Physics Doppler Effect
what is the cosine of 180
-1
the functions of a doppler detector include
-amplification -phase quadrature detection -doppler shift detection -sign determination
stenosis effects what in the spectral analysis?
-peak systolic flow speed -end diastolic flow speed -spectral broadening
as a stenosis progresses, what increases?
-systolic doppler shift -diastolic doppler shift -spectral broadening
What is the cosine of 60?
.5
at what angles between the sound beam and the direction of motion will the doppler shift be the highest?
0 or 180
b-mode imaging requires at least ____ pulse
1
the cosine of 0 is
1
What are the two terms used to describe PW's ability to select the exact location where velocities are measured?
1) range resolution 2) range specificity
What 5 pieces of information does Autocorrelation contain?
1. Doppler Shift 2. Mean 3. Sign 4. Variance 5. Power
What are five techniques that can be used to avoid aliasing artifact?
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
The functions of a doppler detector include what 4 things?
1. amplification 2. phase quadrature detection 3. doppler shift detection 4. sign determination
What are the 4 forms of doppler presentation?
1. audible 2. color doppler 3. spectral doppler 4. strip-chart recording
the doppler sample volume is determined by what three things?
1. beam width 2. gate length 3. emitted pulse length
3 factors that create more aliasing
1. faster blood velocity 2. higher frequency transducer 3. deep gate (low PRF)
Lack of color in a vessel containing blood flow may be attributable to what 3 things?
1. low color gain 2. a high wall filter setting 3. a low priority setting
Compared with doppler-shift imaging, doppler-power imaging is what 4 things?
1. more sensitive 2. angle independent 3. aliasing independent 4. speed independent
flow information provides what 4 things?
1. presence of flow 2. direction of flow 3. speed of flow 4. character of flow
What are two ways to avoid aliasing?
1. raise the Nyquist limit 2. lower baseline
3 factors that create less aliasing
1. slower blood velocity 2. lower frequency transducer 3. shallow gate (high PRF)
the amount of increase or decrease in doppler frequency depends on what 3 things?
1. speed 2. angle 3. frequency of the wave emitted
Autocorrelation produces what 2 things?
1. the mean value of the Doppler shift 2. variance
What are the two things that a good doppler angle is critical for?
1. to determine flow direction 2. to accurately determine speed
dedicated continuous wave transducers do not use backing material, which results in the following 4 things:
1. undampened transmitted signal 2. narrow bandwidth 3. high quality factor 4. higher sensitivity
In a single frame, color can change in a vessel because of what 4 things?
1. vessel curvature 2. sector format 3. helical flow 4. diastolic flow reversal
In practice, approximately _____ pulses are required to obtain one line of color info
10
What is the average ensemble length?
10-20
how many crystals are present in CW doppler?
2
If angle correction is set at zero degrees but should be 60 degrees, the display indicates a flow speed of 100 cm/s. The correct flow speed is
200 cm/s
What is the minimum ensemble length?
3
How many frames per second are produced by a color Doppler instrument?
40 or less
doppler instruments have pulse lengths of how many cycles?
5-30
If a 5-kHz Doppler shift corresponds to 100 cm/s, then a 2.5 kHz shift corresponds to _____ cm/s
50
If angle correction is set at 60 degrees but should be zero degrees, the display indicates a flow speed of 100 cm/s. The correct flow speed is.
50 cm/s
Frame Rate in Color
77,000/depth x LPF ensemble length
Grayscale imaging is best performed at what angle?
90 degrees
what is the duty factor in PW doppler?
<1%
Color Doppler instruments use an _______ technique to yield Doppler info in real time
Autocorrelation
What is the typical range of Doppler shift found in diagnostic imaging examinations?
Between 20 Hz and 20 kHz (in the audible range)
A region of bright color on a doppler shift display always indicates the highest flow speeds. True or false.
False
Angle is not important in transverse color Doppler views through vessels. True or False.
False
doppler ultrasound provides information about flow conditions only at the site of measurement. True or False.
False
Higher flow speed always produces a higher doppler shift on a spectral display. True or False.
False (remember the doppler angle)
How is "doppler shift" represented in equations?
Fd
What is the doppler equation?
Fd = 2x Fo x v x cosθ /c
Color Doppler instruments present two-dimensional, color-coded images representing ___________ that are superimposed on gray-scale images representing __________
Flow or motion, anatomy
How is the operating frequency represented in equations?
Fo
What is presented in real time on a color Doppler display
Gray-scale anatomy and flow direction
What is the unit of doppler shifts?
Hz
The larger the Doppler angle the [lesser, more] the Doppler shift
Lesser
The very top of a doppler display is called the
Nyquist limit
What is the Nyquist Limit (formula)
PRF(Hz)/2
What type of Doppler is not angle dependent?
Power Doppler
what formula is used to yield effective sample volume length?
SPL/2
formula to determine velocity
V = Fd x C/ 2 x Fo x cos(theta)
When is the blood cell velocity 100% accurate?
When blood cells are moving parallel to the sound beam
What is the doppler effect?
a change in frequency caused by the motion of a sound source
an earlier gate time means _____ sample volume depth
a shallower
What is the primary advantage of PW doppler?
ability to select the exact location where velocities are measured
What is the advantage of CW doppler?
able to display high velocities without aliasing
What is the biggest disadvantage of PW doppler over CW?
aliasing
the part of a doppler spectrum that wraps around and appears at the bottom of the screen is called
aliasing
Which is typically higher, arterial or venous velocities?
arterial
The _____ technique is commonly used to detect echo Doppler shifts in color Doppler instruments?
autocorrelation
power doppler uses _____ to detect the strength of the doppler shift
autocorrelation
the width of the sample volume is equal to the
beam width
Why is the measured velocity zero when the direction of flow is perpendicular to the sound beam?
because the cosine of 90 is zero
If an instrument distinguishes between positive and negative doppler shifts, it is called
bidirectional
Color map baselines are always represented by what color?
black
the doppler shift is proportional to the
blood flow
doppler signal power is proportional to
cell concentration
Can you control PRF in B-mode or Color?
color
Ensemble length AKA
color packet
for blood flowing in a vessel with a plug flow profile, the doppler shift is ____ across the vessel
constant
when incident sound direction and reflector motion are not parallel, calculation of the reflected frequency involves the ______ of the angle between these directions
cosine
With constant flow speed and source frequency an increase in doppler angle creates a _____ in doppler shift
decrease
If cosine is increased velocity is
decreased
If doppler angle is doubled, the doppler shift is
decreased
Increasing the ensemble length _____ the frame rate
decreases
as the angle indicator increases, the cosine
decreases
the process of extracting the low Doppler frequency from the transducer's carrier frequency is called
demodulation
range gating enables what two things?
depth selectivity and a small doppler sample volume
echo voltages from the transducer are processed in the
detector
What does zero crossing do?
detects motion above or below the baseline
The measurement of the doppler frequency depends on the relationship between what two things?
direction of blood flow and the direction in which the sound wave propagates
how is speed reported?
distance over time
to proceed from a measurement of doppler shift frequency to a calculation of flow speed, what must be known or assumed?
doppler angle
What do you call the many different doppler shifts received from the vessel by the system?
doppler frequency spectrum
the y-axis of a doppler spectrum represents
doppler shift or velocity
How do you acquire flow information?
doppler ultrasound
the ______ effect is used to detect and measure ______ in vessels
doppler, flow
If a transducer's frequency is doubled the measured doppler shift will
double
simultaneous anatomic imaging and doppler is called
duplex imaging
What is increased if doppler angle is increased?
effect of angle error
Doppler shift displays are not dependent on Doppler angle. True or False?
false
If a color Doppler instrument shows two colors in the same vessel, it always means flow is occurring in opposite directions in the vessel. True or false?
false
True or False. Shallower sample volumes result in more aliasing
false
the [slower, faster] the velocity, the greater the doppler shift
faster
Doppler shift is directly related to the ______ of the transmitted sound
frequency
motion of an echo-generating structure causes an echo to have a different _____ from that of the emitted pulse
frequency
The doppler effect is presented as a ______ when the source and the receiver are ________
frequency shift, in motion relative to each other
the faster the velocity the [lesser, greater] the doppler shift
greater
doppler measurements are not reliably achieved at doppler angles
greater than 60 degrees
When the measured velocity is at 60 degrees it is what percent of the actual velocity?
half
Doubling the width of a color window produces a ______ frame rate
halved
If the cosine (theta) is reduced by half the doppler shift will be
halved
When the velocity is halved the doppler shift is
halved
flow reversal in diastole indicates
high distal resistance
When the sample volume is shallow, the PRF is [low, high] and the Nyquist limit is [low, high]
high, high
In a positive doppler shift the received frequency is [lower, higher] than the transmitted frequency
higher
what is slowed to allow for the acquisition of doppler information between frames?
imaging frame rates
A shorter gate length does what in a spectral display?
improves the quality
Increasing ensemble length _____ color sensitivity and accuracy and _____ frame rate
improves, decreases
With constant flow speed and source frequency a decrease in doppler angle creates a ______ in doppler shift
increase
decreased distal resistance normally causes end diastolic flow to
increase
if doppler angle is increased then velocity is
increased
doppler angle and cosine have a(n) _______ relationship
inverse
What is the doppler equation for?
it is used to quantify the doppler effect
What type of situation does the lack of TGC's create in CW ultrasound?
it may falsely appear that deeper vessels have less flow
What is another disadvantage of CW ultrasound besides range ambiguity?
lack of TGCs
the doppler sample volume in CW is [small, large]
large
When an angle exists between the direction of flow and the sound beam, the measured velocity is [less, more] than the true velocity
less
color doppler frame rates are ____ gray-scale rates
less than
Aliasing occurs when the doppler sampling rate is too ______ in comparison to the measured blood velocities
low
When the sample volume is deep, the PRF is [low, high], and the Nyquist limit is [low, high]
low, low
Pulsed wave doppler has ______ quality factor, _____ sensitivity, and _____ bandwidth pulses
low, low, wide
In a negative doppler shift the received frequency is [lower, higher] than the transmitted frequency
lower
speed is purely a
magnitude
velocity is defined by a
magnitude and a direction
Color doppler measures ____ velocity while spectral doppler measures ______ velocity
mean, peak
higher frequency transducers create [less, more] aliasing
more
what is required for doppler effect?
motion
When blood cells move away from the transducer, the Doppler shift is [negative, positive]
negative
Disadvantages of Power Doppler
no flow direction info no speed info no flow characteristics info
What is the disadvantage of CW doppler?
no imaging
what is the main disadvantage of selecting a new ultrasonic view with a shallower sample volume to reduce aliasing?
none
Advantages of Power doppler
not dependent on doppler shift formula no aliasing no angle dependence more sensitive to slow flow and small vessels
What is the job of the two crystals in a CW transducer?
one continuously sends, the other continuously receives
doppler shifts are typically ________ of the operating frequency
one thousandth
in CW, doppler shifts are determined by mixing the returning voltages with the CW voltage from the
oscillator
The cosine gives the component of the flow velocity vector that is _______ to the sound beam
parallel
What is the commonly used signal processing technique for bi-directional doppler?
phase quadrature, or quadrature detection
When blood cells move toward the transducer, the Doppler shift is [negative, positive]
positive
Doppler-power imaging indicates (with color) the ____ of flow
presence
Doppler-shift imaging indicates (with color) the _(4)__ of flow
presence, direction, speed, character
spectral analysis does what with doppler shift frequencies?
presents them in frequency order
What is the Pulsed Wave operating frequency equation?
propagation speed / 2 x crystal thickness
color doppler has high ______ resolution
range
What do you call the overlap between the transmit and receive beams in CW ultrasound?
range ambiguity
through _______, pulsed wave doppler has the ability to select information from a particular depth along the beam
range gating
the doppler shift is the difference between _____ and _____ frequencies
received, emitted
what are the reflectors in human circulation?
red blood cells
what is the main disadvantage of adjusting the scale to a maximum to avoid aliasing?
reduces sensitivity to low velocities
what does the wall filter do?
rejects frequencies below an adjustable value
range gate aka
sample volume
What do you call a marker that shows where doppler information is being obtained from?
sample volume/gate
longer gate lengths are used when
searching for a desired vessel and flow location
most importantly, continuous wave transducers have increased
sensitivity
the spectral display is a presentation of doppler _____ versus ______
spectra, time
shorter gate lengths are used for
spectral analysis and evaluation
In continuous wave, doppler shifts are commonly sent through a _______ to a _______
spectrum analyzer to a spectral display
power doppler uses autocorrelation process to detect the _____ of doppler shift instead of the _______ that color doppler uses
strength, mean value
Is the system able to more accurately measure velocities in deep or superficial structures?
superficial
The frequency at which aliasing occurs is called
the Nyquist Limit
What can be thought of like a speed limit?
the Nyquist limit
The sampling rate of pulsed doppler is the same as
the PRF
What is the doppler angle defined as?
the angle between the sound beam direction and the direction of flow
what is the doppler effect?
the change in frequency/wavelength of a sound that results from motion
What determines the strength of power doppler?
the concentration of moving reflectors producing Doppler shift
What is the unit of power doppler?
the concentration of reflectors
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
How do you determine the doppler shift?
the difference between the received frequency and transmitted frequency
Steering the color window to the right or left changes what two things?
the doppler angle and the doppler shift
what determines the frequency in a CW transducer?
the drive voltage frequency
Modern ultrasound systems actually measure what in hertz between the received and transmitted sound waves?
the frequency difference
What is Fast Fourier Transform?
the mathematical technique the instrument uses to derive the doppler spectrum
What is ensemble length?
the number of pulses used for each scan line
In CW, the sample volume is where?
the overlapping region of the transmitting and receiving beams
Which portion of the motion contributes to the doppler effect?
the portion that is parallel to the sound beam
What is doppler shift?
the quantitative measurement of doppler effect
Where does auto-correlation occur?
the signal processor
what removes clutter?
the wall filter
Why is there a "2" in the doppler equation?
there are actually two doppler shifts
how are frequency shift and doppler shift related?
they are the same thing
what determines the frequency in a pulsed wave transducer?
thickness of crystal
If all the cells in a vessel were moving at the same constant speed, the spectral trace would be a
thin horizontal
color doppler requires at least _____ pulses
three
The x-axis of a doppler spectrum represents
time
True or False. The higher the emitted frequency, the more likely a pulsed wave signal is to alias
true
True or false. Only pulsed wave doppler exams have a sample volume.
true
measured velocity =
true velocity x cos(θ)
What type of flow is synonymous with spectral broadening?
turbulent flow
common causes of spectral broadening
turbulent or disturbed flow over gain sample volume too wide incorrect placement of sample volume
Steering the color window to the right or left produces a _____ frame rate?
unchanged
doppler shift is directly related to the _____ of the blood cells
velocity
What two major things is doppler shift directly related to?
velocity and transducer frequency
Doppler frequencies indicate _____ , not _______
velocity, not speed
Spectral broadening is a ______ of the spectral trace
vertical thickening
What control can be used to help with clutter?
wall filter
Name two things that will reduce the frame rate of a color Doppler image.
wider color window or increased ensemble length
if there is no doppler effect can there still be sound?
yes