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Physics 3 > M7: Doppler > Flashcards

M7: Doppler Flashcards

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1
Q

block diagram for doppler instrumentation

A

TX > beam former > signal processor > image processor and sound production

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2
Q

2 parts of the signal processor for doppler

A

doppler shift detector

spectrum analyzer

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3
Q

how does frequency change when we scan in 2D vs doppler

A

frequency is lower for doppler than for 2D due to more attenuation from the RBCs (Rayleigh scatter)

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4
Q

how many cycles per pulse does the pulser generate for doppler

why

A

5-30 cycle pulses

to improve accuracy and interact w/ the cardiac cycle longer

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5
Q

doppler shift detection is the result of which process

how does it work

A

quadrature phase detector

the transmitted signal (reference signal) is duplicated and stored… the duplicated reference signals are the same except they are 1/4 out of phase w/ each other…
… the returning signal is compared to the reference signal to determine a + or – shift, which determines the resultant signal

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6
Q

when you see quadrature phase detector what do you think

A

think 1/4 out of phase, and trying to detect + or – changes

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7
Q

where is the resultant signal sent

A

to a speaker or spectral analyzer for image processing

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8
Q

what does the overall doppler shift signal contain

A

contains all the shifts that occur in the sample gate

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9
Q

how does the spectrum analyzer work

A

it separates the individual doppler shifts from the complex beat frequency… (this is where fast Fourier transformation occurs)

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10
Q

the phenomenon of fast Fourier transformation is synonymous to what

A

a prism separating the different hues that comprise white light

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11
Q

how is the doppler signal separated into the doppler shifted frequencies (e.g. how is the spectral doppler tracing produced)

A

fast Fourier transformation

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12
Q

describe how fast Fourier transformation works

A

it analyzes the complex doppler shift signal and breaks the signal into the component or separate frequencies displaying both magnitude and amplitude

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13
Q

which axis displays magnitude

which displays time

A

mag: Y
time: X

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14
Q

what is the magnitude of the doppler shift

A

the amount of + or – shift

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15
Q

is a + 4 kHz doppler shift the same magnitude as a – 4 kHz shift

A

yes… refers to ‘distance’ from the baseline only… magnitudes are the same, but the sign is opposite

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16
Q

factors affecting magnitude

A

RBC velocity, angle of insonation, operating frequency

e.g. everything in the doppler shift formula except C which is constant

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17
Q

relationship b/w doppler shift and magnitude

A

greater doppler shift = greater magnitude

directly related

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18
Q

what is the amplitude of the doppler shift

A

represents the number of RBCs that are contained w/in the SV

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19
Q

other names for amplitude

A

brightness, power, intensity

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20
Q

will two individual doppler shifts of 4 kHz have the same amplitude

A

no. they have the same magnitude but may not have the same # of RBCs moving through the SV… depends on RBC density

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21
Q

how is amplitude displayed on the spectral tracing

A

Z-axis or brightness of the tracing

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22
Q

factors affecting amplitude

A

RBC density
attenuation
power
gain

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23
Q

what is the SV comprised of

A

beam width, gate length (SV size) and pulse length

as well as size in the Z axis

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24
Q

is the SV range gated?

what does that mean

A

yes

we know how long it takes the sound to go out the sampled depth and come back because we are able to specify a desired depth

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25
Q

is the SV of PW doppler three dimensional

A

yes

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26
Q

does the SV size change relative to depth

what happens when its placed in the far field

A

yes…

it becomes very large due to the divergence in 3 dimensions…. there is the potential for spectral broadening and picking up signals from other vessels

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27
Q

why does aliasing occur

A

the velocity of the RBCs are exceeding the sampling limit of the machine w/ PW… we are exceeding the nyquist limit which is 1/2 the PRF

MUST BE OVER HALF THE PRF

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28
Q

what limits the amount of sampling

A

depth (PRF)

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29
Q

formula for nyquist limit

A

NL = 1/2 PRF

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30
Q

5 ways to correct for aliasing

which ones do we do

A 
change the baseline
increase the PRF (scale)
lower the frequency
increase the doppler angle
switch to CW

the first 2

31
Q

why cant we lower the frequency, increase the doppler angle or switch to CW to correct for aliasing

A
  1. usually defaulted to lowest frequency already
  2. increase % error
  3. dont usually have correct probe, but good in the echo lab
32
Q

describe high PRF mode

A

can be used when the velocities exceed the nyquist limit… allows the machine to fire the next pulse before the echos from the first pulse have come back

33
Q

purpose of high PRF mode

possible consequence

A

can help raise the nyquist limit

range ambiguity

34
Q

how can range ambiguity affect the spectral tracing

A

can cause you to pick up flow from multiple depths resulting in a complex spectral tracing that displays multiple flow profiles of vessels

35
Q

with what type of measurements should you use a fast sweep speed

why

A

intracycle measurements (AR, PR) due to easier caliper placement and increased accuracy for systolic events since the waveform is more spread out

36
Q

with what type of measurements should you use a slow sweep speed

why

A

intercycle measurements like rate and rhythm

37
Q

describe how CW works

A

CW has an oscillator that produces a continuous AC voltage for motion detection.. the echo signal can be sent to a speaker to processed into an image/spectra tracing just like PW

38
Q

newer development w/ CW probes

A

fire half of the crystals with pulsed to get a 2D image and the other half of the crystals w/ CW to get a CW signal… allows for 2D and CW simultaneously

BUT CW STILL DOESN’T PRODUCE AN IMAGE

39
Q

describe wall filter

why is it needed

A

used to reject low frequencies from wall motion of pulsatile vessels…. e.g. gets rid of noise but

needed b/c doppler is sensitive to ANY motion, not just moving RBCs

40
Q

risk w/ a high wall filter

A

could reject real, low level echos in the lower frequency range if set to high

41
Q

what type of info does colour doppler provide

what does it include in its assessment

A

complete qualitative blood flow analysis over a wide field of view…

combines anatomical detail from the 2d image and physiological hemodynamics w/ the colour overlay

42
Q

other names for colour doppler (CD)

A

colour doppler imaging (CDI)
colour flow imaging (CFI)
colour flow mapping (CFM)

43
Q

two processes required to display colour doppler

what does mean for the # of pulses per scan line, whats the effect of this

A

first, the standard 2D process - 1 pulse per scan line
second, the doppler processing - at least 3 pulses per scan line

we need several pulses per scan line which has a huge affect on FR

44
Q

what processing technique is used to produce colour doppler

A

autocorrelation

45
Q

describe autocorrelation

A

mathematical means of rapidly determining the sign, mean, power and variance of the returning signal

46
Q

what does the term “sign” refer to w/ colour doppler

A

direction of flow

47
Q

how many doppler samples are displayed on the monitor per scan line

how does this affect FR and why

A

100-400

dramatically reduces it b/c autocorrelation must occur at each depth along the scan line

48
Q

typical FRs w/ colour doppler

what does it depend on

A

5-50 Hz

depends on depth and size of the field of view

49
Q

what is ensemble length

A

of pulses used for each scan line for colour doppler

50
Q

another name for ensemble length

A

packet size

51
Q

another name for scan line

A

A-line

52
Q

what does the colour doppler that we see represent

A

mean or average of the velocities present

53
Q

how many pulses per scan line are commonly used to calculate the mean velocity for colour doppler

A

10-20…. need a minimum of 3 pulses

54
Q

relationship b/w ensemble length and FR

A

longer ensemble length = slower frame rate

55
Q

what 4 pieces of information does autocorrelation provide about blood flow

A

sign (Hue)
magnitude (saturation)
power (luminance)
variance

56
Q

what is hue

A

the colour that you see that represents the sign or direction of flow

57
Q

If you arent getting reversal in arteries, how should you change the colour scale

A

lower it

58
Q

describe saturation

is it a mean

A

saturation is the amount of colour in a mix of white and represents the magnitude of the doppler shift or the velocity of blood

yes

59
Q

what does more white mean w/ saturation

A

more white = less saturated and faster flow/velocity

e.g. red is more saturated than pink

60
Q

are people more sensitive to changes in hue or changes in saturation

A

hue

61
Q

describe luminance

A

the brightness of the hue and saturation present… it represents the power of the flow and tells us the RBC density in the field of view

62
Q

relationship b/w # of RBCs and luminance

A

more RBCs travelling at the same velocity in a sample will look brighter on the display

63
Q

what is variance

how does it appear on CD

A

the # of velocities that diverge from the mean velocity

will show all difference colours/hues based on the actual velocity, not the mean

64
Q

advantage of a variance map

A

can give greater detail than the averaged velocities in a hue or saturation map (can help pin point disease)

65
Q

what is priority

A

another CD control that determines whether or not the colour overlay will overwrite the low level echos

66
Q

what happens if priority is set low

high?

A

low: precedence is given to non-doppler shifted weak echos
high: allows the colour to overwrite the low 2D echos

67
Q

risk if priority is set too high

A

colour could overwrite plaque and you miss pathology

68
Q

describe power doppler

does it display magnitude or amplitude

A

represents the strength of the doppler shift rather than velocity

amplitude ONLY

69
Q

advantages to power doppler

A

no aliasing, less dependant on angle, very sensitive to low flow states

70
Q

disadvantages to power doppler

A

no direction flow speed or character information

susceptible to blooming artifact

71
Q

other names for power doppler

A

energy mode

power-angio

72
Q

w/ what type of exam might power doppler be very useful

A

EV, looking for stalk connecting to poly, cant get an angle

73
Q

Another name for scale

A

Velocity range

Physics 3 (5 decks)

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