Chapter 19 Flashcards
Doppler ultrasound can detect
_
The presence,
direction, velocity, and properties of blood
flow in vessels
The Doppler effect is a phenomenon in
which
an apparent change in the frequency
of sound is observed if there is relative
motion between the source of the sound and
the receiver
the change in frequency
between the transmitted frequency and the
received frequency
Doppler shift
Doppler shifts are created when _
transmitted
sound waves strike moving red blood cells
_ doppler shift: when blood cells
move toward the transducer; reflected
intensity is _ than the transmitted
frequency
positive
higher
_ doppler shift: when blood cells
move away from the transducer; reflected
intensity is _ than the transmitted
frequency.
negative
lower
If the transmitted frequency is higher than the reflected frequency,
what type of shift occurs?
negative
If the reflected frequency is higher
than the transmitted frequency,
what type of shift occurs?
positive
Doppler frequencies indicate _ not _
velocity
speed
Speed or velocity:
Purely magnitude
speed
Speed or velocity: Indicates the distance that a red blood cell
moves in 1 second
speed
Speed or velocity: cm/s or other units of distance/time
speed
Speed or velocity: Magnitude and direction
velocity
The doppler equation
Doppler shift= (2 X velocity of blood X transducer frequency X cos) / propagation speed
The two in the equation represents
that there
are actually two Doppler shifts during an
exam
First doppler shift _
Second doppler shift _
when sound strikes moving blood cells
transducers reception of the sound wave from moving red blood cells
Relationship between doppler shift and velocity of RBC’s
direct
The faster the velocity, the _ the doppler frequency
greater
Modern ultrasound systems actually measure
the frequency difference (in hertz) between
received and transmitted waves
The Doppler equation is programmed into the
ultrasound system’s computer. The computer
uses the data to determine _
the velocity of blood
Doppler equation programmed into US system: Actual data is _, with units of _. Derived information is _, with units of _
frequency
hertz
velocity
m/s
Relationship between doppler shift and frequency of transmitted sound
direct
If transducer’s frequency is doubled, the
measured Doppler shift will
also be doubled
The measurement of the Doppler frequency
depends on
the relationship between the
direction of blood flow and the direction in
which the sound wave propagates
When blood cells are moving parallel to the
sound beam, the _ is measured
entire velocity
When an angle exists between the direction
of flow and the sound beam, the measured
velocity is _ than the true velocity
less
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.
Angle: 0
Cosine:
1.0
Angle: 30
Cosine:
0.87
Angle: 60
Cosine:
0.5
Angle: 90
Cosine:
0
Angle: 120
Cosine:
-0.5
Angle: 150
Cosine:
-0.87
Angle: 180
Cosine:
-1.0
Realtionship between doppler shift and cosine of the angle
direct
Relationship between doppler shift and the direction of sound
direct
If the cosine Θ is reduced by half, the
measured Doppler shift will _
be halved
If flow is parallel to the sound beam, the angle between the direction of motion and sound is
0 degrees or 180 degrees
If flow is parallel to the sound beam, the measured and true velocities are _
equal
cosine 0 degrees is _, indicating flow _
1
toward the transducer
cosine 180 degrees is _, indicating flow _
-1
away from the transducer
If flow is perpendicular to the sound beam,
the measured velocity is _. Cosine 90 degrees is _
zero
zero
Doppler shifts and velocities cannot be
measured with
perpendicular incidence
Most accurate measured velocity
Flow is parallel to the sound beam 0 degrees or 180 degrees
if blood moves at a 60° angle to the
beam, the measured velocity is _ the actual velocity. Why?
1/2
the cosine of 60 is 0.5
Bidirectional Doppler devices distinguish _
the
direction of flow toward or away from the
transducer.
A positive Doppler shift indicates flow _ the transducer
toward
A negative Doppler shift indicates flow _ the transducer
away from
Audio of bidirectional Doppler requires _
headphones or speakers, where sound from one
speaker represents flow towards the transducer, and the other represents flow away from the transducer.
Flow toward the transducer is displayed _ the baseline
above
flow away from the
transducer is displayed _ the baseline
below
Continuous Wave
Doppler requires _ crystals
2
In continuous wave trandsucers, one crystal _ while the other _
constantly transmits
continuously receives
reflections from blood cells
Greatest advantage of continuous wave doppler is its ability to _
accurately measure very high velocities.
Disadvantages of continuous wave doppler
range ambiguity
Lack of TGC
Range ambiguity
Exact location of the moving blood cells
cannot be determined
Do continuous wave transducers use backing material?
No
Do continuous wave transducers have a matching layer?
Yes
In pulsed wave Doppler, _ PZT crystal is
necessary.
only one
With pulsed wave doppler, sonographers position _. The US system calculates _
a small marker called
the sample volume or
gate on a 2-D image.
time of flight for a sound pulse traveling to and from the gate.
Advantage of pulsed wave doppler:
the ability to select the exact location where velocities are measured.
AKA range resolution, range specificity, or
freedom from range ambiguity artifact.
the ability to select the exact location where velocities are measured
Disadvantages of pulsed wave doppler
Inaccurate measurement of high velocity
signals.
High velocity flow in one direction is incorrectly
displayed as traveling in the opposite direction.
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 exams
pulsed wave transducers: _ crystals, contains _, _ Q factor, _ sensitivity, _ bandwith
single backing material low low wide
Most common error associated with Doppler
ultrasound.
Aliasing
Aliasing
Very high velocities in one direction are
incorrectly displayed as going in the opposite
direction.
Velocity of blood cells reach the very top of the spectral display and incorrectly wrap around and appear at the bottom of the display
Aliasing
The very top or bottom of the display
nyquist limit
Can aliasing occur in the opposite direction?
Yes
Aliasing only occurs with _ doppler
pulsed
Aliasing occurs when doppler sampling rate is _
too low in comparison to the measured blood
velocities
Nyquist limit = _ PRF
1/2
Nyquist limit = _
PRF/2
Two ways to avoid aliasing:
Raise the Nyquist limit
Reduce the Doppler shift
When the sample volume is deep, the
PRF is _ and the nyquist limit is _. This can create _
low
low
aliasing
When the sample volume is shallow,
the PRF is_ and the nyquist limit is _. The system
high
high
accurately measures
high velocity without aliasing.
_ frequency transducers create more
aliasing.
higher
Relationship between doppler shift and transducer frequency
directly
At a particular velocity, higher frequency
transducers create _ doppler shifts and lower frequency transducers create _doppler shifts
higher
lower
Aliasing artifacts are less common with _ frequency transducers due to _
lower
the Doppler
shifts being less likely to exceed the Nyquist
limit.
Less aliasing: _ velocity, _ frequency transducer, _ gate, _ PRF
slower, lower, shallow, higher
More alaiasing: _ velocity, _ frequency, _ gate, _ PRF
faster, higher, deep, low
Five techniques may be used to avoid
aliasing artifact:
- Adjust the scale to its maximum
- Select a new ultrasonic view with a shallower sample volume
- Select a lower frequency transducer
- Use baseline shift
- Use continuous wave Doppler
Advanages of adjusting scale to its maximum
raises the nyquist limit and aliasing
is less likely to appear
Disadvantages of adjusting scale to its maximum
higher PRF reduces sensitivity to low velocities.
With very high velocities, aliasing artifact persists,
even when the scale is maximized.
If the scale is adjusted to its maximum, the PRF is _
also adjusted to its
maximum. (increase PRF increases Nyquist limit)
Advantages of selecting a new ultrasonic view with a shallower sample volume
PRF is increased, Nyquist limit increased,
aliasing is reduced
Disadvantages of selecting a new ultrasonic view with a shallower sample volume
None
Advantages of selecting a lower frequency transducer
Lower doppler shifts occur with lower frequency
transducers and reduces the height of the doppler
spectrum.
Lower doppler shifts are less likely to exceed the
Nyquist limit and less likely to alias
Disadvantages of selecting a lower frequency transducer
No significant disadvantages other than producing
a lower quality anatomic image
Baseline usually appears _
in the middle of the display
_ simply slides the display baseline up or down so
that the entire velocity scale is devoted to one direction.
Baseline shift
Advantages of baseline shift
High velocity flows are displayed in the proper direction.
Measurements remain accurate even after shifting the baseline
Disadvantages of baseline shift
Only changes the visual appearance of flow
If audio is used, the sound will still be heard from incorrect
speaker
Will be ineffective when the doppler shift is so high that the
signal completely wraps around itself.
Advantages of continuous wave doppler
Aliasing never appears
with continuous wave Doppler.
Disadvantages of continuous wave doppler
Range ambiguity Velocities along the entire region of overlap between the transmit and receive beams blend together to form the spectrum
Techniques that eliminated aliasing:
Method: Adjust scale
Strategy: _
Increase nyquist limit
Techniques that eliminated aliasing:
Method: new, shallower view
Strategy: _
increase nyquist limit
Techniques that eliminated aliasing:
Method: lower transducer frequency
Strategy: _
decrease doppler shift
Techniques that eliminated aliasing:
Method: zero baseline shift
Strategy: _
aliasing remains but display is more appealing
Techniques that eliminated aliasing:
Method: Continuous wave doppler
Strategy: _
never aliases, but range ambiguity
Gray shades
on a doppler
spectrum are
related to
amplitude of the reflected signal or number of blood cells creating the reflection
Pulsed doppler transducers: _ crystals, (dampened or undampened), _ Q factor, _ bandwidth, _ sensitivity
ateast one, dampened, low, wide, lower
Continuous wave doppler transducers: _ crystals, (dampened or undampened), _ Q factor, _ bandwidth, _ sensitivity
atleast 2, undampened, high, narrow, higher
Imaging vs doppler: Imaging: _ incidence, _ frequency, (pulsed or continuous wave), _ crystals
normal, higher, pulsed only, minimum of 1
Imaging vs doppler: doppler: _ incidence, _ frequency, (pulsed or continuous wave), _ crystals
0 or 180 degrees, lower, pulsed or continuous, minimum of 1 (pulsed) or 2 (continuous)
Color flow is a form of _
doppler
2D
Color flow doppler: Velocity info is coded into _ and _
colors
superimposed on a 2D gray scale anatomic image.
Color flow doppler is a _ ultrasound technique
pulsed
Is color flow doppler subject to aliasing?
Yes
Color flow doppler is associated with range _
resolution/specificity
IS the angle between direction of sound and flow more or less important with color than with pulsed or continuous wave doppler?
less
Color flow doppler reports _
average or mean velocities
Pulsed and continuous wave doppler reports _
Peak velocities
“Dictionary” used to convert
measured velocities into
colors that appear on the
image.
Color maps
Color maps are used to convert _ into _
measured velocities
color
Velocity mode: Colors provide information on
flow direction and velocity
Black region in the middle of a color map indicates _
no doppler shift
velocity mode: Colors above black line
indicate
flow towards
transducer
Velocity mode: Colors below black line
indicate
flow away from
transducer
Velocity mode: Color change
always up and down, never side to side
Variance Mode: provides information on _
speed, direction, and distinguishes between turbulent and laminar flow
Variance mode: colors on the left side indicate _
laminar
Variance mode: colors on the right indicate _
turbulence
With color Doppler, multiple ultrasound pulses are
used to accurately determine blood velocities
(called _)
a packet or ensemble
Larger packets have two advantages:
More accurate velocity measurement
Increased sensitivity to low flow
Disadvantages of larger packets:
More time needed to acquire data
Reduced frame rate
Decreased temporal resolution
_ Only identifies presence of a
Doppler shif
power doppler
Does power doppler evaluate speed or direction?
No
Power doppler: All vessels have _
the same
color, regardless of the
direction of the blood flow
Power doppler AKA
energy mode or
color angio
Power doppler: Strength of reflected signal is processed (with or without) regard to the blood’s direction or speed.
without
Advantages of power doppler
Increased sensitivity to low flow or velocity
Unaffected by Doppler angles, unless the angle is exactly 90°
No aliasing (velocity information is ignored)
Disadvantages of power doppler
No measurement of velocity or direction
Lower frame rates (reduced temporal resolution)
Susceptible to motion of the transducer, patient, or
soft tissues, which may result in flash artifact
Doppler shifts generally arise from
moving blood
Slowly moving anatomy may also create
very low frequency doppler shifts
low frequency Doppler shift
artifacts on a spectral display
Clutter
low frequency
Doppler shift artifacts with color doppler
ghosting
Clutter
low frequency Doppler shift
artifacts on a spectral display
Ghosting
low frequency
Doppler shift artifacts with color doppler
A _ is used to eliminate low frequency Doppler shifts from moving anatomy rather than moving blood cells
wall filter
Wall filter serves as a _ for doppler
“reject”
_wall filter eliminates _ but has no effect on _. Called _
low frequency doppler shifts around the baseline of a spectral display.
the higher frequency doppler shifts.
high pass filter
With color, wall filters eliminate _
color arising from slow velocity reflectors.
A special form of “mirror image” artifact that
arises only with spectral Doppler.
cross talk
Crosstalk results from either:
Doppler gain is set too high
(electronic crosstalk)
or
The incident angle is near 90° between the sound beam and the flow direction, when flow is at the beam’s focus
All blood cells within a sample volume (do or do not) travel at the same speed or direction.
do not
_ is a tool that breaks the
complex signal into basic building blocks and
identifies the individual velocities that make
up the reflected Doppler signal
Spectral analysis
Two methods of spectral analysis:
Fast Fourier Transform (FFT)
Autocorrelation
A digital technique that is used to process both
pulsed and continuous wave Doppler signals
Fast Fourier
Transform
_ is the process of separating a
waveform into a series of single-frequency sine-
wave components. When algebraically combined,
these components yield the original waveform
FFT
2 advantages of FFT
Exceedingly accurate
Displays all individual velocity components that
make up the complex reflected signal.
FFT distinguishes _
laminar from turbulent flow
Fourier analysis is the process of _. When algebraically combined, these components _
separating a
waveform into a series of single-frequency sine-
wave components.
Yield the original waveform
With laminar flow, most blood cells within the
sample volume travel at _ velocities
similar
Spectral trace is a_ line
narrow, well defined
FFT: Laminar flow: Region between the baseline and the
spectrum, called the _, is _
window
clear
FFT: with turbulence, _ directions and speeds are within the sample volume
many different
FFT: turbulence: pulsed soppler spectral window is _
filled in
Spectral broadening
wider range of velocities and shifts in the sample volume
spectral broadening can be caused by _
large gate
Autocorrelation AKA
correlation function
Digital technique used to analyze color flow
Doppler
Autocorrelation
Is autocorrelation less accurate than FFT?
yes, somewhat
Is autocorrelation faster than FFT?
Yes, substantially
Autocorrelation is used with _ due to the enormous
amount of data that is processed.
color doppler
