Flashcards in Chapter 19 Deck (100):
What is the change in frequency called?
Doppler shift or Doppler freq
When does the freq of sound changes?
when the sound source and receiver move closer together or farther apart
When does the freq of sound doesn't change?
when the distance between the sound source and receiver remains constant
The process of extracting the low Doppler freq from the transducers carrier freq is called what?
When is the Doppler shift positive?
when the reflected sound has a higher freq than the transmitted sound
What is the Doppler shift formula? (2)
Hz=reflected freq- transmitted freq
Doppler shift= 2vf (cos)/c
When is the Doppler shift negative?
when the reflected freq is lower than the transmitted freq
When are Doppler shifts created?
when transmitted sound waves strike moving red blood cells
Doppler frequencies indicate___ not speed
What is the speed of Doppler defined as?
speed which a purely a magnitude, indicates the distance that a red blood cell moves in 1 second
What is velocity defined as?
defined by magnitude and a direction
What is the relationship between velocity of blood and Doppler shift measured in hertz?
What does the x-axis of a Doppler spectrum represent?
what does the y-axis of a Doppler spectrum represent?
Doppler shift or velocity
What is the frequency difference?
difference between the received and transmitted sound waves
What is the relationship between the transmitted freq and the Doppler shift measured in hertz?
When is the entire velocity measured?
when the blood cells are moving parallel to the sound beam
If the sound beam and flow direction are not parallel, what determines how much of the velocity is measured?
the percentage of the true velocity that is measured depends on the cosine of the angle between the sound beam and direction of motion.
What is the formula for the measured velocity for cosine?
=true velocity x cosine angle
What is the relationship between the cosine angle and Doppler shift?
What angles are vessels parallel?
zero or 180 degrees
What is the cosine of zero?
What is the cosine of 180?
What happens to the measured velocity at angle other than zero or 180?
only a portion of the true velocity is measured
What are the values for Doppler shift ranges?
20 hz-20,000 hz
What are typical transducer frequencies?
What is the most common clinical doppler device?
they are bidirectional, which distinguish the direction of low toward or away from the transducer
What is commonly used for signal processing technique for bidirectional Doppler?
phase quadrature, or quadrature detection
True or false: arterial velocities are higher than venous velocities
how many crystals are required for a continuous wave Doppler?
one to transmit and another to receive
what is the advantage of a continuous wave Doppler?
its ability to accurately measure very high velocities.
What are some disadvantages of a continuous wave Doppler?
primary disadvantage is that the exact location of moving blood cells cannot be determined.
lack of TGC
What is range ambiguity?
signal arise from all blood cells in the region of overlap between the transmit and receive beams. this limitation is called range ambiguity
What is duplex imaging?
anatomic imaging and Doppler
Do continuous wave transducers have a backing material?
Since continuous wave transducers do not have a backing material what are the results?
undampened transmitted signal
high quality factor
What does the matching layer do in transducers?
increase the efficiency of sound transmitted in and out of the body
What is the most important thing to remember about dedicated continuous wave transducers?
they have increased sensitivity and they are able to detect low amplitude reflections and small Doppler shifts
in a pulsed wave Doppler, how many PZT crystals are necessary?
they alternate between sending and receiving sound pulses
what are advantages of pulsed wave doppler?
being able to select the exact location where velocities are measured
What is it called when you are able to select the exact location where velocities are measured?
range resolution, range specificity, or freedom from range ambiguity artifact
What is a small marker called in pulsed doppler?
a gate or sample volume
What is a disadvantage of pulsed Doppler?
inaccurate measurement of high velocity signals.
What is aliasing?
high velocity flow in one direction is incorrectly display as traveling in the opposite direction. this inaccuracy is called aliasing, a false identity
What are the characteristics of pulsed wave Doppler transducer?
low quality factor
wide bandwidth pulses
The very top of the display in an aliasing spectrum is what?
What conditions create aliasing?
aliasing occurs only with pulsed Doppler, not with CW doppler
aliasing occurs when the Doppler sampling rate is too low in comparison to the measure blood velocities
What is the formula for nyquist limit?
Hz= PRF (Hz)/2
What is the nyquist limit?
the highest Doppler freq or velocity that can be measured without the appearance of aliasing
What are two ways to avoid aliasing?
raise the Nyquist limit
reduce the Doppler shift
do shallow or deep sample volume have more chance of creating aliasing?
b/c PRF is low and nyquist limit is low
What does shallow gate mean?
What does a deep gate mean?
What is seen with less aliasing?
slower blood velocity
lower freq transducer
shallow gate (high PRF)
What is seen with more aliasing?
faster blood velocity
higher freq transducer
deep gate (low PRF)
What are five techniques to avoid aliasing?
1. adjust the sale to its max
2. select a new ultrasonic view with a shallower sample volume
3. select a lower freq transducer
4. use baseline shift
5. use a CW Doppler
What is the advantage of increasing the PRF?
raises the nyquist limit and reduces aliasing
What is the disadvantage of high PRF?
reduced sensitivity to low velocities
What advantage of selecting a new ultrasonic view with a shallower sample volume do?
Increasing PRF, increases nyquist limit= reduces aliasing
What is a disadvantage of selecting a new ultrasonic view with a shallower sample volume?
What never occurs with CW Doppler?
What occurs as a result of no aliasing on CW Doppler?
What does eliminating aliasing improve?
improves the ability to measure the maximum velocity with Doppler
What is the strategy for adjusting the baseline?
aliasing remains but the display is more appealing to the eye
What are gray shades on a Doppler spectrum related to?
amplitude of the reflected signal
number of blood cells creating the reflection
What does color doppler provide?
is a pulsed US technique
has range resolution
is subject to aliasing
What information does color Doppler provide?
info regarding direction of flow and is considered semi-quantitative
What does color doppler report?
mean or average velocities
What does spectral (pulsed and CW) doppler report?
What are color maps for?
to convert measured velocites into color that appear on the image
What are two type of dictionaries for color maps?
What do the colors in a velocity mode map mean?
the color provide info on flow direction and velocity
What does the black region on a color map mean?
no Doppler shift
What info do variance mode provide?
in addition to direction and speed info, variance mode distinguishes laminar flow from turbulent flow
What does colors on the left side represent on a variance map?
What does colors on the right side represent on a variance map?
turbulent flow, which is associated with pathology
With variance maps, how do the colors change on the map?
side to side as well as up and down
With color Doppler, multiply pulses are used to accurately determine blood velocities. These multiply pulses are called what?
packet or ensemble
What are the advantages for packets composed of a larger number of pulses?
more accurate velocity measurement
increased sensitivity to slow flow
What are disadvantages of packets with more pulses?
more time needed to acquire data
reduced frame rate
decreased temporal resolution
What does power mode identify?
presence of Doppler shift
it doesn't evaluate speed or direction
What is power mode?
nondirectional color Doppler
What are other names for power Doppler?
energy mode or color angio
What is processed in power mode?
the strength (amplitude) of the reflected signal is processed without regard to the bloods direction or speed
What are the three advantages of power mode?
1.increased sensitivity to slow flow or velocity, such as venous flow or flow in small vessels
2. unaffected by Doppler angles, unless the angle is exactly 90 degrees
3. no aliasing, since the velocity info is ignored
What are three disadvantages of power mode?
1. no measurement of velocity or direction
2. lower frame rates than conventional color flow Doppler
3. susceptible to motion of the transducer, patient, or soft tissues, which may result in a burst of color or flash artifact
What are low frequency Doppler shift artifacts called?
What are color doppler artifact called?
How is clutter or ghosting artifact eliminated?
a wall filter set to high
What is another word for wall filter?
high pass filters
Wall filter affects what?
low freq Doppler shifts but doesnt affect high freq Doppler shifts
What is a special form of mirror image artifact that arises only with spectral Doppler?
What does crosstalk results from?
Doppler gain is set too high or
the incident angle is near 90 degrees between the sound beam and the flow direction, when flow is at the beams focus
What is spectral analysis?
a tool that breaks the compelx signal into its basic building blocks and identifies the individual velocities that make up the reflected Doppler signal
What are two methods of spectral analysis that are commonly used?
Fast Fourier Transform (FFT)
What is a digital technique that is used to process both pulsed and CW Doppler signals?
What are the two advantages of FFT?
displays all individual velocity components that make up the complex reflected signal
The ___ display of the FFT distinguishes laminar flow from turbulent flow?
What is the wider range of velocities and Doppler shift within the sample volume?