Ch 19: Doppler Flashcards
Relative motion between the sound source and the receiver creates…
a frequency change.
The frequency of sound changes when the sound source and the receiver move…
closer together or farther apart.
The process of extracting the low Doppler frequency from the transducer’s carrier frequency is called…
demodulation.
When the Doppler shift is positive, blood cells are traveling __ the transducer.
towards
Doppler shifts range from __ and are in the range of audible sound.
20 Hz to 20,000 Hz
reflected frequency - transmitted frequency =
Doppler shift (Hz)
When blood cells move away from the transducer, the Doppler shift is…
negative.
When the Doppler shift is negative, the reflected frequency is __ than the transmitted frequency.
lower
T/F? Speed is magnitude and direction.
FALSE, just magitude
This indicates the distance that a red blood cell moves in one second.
speed
Speed is reported in units of…
distance divided by time (cm/s).
This is defined by magnitude and direction.
velocity
Doppler shift is __ related to the velocity of the RBCs.
directly. The faster the veloctiy, the greated the Doppler frequency.
How many Doppler shifts are there during an ultrasound exam?
two
How do ultrasound systems report blood velocity?
in cm/s, not Hz
The Doppler shift is __ related to the frequency of the transmitted sound.
directly. If the frequency is doubled, the shift will also be doubled.
When the blood cells are moving __ to the sound beam, the entire velocity is measured.
parallel
When an angle exists between the direction of the flow and the sound beam, the measured velocity is…
less than the true velocity.
The percentage of the velocity that is measured depends on the __ between the sound beam and the direction of motion.
cosine of the angle
The Doppler shift is __ related to the consine of the angled between the direction of flow and the sound.
directly. If the cosine is reduced by half, the measured shift will be halved.
What is the relationship between the true velocity and the measured velocity when blood moves parallel to the beam?
They are equal.
What is the relationship between true and measured velocities when blood moves perpendicular to the sound beam/
Trick question! Cannot be measured.
What is the relationship between true and measured velocities when blood moves at a 60 degree angle to the beam?
Measured will be one half the true velocity.
This is a commonly used signal processing technique for bidirectional Doppler.
phase quadrature or quadrature detection
What is the advantage of continuous wave Doppler?
Its ability to accurately measure very high velocities.
What is the primary disadvantage of continuous wave Doppler?
The exact location of the moving blood cells cannot be determined.
The limitation of continuous wave Doppler wherein signals arise from all blood cells in the region of of the beam is called…
range ambiguity.
What is the secondary disadvantage of continuous wave Doppler?
Its lack of time gain compensation.
Doppler shifts produced by continuous wave from deeper blood cells may be incorrectly interpreted as having been created by __ blood cells.
fewer
Dedicated continuous wave transducers are made with how many elements?
two
Simultaneous anatomic imaging and Doppler is called…
duplex imaging.
Dedicated continuous wave transducers do not use backing material which results in the following…
- undampened tranmitted signal
- narrow bandwidth
- high quality factor
- high sensitivity
Dedicated continuous wave transducers have increased…
sensitivity.
T/F? Dedicated continuous wave transducers do not have a matching layer.
FALSE, yes they do.
How many crystals are necessary in pulsed wave Doppler?
one
What is the greatest advantage of pulse Doppler?
Being able to select the exact location where velocities are measured.
This is the ability to select the exact location where velocities are to be measured.
range resolution, range specificity, freedom from range ambiguity artifact
What is the primary disadvantage of pulsed Doppler?
Inaccurate measurement of high velocity signals.
This is when high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction.
aliasing
Studies that simultaneously perform imaging and pulsed Doppler are called..
duplex examinations.
T/F? Pulsed Doppler transducers contain backing material.
true
This is the very top and bottom of the spectral display.
Nyquist limit
What conditions create aliasing?
- pulsed (not continuous wave) Doppler
2. the samping rate being to low
The Nyquist limit is __ of the pulse repetition frequency.
one half
Two ways to avoid aliasing are…
- raise the Nyquist limit
2. reduce the Doppler shift
When the sample volume is deep, the PRF is __ and the Nyquist limit is __.
low, low
Because the velocity is sampled fewer times at deeper volumes, the system is unable to…
accurate measure high velocities and creates aliasing.
When the sample volume is shallow, the PRF is __ and the Nyquist limit is __.
high, high
Because the velocity is sampled more times at shallower volumes, the system is able to…
meaure high velocities without aliasing.
How does transducer frequency affect aliasing?
The higher the frequency, the more aliasing created.
T/F? Aliasing artifacts are less common with higher frequency transducers.
FALSE, lower
Five techniques to avoid aliasing…
- adjust the scale to its max
- select a new view with a shallower sample volume
- select a lower frequency transducer
- use baseline shift
- use continuous wave Doppler
When the PRF is increased, the Nyquist limit is…
increased, too.
What is the disadvantage to maximizing the scale to avoid aliasing?
Loss of sensitivity to low velocities. Aliasing may persist anyway.
When a shallower sample volume is selected, the PRF is…
increased.
What is the disadvantage to selecting a shallower sample volume?
none
Why does a lower frequency transducer reduce aliasing?
Because lower frequencies create lower Doppler shifts that are more likely to be less than the Nyquist limit.
What is the disadvantage of selecting a lower frequency transducer to reduce aliasing?
The anatomic image may be of lower quality.
What is the advantage of shifting the baseline to avoid aliasing?
High velocity flows are displayed in the proper direction.
What is the disadvantage of shifting the baseline to avoid aliasing?
The sound still comes from the wrong speaker. The shift may be so high that it doesn’t help.
T/F? Aliasing never appears with continuous wave Doppler.
true
What is the disadvantage of using continuous wave Doppler to avoid aliasing?
Range ambiguity
Gray shades on a Doppler spectrum are related to…
- amplitude of the reflected signal
2. number of blood cells creating the reflection
This is a form of two dimensional Doppler.
color flow
T/F? Color Doppler is a continuous wave technique.
FALSE, pulsed
T/F? Color Doppler has range resolution.
true
T/F? Color Doppler is not subject to aliasing.
FALSE, is too
T/F? Color Doppler measures average velocities.
true
Pulsed and continuous wave Doppler report __ velocities.
peak
This is displayed as a vertical bar of various colors, with a black region in the center, during color flow imaging.
the dictionary, or lookup table
These are the two most commonly used dictionaries in color flow.
- velocity mode
2. variance mode
The colors on the color flow dictionary provide information about __ and __.
flow direction and velocity
The black region in the middle of the color ma indicates…
no doppler shift.
Does variance mode or velocity mode provide more information?
variance mode
Variance mode provides information on…
direction, speed, and laminar vs turbulent flow.
Colors on the left side of a variance mode dictionary indicate __ flow.
laminar
Colors on the right side of a variance mode dictionary indicate __ flow.
turbulent
This is a collection of multiple ultrasound pulses used to accurately determine blood velocities.
packet or ensemble
What are the advantages of larger doppler packets?
- more accurate velocity measurement
2. increased sensitivity to low flow
What are the disadvantages of larger doppler packets?
- more time needed to acquire data
- reduced frame rate
- decreased temporal resolution
This is non-directional color Doppler.
power Doppler
Power Doppler is also known as…
energy mode or color angio.
T/F? Power Doppler does not evaluate speed or direction.
true
What are the advantages of power mode?
- increased sensitivity to low flow or velocity (venous)
- unaffected by Doppler angles (unless it’s 90 exactly)
- no aliasing, since velocity is ignored
What are the disadvantages of power mode?
- no measurement of velocity or direction
- lower frame rates
- susceptible to flash artifact
Low frequency Doppler shift artifacts are called…
clutter.
Low frequency color Doppler shift artifacts are called…
ghosting artifact.
This is used to eliminate low frequency Doppler shifts from moving anatomy rather than from moving blood cells.
wall filter
Wall filters are also known as…
high pass filters.
This special form of ‘mirror image’ artifact arises only with spectral Doppler.
crosstalk
Crosstalk results from either…
- Doppler gain set too high
2. incident angle near 90
This is a tool that breaks the complex signal into its basic ‘building blocks’ and identifies the individual velocities that make up the reflected Doppler signal.
Spectral analysis
The two methods of spectral analysis currently used are…
- fast fourier transform (FFT)
2. autocorrelation
This is a digital technique, used to process both pulsed and continuous wave Doppler signals.
Fast Fourier Transform (FFT)
What are the advantages of FFT?
- exceedingly accurate
2. displays all individual velocity components
The spectral display of an FFT distinguishes __ flow from __ flow.
laminar, turbulent
A wider range of velocities and shift within a FFT sample volume as seen during turbulent flow is called…
spectral broadening
This is a digital technique used to analyze color flow Doppler because of the enormous amount of data being processed.
autocorrelation or correlation function
