Physics Ch. 19 & 20 Flashcards
A change in frequency that comes from motion between sound source and the receiver as they move closer and farther apart
Doppler shift
Another name for doppler shift
doppler frequency
What is doppler used to measure
velocity of blood in the circulation
Process of extracting the low doppler frequency change from the transducers transmitted frequency
demodulation
Doppler shift (HZ)=
reflected frequency - transmitted frequency
Doppler shifts are in the _____ range of frequencies (_______)
audible, 20Hz-20kHz
Doppler shifts are created when transmitted sound wave strikes
moving red blood cells
Blod flow moving toward the transducer is
positive doppler shift
Positive doppler shift: the _____ frequency is HIGHER than the _____ frequency
reflected, transmitted
Blood flow is moving away from the transducer
negative doppler shift
Negative doppler shift: the reflected frequency is _____ than the transmitted frequency
lower
Speed is _____ only
magnitude
Velocity is ______ and ______
magnitude, direction
Doppler shift is directly related to
velocity, frequency, cosine
Doppler shift is inversely related to
speed of sound in soft tissue
The faster the velocity, the _____ the doppler shift
greater
What is the doppler equation
2 x vel of blood x transducer frequency x cosine / prop speed
Why is there a 2 in the doppler equation
First when sound from transducer hits the moving blood cells / Second when the transducer receives the returning sound wave
Doppler shift is measured in units of
frequency
Velocity is measured in units of
cm/sec
If the transducers frequency is doubled, the measured ______ will also be doubled
doppler shift
What about velocity changes with transducer frequency
the calculated velocities will be the same regardless of the frequency of the transducer
The measurement of the doppler depends on the relationship between the
direction of blood flow and direction in which the sound wave propagates
When the blood cells are moving parallel to the sound beam (__ to __) the
0 degrees- 180 degrees, entire velocity is measured and 100% accurate
When blood cells are moving perpendicular (____) to the sound beam, there is
no doppler shift and no velocity measurement is attainable
If the sound beam and flow direction are not parallel, only a percentage of the true velocity is measured and that depends on the
cosine of the angle between the sound beam and the direction of flow
Measured velocity=
true velocity x cosine
Positive and negative doppler shift. Flow toward and away from the transducer
bidirectional doppler
Signal processing technique for bidirectional doppler
phase quadrature/quadrature detection
Doppler that requires TWO crystals
continuous wave doppler
What do the two CW doppler crystals do
one constantly transmits and the other constantly receives
What is the advantage of CW
ability to accurately measure very high velocities
Disadvantages of CW and what is the name for it
cannot determine exact location of the highest velocity, range ambiguity
Reflections created during CW from blood cells located deeper will have
lower amplitude (weaker signal) than reflections from shallower depths
What are CW transducers also called
dedicated CW transducer, blind transducer, pedoff transducer
CW transducers do not have
backing material
What are the 4 qualities of a dedicated CW transducer
undampened transmitted signal, narrow bandwidth, high Q factor, higher sensitivity
Simultaneous anatomic imaging and doppler is called
duplex imaging
Only one crystal is necessary, alternates between sending and receiving
Pulsed wave doppler
2 advantages of PW
select exact location for velocity measurement / use of sample volume or gate
When choosing the exact location for PW, what is this called
range resolution, range specificity, range specific
What does the sample volume/gate use
time of flight calculation
What is the disadvantage of PW
inaccurate measurement of high velocity signals (aliasing)
PW doppler transducers have backing material so they have
low Q factor, low sensitivity, wide bandwidth
Doppler gain settings alter the gray scale content of the spectral display
spectral doppler gain
Spectral doppler wall filters selectively eliminates ____ frequency doppler shifts near the ____ of the spectral display
low, baseline
The wall filter does not affect the appearance of
higher velocity flows
Most common error associated with doppler ultrasound
aliasing
An alias is a ______ which is very _____ velocities in one direction are ____ displayed as going in the ______ direction
false identity, high, incorrectly, opposite
The very top of the display is called
nyquist limit
The part of the spectrum appearing at the bottom is called
aliasing
Aliasing occurs when the
doppler PRF is too low in comparison to the measured blood velocities (nyquist limit surpassed)
Nyquist limit=
PRF/2
highest doppler shift or velocity that can be measured without the appearance of aliasing
nyquist limit
Two ways to avoid aliasing
raise the nyquist limit (PRF), reduce the doppler shift
The sampling rate of PW doppler is the
PRF
When the sample volume is deep, the PRF is
low
Deeper sample volume shave a lower _____ and create _____
PRF, aliasing
When the sampler volume is shallow, the PRF is _____, Nyquist limit is _____
high, high
Five techniques to avoid/correct aliasing
- adjust scale (PRF) to max 2. Use a different u/s window and shallower sam volume 3. lower frequency trans 4. adjust baseline 5. use cw doppler
Gray shades of spectral display are related to
amplitude of reflected signal, number of blood cells creating the reflection
Velocity info is coded into colors and superimposed on a 2D gray scale anatomic image
color flow doppler
Color doppler provided info regarding ______ and is considered _____
direction of flow, semi-quantitative
Color doppler reports/measures ____ or ____ velocities
average, mean
Two most commonly used maps/dictionaries are called
velocity mode, variance mode
The colors provide info on flow direction and mean velocity and is shown on the color map in which direction
velocity mode, top to bottom
Provides more info, can distinguish between laminar and turbulent flow. What direction is the color map read
Cariance mode, side to side
The multiple ultrasound pulses for blood velocities is called
packet/ensemble
To correct aliasing on color doppler you
increase scale/PRF/Nyquist limit
What does the color look like with aliasing
color wrap around reds to blues
What does color look like with flow reversal
red to black to blue
The wall filter selectively eliminateds _______ doppler shifts. The wall filter leaves the _________ unchanged.
low frequency, high velocity flows
Identifies only the presence of a doppler shift
power doppler
Does power doppler evaluate velocity or direction
no
Power doppler is also called
energy mode or angio
In power mode, the amplitude of the reflection is directly related to the
number of moving blood cells
Advantages of power mode
increased sensitivity to low flow, unaffected dopp angles, no aliasing
Disadvantage to power mode
no velocity or flow direction
On spectral analysis, the artifacts are called
clutter
With color doppler, the artifacts are called
ghosting
How do we eliminate ghosting or clutter artifacts
wall filter
Used to eliminate low frequency doppler shifts from moving anatomy rather than from moving blood cells (“reject”)
wall filter
Special form of mirror image artifact that only arises with spectral doppler
crosstalk
Appears as an identical doppler spectrum both above and below the baseline
crosstalk
Tool that breaks the complex signal into basic building blocks and identifies the individual velocities that make up the reflected doppler signal
spectral analysis
What are the two methods of spectral analysis
fast fourier transform (FFT), autocorrelation
A digital technique used to process both pulsed and continuous wave doppler signals
fast fourier transform
Spectral display of an FFT distinguishes
laminar flow from turbulent flow
Digital technique used to analyze color doppler, less accurate but faster
autocorrelation
