Doppler Artifacts

Question 1

Aliasaing

Answer 1

Aliasing results when the Nyquist criterion is violated. When the Doppler frequency shift is greater than one half of the PRF, the Doppler signal wraps around either the spectrum (spectral Doppler) or the color scale (Color Doppler).

Question 2

what are the two types of aliasing

Answer 2

True and Display

Question 3

what steps should be taken to determine if something is display or true aliasing

Answer 3

1. Change baseline
2. Change scale
3. Switch to CW or change the PRP, window, depth

Question 4

will CW have aliasing

Answer 4

No because CW Doppler has an infinite max velocity as opposed to PW, which will have aliasing and range ambiguity

Question 5

Does CW have aiasing

Answer 5

NO

Question 6

When aliasing is true aliasing

Answer 6

Note that in this case, the aliasing is a true alias in that it is not possible to determine the actual peak velocity.

Question 7

what must happen if aliasing is true

Answer 7

Have to change transducer to a lower frequency transducer

Question 8

when is color aliasing more likely to occur

Answer 8

Center of a vessel 
angle closer to 0 or 180
Low scales
high transmit frequency
Bend, branch etc

Question 9

range ambguity

Answer 9

potential to detect signals from deeper depths as the echo from the previous transmit events overlaps the echoe from the current trasnmit event

Question 10

all pulsed wave modalities

Answer 10

exhibit range amnguity

Question 11

what modality uses range ambiguity to its advantage

Answer 11

spectral doppler modality of HPRF

Question 12

Spectral mirroring

Answer 12

artificial doppler signal displaying the opposite direction of the true flow

Question 13

What can result in Spectral mirroring

Answer 13

• Excessive transmit
• Excessive receive gain
• Superficial doppler with high frequency transducer
• insonification angle close to 90 degrees
• poor electronic design

Question 14

when insonification angle is close to 90 degrees

Answer 14

elements on one side of the steer line will see flow towards the transducer whereas elements on the other side of the steer line will see flow away-spectral mirror

Question 15

Spectral Spread (Broadening)

Answer 15

• spectral broadening occurs normally from differing blood velocities
• Can have artificial broadening

Question 16

Artificial spectral spread can cause what

Answer 16

can smear out the spectrum and INCREASE The peak velcoity

Question 17

what creates spectral spread/broandening

Answer 17

insonification angles greater then 7- degrees at more superficial gate depths cause this artifact
Principle cause is that the angle to flow from each element of an array is slightly different

Question 18

Spectral spread causes what

Answer 18

Overestimation of the peak velocity as well as the diminishment of a spectral window

Question 19

what is spectral spread exacerbated by

Answer 19

• Large array transducers (linear arrays)
• Superficial gate location
• Large insonification angles (esp over 60)
• Excessive gain

Question 20

Blossoming artifact

Answer 20

Occurs when signal is over gained

-higher then true peak velocity as well as the potential loss or decrease of the spectral window in PW Doppler

Question 21

Wall Filter saturation

Answer 21

When dynamic range is not adequately reduced by wall filters, circuit saturation occurs

Question 22

wall filter saturation occurs when

Answer 22

Clutter signals are not adequately attenuated

Question 23

what does wall filter saturation appear as

Answer 23

Bright, white line within the spectrum

Question 24

Total internal reflection

Answer 24

Can result in a loss in spectral signals as indicated by the diagram

Question 25

Twinkle artifact

Answer 25

Result of instrinic machine noise seen with colour Doppler

Question 26

what does twinkle appear as

Answer 26

Focus of alternating colours on Doppler signal behind a reflective object (like calculi) which gives the appearance of turbulent blood flow

Question 27

Twinkle can detect what?

Answer 27

More sensitive for the detection of small stones and is highly dependant on machine settings

Question 28

When is twinkle most pronounced

Answer 28

When reflecting surface is rough

Question 29

Colour and Power Doppler Flash

Answer 29

As a short term flash of colour within the colour box related to relative motion between the patient and the transducer, not the actual flow

Question 30

Doppler Spectral Dropout

Answer 30

Occurs due to - Poor Doppler angle - Inadequate sensitivity - high wall filter settings relative to low flow velocities - inappropriate settings - low receiver gain - Refraction artifact - Shadowing artifact

Question 31

HPRF

Answer 31

Comporomise between CW and PW doppler that uses the artifact of range ambiguity to increase the max detectable velocity without aliasing (at the expense of some range resolution)

Question 32

In PW what determines the minimum PRP (therefore the max PRF)

Answer 32

The Doppler gate depth

Question 33

a doppler gate depth results in what

Answer 33

A longer travel time and therefore a lower maximum PRF

Question 34

Colour Doppler Dropout

Answer 34

- Poor Doppler angle (close to 90 degrees) - high colour scale settings (PRF) - Inadequate sensitivity (transmit frequency to high with deeper depths, transmit power too low, weak signal from insignificant number of RBC, packet size small when imaging low flow, small, deeper vessels) - Inappropriate settings such as low receiver gain - Low colour priority setting (esp if 2D receiver gain is high) - refraction - Shadowing artifact

Question 35

______ _______occurs at shallow depth settings in PW doppler, this results in an echo from a deeper depth registering as if from a shallower depth

Answer 35

Range ambguity

Question 36

Spectral mirroring is evident as a mirrored spectrum in the _______direction of strong flow

Answer 36

Reverse

Question 37

Spectral broadening generally becomes a significant concern when using large ________arrays and insonifcation angles of ______or greater

Answer 37

linear, 70

Question 38

what can spectral broandening result in

Answer 38

inaccurate measurements leading to PSV overestimation and pressure gradient overstimulation