Flashcards in Doppler Chapter 19 full chapter Deck (110):
what is demodulation?
The process of extracting the low Doppler frequency from the tranducer's carrier frequency
*(Doppler shift is a low frequency that "rides" on top of a much higher transducer frequency)
What is the equation for doppler shift?
doppler shift = reflected frequency - transmitted frequency
doppler shift = 2 x velocity of blood x transducer freq x cos angle/
Doppler shifts are created when transmitted sound waves strike moving _______
red blood cells
When blood cells move toward the transducer, the doppler shift is ______
the reflected frequency is HIGHER than the transmitted frequency
blood cells moving toward the transducer reflect sound with a higher frequency....true or false
When blood cells move away from the transducer, the Doppler shift is _______
the reflected frequency is LOWER than the transmitted frequency
blood cells moving away from the transducer reflect sound with a lower frequency...true or false
Do doppler frequencies indicate velocity or speed?
what is speed?
purely a magnitude
indicated the distance that a red blood cell moves in 1 second
units of cm/s or any distance divided by time
what is velocity?
a magnitude AND a direction
What is the relationship between doppler shift and velocity?
directly related to the velocity of blood cells
the faster the velocity the greater the doppler shift
Why is there a 2 in the doppler equation?
there are actually 2 doppler shifts during a clinical ultrasound exam
first: when the sound wave from the transducer strikes moving blood vessels
second: after the sound wave strikes blood cells, reflections return to the transducer
The x-axis of a doppler spectrum represents ________. The y-axis represents _______ or ________
doppler shift, velocity
what unit is frequency measured when it comes to doppler?
the actual data is ________, with units of _______. the derived information is ___________ with units of ________
What is the relationship between transmitted frequency and the Doppler shift?
doppler shift is directly related to the frequency of the transmitted sound
if one is doubled, the other is doubled
Doppler shift measured in Hz has what relationship to velocity and transducer frequency?
directly related to velocity
directly related to transducer frequency
What does the measurement of the doppler frequency depends on the relationship between what?
the direction of blood flow and the direction in which the sound wave propagates
What happens when the blood cells are moving PARALLEL to the sound beam?
the ENTIRE velocity is measured
when the blood cells move either directly toward or directly away from the transducer the velocity measurement is 100% accurate
What happens when an angle exists between the direction of flow and the sound beam?
the measured velocity is less than the true velocity
If the sound beam and flow direction are not parallel, what determines how much of the velocity is measured?
the cosine of the angle between the sound beam and the direction of motion.
**measured velocity = true velocity x cos angle**
What is the relationship between the cosine angle and the doppler shift?
the doppler shift is directly related to the cosine of the angle between the direction of flow and the direction of sound
if one is reduced by half so is the other
Doppler shift is ________related to ________. when the cosine doubles, the doppler _______doubles. The cosine ________ as angles approach _______degrees
What is the relationship between actual velocity and measured velocity when blood moves parallel to the sound beam?
When the angle between the direction of motion and sound is 0 degrees or 180 degrees, flow is parallel
the measured and true velocities are equal
cosine 0 = 1, flow toward the transducer
cosine 180 = -1 flow is away from the transducer
What is the relationship between actual and measured velocities when blood moves perpendicular to the sound beam?
when the direction of flow is perpendicular (90 degrees to the sound beam), the measured velocity is zero because the cosine of 90 degrees is 0
doppler shifts and velocities cannot be measured with perpendicular incidence
What is the relationship between actual velocity and measured velocity when blood moves at a 60 degree angle to the beam?
since the cosine of 60 degrees is 0.5, the measured velocity at 60 degrees is one-half the actual velocity
When blood travels at 2 m/s at a 60 degree angle it is reported as 1m/s because it is halved
What does a positive doppler shift indicate?
flow TOWARD the transducer
what does a negative doppler shift indicate?
flow AWAY from the transducer
In relation to the baseline, how is flow toward the transducer displayed? away from the transducer??
toward: above the baseline
away: below the base line
What is phase quadrature or quadrature detection?
commonly used signal processing technique for bidirectional doppler
(displays flow above the base line AND below the baseline)
arterial velocities are ______than venous velocities
How many crystals are required in the continuous wave doppler?
one to constantly transmit ultrasonic energy, while the other constantly receives reflections from blood cells
what is the greatest advantage of continuous wave doppler?
its ability to accurately measure very high velocities
what is the primary disadvantage of continuous wave doppler?
that the exact location of the moving blood cells cannot be determined.
With a continuous wave doppler, what is range ambiguity?
signals arise from all blood cells in the region of overlap between the transmit and receive beams, these are the velocities that are measured
it is a disadvantage and limitation
Why is continuous wave doppler's lack of time gain compensation (TCG) a disadvantage?
reflections created from blood cells located DEEPER will have a LOWER amplitude than reflections that are at SHALLOWER depths
as a result doppler shifts produced by DEEPER blood cells may be incorrectly interpreted as having been created by FEWER blood cells.
What can a dedicated continuous wave transducer NOT do?
create anatomic images
Without the ability to create images of anatomy, short duration pulses are unnecessary
What is the shape of the two elements in the continuous wave transducer?
What is duplex imaging?
simultaneous anatomic imaging and doppler
Dedicated continuous wave transducers do not use backing material, what does that result in?
undampened transmitted signal
high quality factor (Q-factor)
What kind of transducers have a matching layer?
both duplex and dedicated continuous wave transducers
(matching layers increase the efficiency of sound transmitted in and out of the body - an advantage to both)
What kind of sensitivity do continuous wave transducer have?
they detect low amplitude reflections and small doppler shifts
In a pulsed wave doppler how many crystals are necessary?
only one PZT crystal is necessary
the crystal alternates between sending and receiving sound pulses
What is the greatest advantage of pulsed doppler?
being able to select the exact location where velocities are measured
aka: RANGE RESOLUTION, RANGE SPECIFICITY or freedom from range ambiguity artifact
What is range resolution?
the ability to select the exact location where velocities are measured
What is range specificity?
the ability to select the exact location where velocities are measured
What is the sample volume or gate?
It is a marker placed where the sonographer wants to measure the velocity.
the system then calculates the time of flight for a sound pulse traveling to and from the gate
What is the primary disadvantage of pulsed doppler?
inaccurate measurement of high velocity signals
What is aliasing?
it means false identity, an artifact
high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction
on spectral display the tip of the wave wraps around the bottom
What are the characteristics of pulsed doppler transducers?
like imaging transducers they contain backing material
low quality factor
wide bandwidth pulses
What happens with high flow when you try to take a sample of it?
the flow is above the baseline indicating flow toward the transducer.
The velocity of these blood cells reaches to the very top of the spectral display and then incorrectly wraps around and reappears at the very bottom of the display -
the very top is called the Nyquist limit, the bottom is aliasing
What is the Nyquist Limit?
high velocity of blood cells that reach the top of the spectral display before wrapping around...this upper limit is the Nyquist limit.
Can the Nyquist limit be underneath the baseline?
when the spectrum reaches the very bottom and then incorrectly wraps around and reappears at the very top....the very bottom is the Nyquist limit
What conditions create aliasing?
occurs only with pulsed doppler (never continuous wave)
occurs when the doppler sampling rate is too low in comparison to the measured blood velocities
What is the limit at which aliasing appears and how is it determined?
acts like a speed limit
aliasing appears when the doppler shift exceeds this speed limit
the Nyquist limit is one-half of the pulse repetition frequency (PRF)
What is the equation for the Nyquist limit?
Nyquist limit (Hz) = PRF (Hz) / 2
What is the Nyquist frequency?
aka: nyquist limit
the highest doppler frequency or velocity that can be measured without the appearance of aliasing
What is PRF?
pulse repetition frequency
The sampling rate of pulsed doppler
is the number of pulses of a repeating signal in a specific time unit, normally measured in pulses per second
What are the two ways to avoid aliasing?
raise the Nyquist limit
reduce the doppler shift
What is the PRF for doppler
velocities are sampled many times per second...the sampling rate is the PRF
When the sample volume is ______ the PRF is _____ and the Nyquist limit is _____
How is the presence of aliasing affected by deep sample volumes?
Velocity is sampled fewer times each second and the system is unable to accurately measure high velocities.
Deeper sample volume have lower PRF and create aliasing
When the sample volume is shallow, the PRF is ______ and velocity is sampled many times per second so the Nyquist limit is ______
the system accurately measures high velocities without aliasing
higher frequency transducers create more or less aliasing?
doppler shift is directly related to the transducers frequency
Smaller Doppler shifts from lower frequency transducers are less likely to exceed _______
they Nyquist limit
therefore aliasing artifacts are less common with lower frequency transducers
What creates less aliasing?
slower blood velocity
lower frequency transducer
shallow gate (high PRF)
what creates more aliasing?
faster blood velocity
higher frequency transducer
deep gate (low PRF)
What 5 technique may be used to avoid aliasing artifacts?
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
What are the advantages and disadvantages of to adjusting the scale to its maximum?
advantages: maxing PRF raises the Nyquist limit and aliasing is less likely to appear
disadvantages: Higher PRF reduces sensitivity to low velocities
What are the advantages and disadvantages to selecting a new ultrasonic view with a shallower sample volume?
a new view with a shallower sample volume eliminates aliasing.
the transducer can be moved to a new location with the same region of interest at a shallower depth...which increases PRF
What are the advantages and disadvantages of selecting a lower frequency transducer?
doppler shift is directly related to frequency of transducer...
the lower frequency will reduce the height of the doppler spectrum because there will be lower doppler shift since they are directly related
lower shifts less likely to alias there fore not exceed Nyquist limit
disadvantages: no significant disadvantages
What are the advantages and disadvantages of using the baseline shift?
baseline shift simply slides the display baseline down so that the entire velocity scale is devoted to one direction,
info remains the same just looks nicer, measurements remain accurate
it only changes the visual appearance.
this will be ineffective when doppler shift is so high it wraps around itself
what are the advantages and disadvantages of using continuous wave doppler?
aliasing never appears with continuous wave doppler...only with pulsed doppler
disadvantages: Range ambiguity
What are the gray shades of a spectrum related to?
amplitude of the reflected signal
number of blood cells creating the reflection
What is color flow doppler?
color flow is a form of two-dimensional doppler
velocity info is coded into colors and superimposed on a 2d gray scale
is a pulsed US technique
has range resolution (range specificity)
subject to aliasing
provides some info on direction of flow
what kind of velocity does color doppler report?
average or mean velocities
continuous wave doppler report peak velocities
spectral doppler measures peak velocities
What is a color map?
it's like a dictionary displayed as a vertical bar of various colors
black region in the center
has two modes: velocity and variance mode
what do the colors in a velocity mode map mean?
the colors provide information on flow DIRECTION and VELOCITY
displays 2 different velocity mode maps
the black regions indicates no doppler shift
above the black = toward transducer, positive doppler shift
colors closest to the black line indicate lower velocity
color change is always up and down, never side to side
What does variance mode do?
distinguishes laminar flow from turbulent flow
What do the colors in a variance mode map mean?
indicate direction of flow
top = toward
different colors side to side = lt side, laminar flow; rt side turbulent flow
colors change up and down and side to side
turblent flow associated with pathology
What is a doppler packet?
with color doppler, multiple ultrasound pulses are used to accurately determine blood velocities.
the multiple pulses are called a packet or ENSEMBLE
Packets composed of a larger number of pulses (larger packets or longer ensemble lengths) have two advantages:
more accurate velocity measurement
increased sensitivity to low flow
packets with more pulses have three disadvantages:
more time needed to acquire data
reduced frame rate
decreased temporal resolution
packet size should be carefully selected to balance what two competing interests of color doppler?
accurate velocity measurements
adequate temporal resolution
what is power doppler
only identifies the presence of a doppler shift - NON DIRECTIONAL COLOR DOPPLER
does NOT evaluate speed or direction
What is power doppler also known as?
the amplitude of the reflection is directly related to the number of moving blood cells
What are the three advantages of power mode?
increased sensitivity to low flow or velocity, such as venous flow or flow in small vessels
unaffected by doppler angles, unless the angel is exactly 90 degrees
no aliasing since the velocity information is ignored
what are the three disadvantages of power mode?
no measurement of velocity or direction
lower frame rates (reduced temporal resolution) than conventional color flow doppler
susceptible to motion of the transducer, patient, or soft tissues which may result in a burst of color or flash artifact
What is ghosting and clutter?
on spectral displays, low frequency Doppler shift artifacts are called clutter with color doppler they are called ghosting artifact
how is clutter and ghosting artifact eliminated?
a WALL FILTER is used to eliminate low frequency doppler shifts from moving anatomy rather than from moving blood cells
wall filters: eliminate low frequency doppler shifts around the baseline of a spectral display...no effect on high frequency
wall filters; eliminate color arising from slow velocity reflectors
wall filters: aka high pass filters
What is crosstalk?
a special form on "mirror image" artifact
only with spectral doppler
appears an identical doppler spectrum both above and below the baseline
the true flow is unidirectional, but it appears bidirectional
Why does crosstalk appear?
doppler gain set too high
the incident angle is near 90 degrees between the sound beam and the flow direction, when the flow is at the beam's focus
What is spectral analysis?
a tool that breaks the complex signal into its basic building blocks
and identifies the individual velocities that make up the reflected doppler signal
what two methods of spectral analysis are currently used?
Fast Fourier Transformation (FFT)
What is FFT?
fast fourier transformation
a digital technique used to process both pulsed and continuous wave doppler signals
what are the two advantages of FFT?
displays all individual velocity components that make up the complex reflected signal
what is the significance of displaying all component frequencies with fast Fourier transforms?
the spectral display of an FFT distinguishes laminar flow from turbulent flow
What is laminar flow?
most blood cells within the sample volume travel at similar velocities.
the spectral trace is a narrow, well defined line
the region between the baseline and the spectrum is called the spectral window (its clear)
What is turbulent flow?
blood flow is chaotic
many different directions and speeds are within the sample volume
the pulsed doppler spectral window is filled in (spectral broading)
Reynolds number >2000 = turbulent flow
what is spectral broading?
a wider range of velocities and doppler shifts within a sample volume
What is autocorrelation?
aka: correlation function
the digital technique used to analyze COLOR FLOW DOPPLER
used with color doppler because of the enormous amount of data that is processed
somewhat less accurate but way faster to perform than FFT
Summary of continuous wave:
identifies highest velocity jets anywhere along the length of the US beam
very good temporal resolution
peak velocity measurements
summary of pulsed wave:
accurately identifies the location of flow
very good temporal resolution
subject to aliasing
peak velocity measurements
Summary of color flow:
provides 2D flow info directly on anatomic image
size of color jet is most affected by color doppler gain settings
reduced temporal resolution due to multiple packets
based on pulsed US, subject to aliasing
mean velocity measurements
Summary of Power mode:
used with low velocity or small volume blood flow
greater sensitivity than color flow
lowest temporal resolution
subject to flash artifact, not to aliasing
no velocity measurements
What is Doppler Shift?
Change in frequency
When the sound source and receiver move closer together or farther apart
*principal used to measure the velocity of blood in circulation*
Modern ultrasound systems actually measure the __________ ________ between the received and transmitted sound waves
Compare and Contrast Pulsed Doppler and CW Doppler
range resolution ---------------------range ambiguity
sample volume -----------------------region of overlap
Nyquist Limit (limited max velocity) ---unlimited max velocity
aliasing ------------------------------no aliasing
Compare and contrast Pulsed Doppler Transducer and CW Doppler Transducer
At least one crystal ------------------ at least 2 crystals
dampened PZT ---------------------- undampened PZT
low Q-factor --------------------------high Q factor
wide bandwidth --------------------- narrow bandwidth
lower sensitivity ---------------------higher sensitvity