRF) is too low compared to doppler shift; grows from top or bottom of a spectrum, never baseline; exhibited with extremely high velocities

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17
Q

what is nyquist frequency? what is the equation (include units)? how can aliasing be eliminated using this value?

A

doppler frequency at which aliasing occurs = 1/2 PRF; nyquist limit (kHz) = PRF/2; can be eliminated by reducing doppler shift or raising nyquist limit

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18
Q

what are 5 ways to eliminate aliasing?

A
  1. use CW doppler
  2. use lower frequ transducer
  3. new view w shallower sample vol
  4. increase the scale, same view
  5. baseline shift (appearance only)
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19
Q

how can lower frequ transducers eliminate aliasing?

A

these reduce the doppler shift and shrink the spectrum

20
Q

how do shallow sample volumes eliminate aliases?

A

provide a diff view, with shallow sample vol PRF and Nyquist frequency is higher

21
Q

increasing the scale (inc PRF) can only be done when system is what? what does this mean?

A

underdriven - eval low velocities

22
Q

when should baseline shifting be used? when is it not best?

A

only alters the display; when wrap around aliasing is present

23
Q

what is the relationship bw sample volume size and doppler spectrum?

A

smaller sample volumes (gates) = doppler spectra w cleaner spectral windows; larger sample vol = filled-in doppler spectra

24
Q

what are gray shades on a doppler spectrum related to?

A
  1. amplitude/strength of reflected signal
  2. concentration of blood cells creating reflection
25
Q

what is color flow doppler? what does it look like?

A

form of 2-D multigated doppler, provides info regarding flow direction; black and white = anatomic structures; color = blood flow velocity

26
Q

color flow is based on pulsed us and is subject to what to things?

A

range resolution or specificity and aliasing

27
Q

color flow measures __________ velocities, pulsed and CW doppler measure _______ velocities

A

mean; peak

28
Q

what is a color map? what do the colors mean?

A

table to convert measured velocities into colors;
1. top color = flow toward transducer
2. bottom color = flow away from transducer

29
Q

when predicting flow direction in a sector image, where should you start?

A

look at the map, in sector the flow direction is ALWAYS from top to bottom color.

30
Q

when predicting flow, what if there is no color present?

A

no doppler shift because of normal incidence

31
Q

what does the color map tell us about velocity?

A

the higher up on the scale the greater the velocity of blood cells towards transducer; the lower on the scale the greater the velocity away.

32
Q

what can the color map tell us when used in variance mode? what do the left and right sides indicate?

A

the extra colors side-to-side tells us presence or absence of turbulence; left = laminar flow; right = turbulent flow

33
Q

what is a doppler packet? why are these used? what is important to consider when determining packet size?

A

multiple pulses (packet or ensemble length); multiple US pulses are needed to accurately determine rbc velocities; must balance size bw accurate velocity measurements and temp res

34
Q

what are 2 advantages when using a large doppler packet as opposed to a smaller one?

A

more accurate doppler:
1. more sensitive to fast velocity or flow;
2. increased sensitivity to low flows

35
Q

what are 2 disadvantages when using a large doppler packet as opposed to a smaller one?

A
  1. requires more time to acquire info
  2. frame rate and temp resolution are reduced
36
Q

what is color power doppler? aka?

A

color doppler where doppler shift is simply colorized, no actual direction or speed calculated; energy mode, color angio

37
Q

advantages and disadvantages of color power doppler?

A
  1. increased sensitivity to slow flows, angle independent, alias-free
  2. very low frame rates = poor temp res, no info on velocity or direction
38
Q

which doppler modality is best suited for this role?
identifies highest velocity jets anywhere along the length of the us beam. range ambiguity. no aliasing.

A

cw doppler

39
Q

which doppler modality is best suited for this role? accurately identifies the location of flow (range resolution). has good temp res. aliasing

A

pw doppler

40
Q

which doppler modality is best suited for this role? provides 2-D flow info directly on anatomic image. size of jet is most affected by doppler gain settings. poorer temp res bc multiple sample vol and packets. pulsed us, subject to range resolution and aliasing

A

color flow doppler

41
Q

which doppler modality is best suited for this role? allows the use of color w low velocities or small volumes of blood flow, great sensitivity

A

color power mode doppler

42
Q

what is spectral analysis? what are the two techniques used to do this?

A

used to identify the individual frequencies making up a complex signal; Fast Fourier Transform (CW and pulsed doppler); autocorrelation (color flow)

43
Q

what is the diff bw autocorrelation and FFT?

A

auto = less accurate but faster than FFT

44
Q

what is a wall filter (what does it do and where)? aka?

A

serve as a reject for doppler. remove low lvl doppler shifts around the baseline, created by moving anatomy rather than rbc; high pass filter

45
Q

what is cross talk? where is it found?

A

special mirror image where doppler spectrum appears above and below the baseline; spectral doppler only

46
Q

what are two causes of cross talk?

A
  1. doppler gain is too high
  2. incident angle near 90 degrees when flow is at focus.

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