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Vascular SON 250 > Doppler Review > Flashcards

Doppler Review Flashcards

(60 cards)

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1
Q

What is Doppler frequency?

A

Change of frequency when there is a change of distance between the sound source and receiver
Use sound frequency to demonstrate this principle
Listening to a siren that gets closer and then passes – a change in the frequency (pitch) of the sound occurs

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2
Q

What is Doppler Shift?

A

Use this principle in blood flow because the red blood cells are moving

So the transmitted frequency will be different from the received frequency

Differences between transmitted and received frequency are either positive or negative

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3
Q

What happens with Doppler shift?

A

During the ultrasound a frequency of 5MHz is sent into the body
The sound strikes the moving red blood cells and the frequency changes
The signal sent back to the transducer is now not 5,000,000 Hz but 5,003,000 Hz
The Doppler shift is 3,000 Hz and the shift is positive because the frequency got larger

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4
Q

What is the Doppler shift formula?

A

fo is the transmitted frequency
fr is the received frequency
Shift in frequency because red
blood cells are moving

Doppler Shift(Hz) = 
reflected frequency - transmitted frequency
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5
Q

What is positive Doppler shift?

A

If blood is moving toward the transducer

Reflected frequency will be higher than transmitted frequency

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6
Q

What is negative Doppler shift?

A

If blood is moving away from transducer

Reflected frequency will be lower than transmitted frequency

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7
Q

What is speed?

A

Only a magnitude

Distance in 1 second

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8
Q

What is velocity?

A

Magnitude and direction
Distance in one second
And in which direction is it moving

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9
Q

What is Doppler shift?

A

Doppler shift =

2 X speed of blood X transducer frequency X cosΘ
propagation speed

Two doppler shifts
First - when wave strikes red blood cell
Second – when reflected wave is received at the transducer
F = frequency shift
f = transducer frequency
v =  velocity
Cos = cosine of doppler angle
C = propagation speed of sound
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10
Q

Do we calculate doppler shifts?

A

No, we do not calculate doppler shifts

Ultrasound machines take the shifts and convert them to velocities

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11
Q

Relationship is the b/w Doppler shift and velocity

A

Direct relationship between velocity and doppler shift

Faster velocity the greater the doppler shift
If velocity is cut in half the doppler shift is cut in half

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12
Q

Relationship is the b/w Doppler shift and frequency

A

Direct relationship between the doppler shift and the transducer frequency

If transducer frequency is doubled the doppler shift will be doubled
(all other factors like angle stay the same)

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13
Q

How accurate is the calculation of velocities?

A

Depends on the angle of the sound beam to the direction of flow
The most accurate velocity is when the sound beam is parallel to the flow

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14
Q

How do we calculate accuracy and cosine

A

The accuracy of velocity comes from the cosine of the angle between the sound beam and the direction of flow
Doppler shift =
2 X speed of blood X transducer frequency X cosΘ
propagation speed

Cosine of 0 degrees or parallel direction is 1 – this gives 100% accuracy of velocity
Cosine of 90 degrees is 0 – this gives us no flow at all

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15
Q

What is the relationship angle and doppler shift?

A

There is an inverse relationship between doppler angle and frequency shift
The larger the doppler angle (ex. 89 degrees) the less of the frequency shift thus an inaccurate velocity

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16
Q

What angle gives us half of the true velocity?

A

The cosine of 60 degrees is 0.5 – this gives us half of the true velocity
Acceptable ranges in diagnostic ultrasound are anything between 0 degrees and 60 degrees
How do we know what our angle is while scanning ? The machine calculates is for us.

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17
Q

What degree is not good?

A

The closer to 90 degree angle the less accurate the velocity

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18
Q

What can you do to decrease doppler angle to flow?

A

heel and toe

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19
Q

What will cause no spectral fill in?

A

bad steering and bad angle alignment

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20
Q

What is Doppler without direction?

A

continuous wave

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21
Q

describe continuous wave

A

Detects blood flow
Without direction
Takes in all signals from path of wave (venous and arterial) – range ambiguity
Just hearing the presence of flow or movement

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22
Q

characteristics of CW doppler

A

True CW transducer has two crystals – one emitted, one receiving
Accurately measures high velocities because of increased sensitivity
No backing material needed b/c not producing an image
Narrow bandwidth
High Q factor

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23
Q

What are disadvantages of CW doppler?

A

Cannot tell exact location of red blood cells because of range ambiguity – receives signals from all red blood cells (or movement) in the area
No TGC ability to compensate for low amplitude signals

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24
Q

What are characteristics pulsed wave doppler?

A

Specific area called the sample volume or gate is chosen on traditional image
Machine calculates velocities only within the sample volume
Performed through the use of pulsed ultrasound so no special crystals needed

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25
What is an advantage of pulsed wave doppler?
Can select the exact location of evaluation Range resolution Range specificity Freedom from range ambiguity
26
What is an disadvantage of pulsed wave doppler?
Does not accurate evaluate very high velocities Produces aliasing Wrapping around the baseline Appears blood is moving in the opposite direction
27
What happens at the nyquist freq?
Aliasing happens at the Nyquist frequency Highest doppler frequency measured without the presence of aliasing ``` Nyquist limit (Hz) = PRF (Hz) 2 If the Nyquist limit involves PRF what could change it???? ```
28
What happens of depth of sample volume at nyquist limit?
Shallow depth has higher PRF (pulse doesn’t have to go as far so it listens less and sends out more pulses) Higher PRF = Higher Nyquist limit Higher Nyquist limit = accurately record higher velocities
29
What happens with transducer freq with nyquist limit?
High frequency transducers produce more aliasing High frequency transducers produce high Doppler shifts To prevent aliasing choose lower frequency transducer
30
What happens less aliasing?
Slow velocity Low frequency transducer Shallow imaging depth which results in shallow gate or sample volume
31
what happens more aliasing?
Faster velocity High frequency transducer Deeper imaging depth which results in deeper gate or sample volume
32
What happens with lower freq and aliasing?
Choose the lowest frequency transducer that still gives diagnostic quality imaging Low frequency produces low doppler shifts which are less likely to create aliasing
33
What happens with changing the depth and aliasing?
Shallow imaging depth Shallow sample position PRF increases Nyquist limit increases
34
what happens with doppler scale and aliasing?
Highest scale available Increases PRF Raises Nyquist limit Disadvantage Reduces sensitivity to low velocities
35
What happens with CW doppler and aliasing?
No aliasing with CW
36
What are disadvantages of CW doppler?
Disadvantage Range ambiguity Uses all velocities within the beam region Not always available
37
how to fix aliasing?
Shift baseline down Entire doppler tracing can be viewed Does not change accuracy just allows viewing of tracing and accurate measurement if needed Does not change the ability of the settings
38
What happens with color doppler?
Represents average velocities through different shades and colors Represents the direction of flow through specified color Shows gray scale anatomy simultaneously Range specificity Is subject to aliasing Uses calculating technique called autocorrelation to obtain color flow info
39
What is velocity mode?
Represents direction of flow Black baseline represents no flow Above black baseline flow is towards the transducer Below black baseline flow is away from transducer
40
What does more colors on the color map mean?
The colors farthest from the black baseline represent higher velocities
41
color optimization
Wrap around of the colors- aliasing Corrected through increasing the scale (increases the PRF) Also need to properly adjust color gain Too high scale - noise/flash Too low scale – won’t pick up weak signals or slow flowing blood
42
color frame rate
Pulses per line (packet size) Density of scan lines Color box width PRF
43
what happens with frame rate versus color box size?
the bigger the box the slower frame rate and more artifact
44
What characteristics for power doppler?
Color angio, energy mode Non directional color doppler All same color – only gives presence of flow and strength of flow
45
what is advantage of power doppler?
Increased sensitivity Not affected by doppler angles expect 90 degrees No aliasing
46
What is disadvantages of power doppler?
No velocity No direction Reduced temporal resolution Susceptible to motion
47
What is spectral analysis?
Blood cells moving at lots of different speeds Spectral analysis is a way to display these different velocities Fast Fourier Transform (FFT) – calculation technique to display spectral tracing
48
What is fast fourier transdorm?
Processes both CW and pulsed wave Accurate Displays all individual velocities Distinguish between laminar and turbulent flow Spectral window is between baseline and tracing Laminar flow the spectral window is clear and free from noise Laminar versus turbulent flow Clear spectral window versus no spectral window spectral broadening
49
doppler sample gate placement in the center
Narrow bandwidth and clear spectral window indicate | that blood is travelling at same speed.
50
doppler sample gate placement near the edge of the vessel
Wider bandwidth and filling in of spectral window indicate that sample gate encompasses greater range of blood speeds.
51
big doppler sample gate
Spectral window filled in because broad range of velocities
52
what is disturbed flow pattern
Disrupted flow layers at: Arterial bifurcations Distal to stenosis CCA at the bulb
53
what can no disturbed flow indicate?
plaque
54
What happens post stenosis and the waveform?
Post-stenotic Doppler waveform shows spectral | broadening because of turbulence.
55
SMA
Eaten-high resistive no eaten-low resistance The superior mesenteric artery follows the same pattern as the superficial femoral artery, except in this case it is a meal that causes vasodilation which changes the waveform from a high resistance to a low resistance flow. Compare diastolic velocities.
56
what is cross talk?
Mirror artifact for doppler Makes it appear that flow is bidirectional Caused by doppler gain too high Caused by near 90 degree incident angle
57
what is doppler artifacts
Ghosting or clutter Noise on image or spectral tracing Created by movement of anatomy, not red blood cells Prevented with use of wall filter (reject for doppler)
58
what is diagnostic indices?
Distinct characteristics of an arterial waveform because of pulsatility Systole and Diastole have their own characteristics Formulas to describe the arterial waveform Resistive Index Pulsatility Index
59
What is RI
``` Resistance of a segment of the vessel Help to diagnosis stenosis Only a ratio (no units) RI = max velocity – min velocity/ max velocity ```
60
what is PI
Uses the average velocity for cardiac cycle PI = max velocity – min velocity/ mean velocity Both RI and PI independent of doppler angle

Vascular SON 250 (5 decks)

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