Ultrasound physics ch 19
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WHat is the Doppler shift?
the shift in frequency when the sound changes as the sound source and the receiver move closer to together or farther apart.
WHat is another name for Doppler shift?
What is the principle of the Doppler shift used for?
used to measure the velocity of blood in the circulation.
What is Demodulation?
The process of extracting the low Doppler frequency from the transducer's carrier frequency.
When are the Doppler shift positive?
they are positive when blood cells move towards the transducer.
The reflected frequency is higher than the transmitted frequency.
When are the Doppler shift negative?
They are negative when blood cells move away from the transducer.
The reflected frequency is lower than the transmitted frequency.
Doppler frequencies indicate what?
How are the Doppler shift and blood cell velocity related?
The faster the velocity the greater the Doppler shift.
HOw many Doppler shifts are during a clinical ultrasound exam?
- 1st- occurs when the sound wave from the transducer strikes moving blood cells.
- 2nd- occurs from the reception of the sound wave from moving red blood cells.
WHat is on the x-axis on a Doppler?
WHat is on the y-axis on a Doppler?
Doppler shift or velocity
What is the relationship between the transmitted frequency and the Doppler shift?
If the transducer's frequency is doubled, the measured Doppler shift will also be doubled.
The measurement of the Doppler frequency depends on the relationship of what two things?
THe relationship between the direction of blood flow and the direction in which the sound wave propagates.
When is the velocity measurement 100 % accurate?
When the blood cells are moving parallel to the sound beam.
When the blood cells are moving either directly toward or directly away from the transducer.
When the sound beam and the flow direction are not parallel what determines how much of the velocity is measured?
the percentage of the true velocity that is measured depends on the cosine of the angle between the sound beam and the direction of motion.
What is the relationship between the Doppler shift and the cosine of the angle?
If the cosine angle is reduced by half, the measured Doppler shift will be halved.
WHat angles do you want when measuring velocity with the Doppler?
0 or 180 degrees
0 degree = 1
180 degree = -1
What angle do you not want when measuring velocity of blood with Doppler?
do not want 90 degrees or perpendicular
The cosine for 90 degrees is zero, which means it will not record any velocity.
A positive Doppler shift indicates movement towards or away from the transducer?
towards the transducer
A negative Doppler shift indicates movement towards or away from the transducer?
away from the transducer
The flow towards the transducer is above or below the baseline?
above the baseline
The flow away from the transducer is above or below the baseline?
below the baseline
Which is has higher velocities?
(arterial or venous)
arterial velocities are higher
Continuous wave Doppler requires what?
two crystals in the transducer
- One constantly transmits
- and one constantly receives reflections from blood cells.
WHat is the greatest advantage of continuous wave Doppler?
It's ability to accurately measure very high velocities.
What are two disadvantages to continuous wave Doppler?
1. Cannot determine the exact location of the moving blood cells.
2. It has a lack in time gain compensation (TGC).
The limitation of where the moving blood cells exact location is.
What is simultaneous anatomic imaging and Doppler is called?
Dedicated continuous wave transducers do not use what?
Which causes what four things?
do not use backing material
- undampened transmitted signal
- narrow bandwidth
- high quality factor
- higher sensitivity
Why is it important for dedicated continuous wave transducer to have increased sensitivity?
They can detect low amplitude reflections and small Doppler shifts.
HOw many crystals do pulsed wave dopplers have?
One, that alternates between sending and receiving sound pulses.
What is the advantage to pulsed wave Doppler?
Can measure the exact location where the velocities are measured.
The exact location of where the velocities are measured is called(3)...
- 1. range resolution
- 2. range specificity
- 3. freedom from range ambiguity artifact
Which Doppler system uses the small marker called the sample volume or gate?
pulsed wave doppler
What is the use of the sample volume or the gate?
The ultrasound system calculates the time-of-flight for a sound pulse traveling to and from the gate.
WHat is disadvantage of pulsed wave Doppler?
Can not accurately measure high velocity signals.
HIgh velocity flow in one direction is incorrectly displayed as traveling in the opposite direction called aliasing.
What is aliasing?
When high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction.
Pulsed Doppler transducer contain backing material. (True or False)
- Characteristics similar to imaging transducers.
- low quality factor
- lower sensitivity
- wide bandwidth pulses.
What is the Nyquist frequency or Nyquist limit?
it is the highest Doppler frequency or velocity that can be measured without the appearance of aliasing.
- It is like a speed limit.
- Aliasing appears when the Doppler shift exceeds this speed limit.
- It is one-half of the pulse repetition freguency.
WHat are the two ways to avoid aliasing?
- 1. raise the Nyguist limit
- 2. reduce the Doppler shift
WHat is the system's pulse repetition frequency when using Doppler?
the sampling rate of pulsed Doppler.
When is the Nyquist limit low?
When the sample volume is deep and the PRF is low
When is the Nyquist limit low?
WHen the sample volume is shallow and PRF is high.
When does the system accurately measure high velocities without aliasing?
When the sample volume is shallow.
What creates more aliasing?
higher frequency transducers- create higher Doppler shifts
lower frequency transducers-- create lower Doppler shifts -- which are less likely to exceed the Nyquist limit.
What are five techniques may be used to avoid aliasing artifact?
Remember each has its advantages and disadvantages!!!
- 1.) adjust the scale to its maximum
- 2.) select a new ultrasonic view with a shallower sample volume
- 3.) select a lower frequency transducer
- 4.) use baseline shift
- 5.) use continuous wave doppler
When the sonographer adjusts the scale to its maximum, what will also change?
- 1.) the system's RPF is also increased to its maximum.
- 2.) When PRF is increased so is the Hyquist Limit.
What is the advantage of adjusting the scale to its maximum?
Maximizing pulse repetition frequency raises the nyquist limit and aliasing is less likely to appear.
What is the disadvantages of adjusting the scale to its maximum?
1.) Higher pulse repetition frequency reduces sensitivity to low velocities.
2.) With very high velocities, aliasing artifact persists, even when the scale is maximized.
What are the advantages of selecting a new ultrasonic view with a shallower sample volume?
1.) PRF is increased so the nyquist limit is increased and aliasing is reduced.
What are the disadvantages of selecting a new ultrasonic view with a shallower sample volume?
None. This technique succeeds if the sonographer can locate an alternative transducer position where the anatomic site is shallower,
What are the advantages of selecting a lower frequency transducer?
Lower frequency sound reduces the height of the Doppler spectrum.
Lower shifts are less likely to exceed the Nyquist limit (or the speed limit) and therefore are less likely to alias.
What are the disadvantages of selecting a lower frequency transducer?
No disadvantages using Doppler only.
But during duplex ultrasound, lower frequency sound produces a lower quality anatomic image.
What is the advantage of use baseline shift?
high velocity flows are displayed in the proper direction.
What is the disadvantages of use baseline shift?
ineffective when the pulsed doppler spectrum has wrapped completely around itself.
What is the advantage of using continuous wave doppler?
Aliasing never appears with continuous wave doppler. Aliasig is associated only with pulsed doppler.
What is the disadvantage of using continuous wave doppler?
- Range ambiguity.
- Velocities along the entire region of overlap between teh transmit and receive beams blend together to form the spectrum.
What does color doppler report?
What does continuous wave doppler report?
color- average or mean velocities
continuous- peak velocities
WHat information does the colors on a velocity mode map include?
- 1.) information on flow direction
- 2.) velocity
What does the black region in the middle of the color map indicate on a velocity mode map?
no Doppler shift
What do the colors ABOVE the black stripe indicate on a velocity mode map?
indicate blood cells moving toward the transducer
OR positive Doppler shifts.
- Colors closer to black line= lower velocities
- Colors farther away from black line= higher velocities.
What do the colors BELOW the black stripe indicate on a velocity mode map?
indicate blood cells moving away from the transducer
OR negative Doppler shifts.
WHat does the variance mode provide on the color flow doppler?
more information than velocity mode.
Variance mode distinquishes laminar flow from turbulent flow.
What do the colors on the left side of the map indicate on the variance mode?
laminar flow (normal flow patterns)
What do the colors on the right side of the map indicate on the variance mode?
indicate turbulent flow
What does the home line indicate?
the imaginary location of the transducer
What are Doppler packets?
they are multiple ultrasound pulses that are used to accurately determine blood velocities.
What are two advantages of larger number of pulses (larger packets or long ensemble lengths)?
1.) more accurate velocity measurement
2.) increased sensitivity to low flow
What are the three disadvantages of larger number of pulses (larger packets or long ensemble lengths) ?
- 1.) more time needed to acquire data
- 2.) reduced frame rate
- 3.) decreased temporal resolution
Why should packet size should be carefully selected to balanced WHAT two competing interests of color doppler?
1.) Accurate velocity measurements
2.) Adequate temporal resolution
What does power mode do?
only identifies the presence of a Doppler shift.
It does not evaluate speed or direction.
All vessels have the same color regardless of the direction of blood flow.
The strength (amplitude) of the reflected signal is processed without regard to the blood's direction or speed.
What two names is power doppler also called?
- 1.) energy mode
- 2.) color angio
What are three advantages of power mode?
- 1.) increased sensitivity to low flow or velocity, such as venous flow or flow in small vessels.
- 2.) unaffected by Doppler angles, unless the angle is exactly 90 degrees.
- 3.) no alising, since the velocity information is ignored.
What are three disadvantages of power mode?
- 1.) no measurement of velocity or direction
- 2.) lower frame rates (reduced temporal resolution) than conventional color flow doppler
- 3.) susceptible to motion of the transducer, patient, or soft tissues, which may result in a burst of color, or flash artifact.
What is one type of doppler artifacts?
ghosting and clutter
Slow moving anatomy such as heart muscle or pulsating vessel walls may create very low frequency Doppler shifts.
- These low frequency Doppler shift artifacts cause
- clutter on spectral displays
- and ghosting artifacts on color doppler
How is clutter or ghosting artifact eliminated?
by using wall filter to eliminate low frequency Doppler shifts from moving anatomy rather than from moving blood cells.
With color doppler, wall filters eliminate color arising from slow velocity reflectors.
What are crosstalk artifacts?
a special form of mirror image artifact araises only with spectral doppler.
What does crosswalk artifacts results from either? (2)
1.) Doppler gain is set two high
2.) the incident angle is near 90 degree between the sound beam and the flow direction, when flow is at the beam's focus.
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