Doppler Flashcards
Doppler Shift (Doppler Frequency)
The change or variation in the freq of sound as a result of motion between the sound source and the receiver
What is the difference between received and transmitted frequencies?
Doppler shift
When reflected frequency is higher than transmitted?
positive change
reflected freq is less than transmitted?
negative change
Doppler measures _____ not _______
frequency shift, not amplitude
velocity, not speed
untis of freq
Hz
Typical audible values created when sound reflects off of RBC’s
20Hz-20 kHz
flow towards the transducers?
increased freq
flow away from the transducers
decreased frequencies
frequency shift is related to what?
velocity
2 MHz-10 MHz transducers are usedto preform doppler ultrasound studies, but doppler shift ranges from?
20Hz-20kHz
The process of extracting the Doppler freq (low) form the transducer frequency (high)?
demodulation
Doppler shift=
received freq - transmitted freq
doppler equation
doppler shift= 2xrefelctor speedxincident frequencyxcos angle/ prop speed
Doppler shift is directly related to:
RBC speed, frequency of transducer, cos of angle between flow and sound beam
Doppler shift is inversely related to:
speed of sound in the medium
velocity (measured)=
true velocityxcos angle
Non-choerent processing used with what
uni-directional doppler
ultrasound system that displays real-time images and Doppler simultaneously is called?
duplex
Phase quadrature processing used in?
bidirectional doppler
number of cyrstals in a CW doppler
2
Advantage of CW doppler
Able to measure very high velocities
Disadvantage of CW Doppler
range ambiguity
Echoes arise from entire length of overlap between the transmit and receive beams
range ambiguity
how manycrystals are needed for PW Doppler
1
Advantage of PW Doppler
Echoes only arise from the area of integration, ample volume or gate. We adjust the receive gate. Range resolution
Disadvantage of PW doppler
Aliasing, errors in measuring high velocities
High velocity measurements are inaccurate if the pulsed Doppler sampling rate is too low in comparison to measured blood velocity?
Aliasing
What should be done to improve the ability to measure the max velocity with Doppler?
Eliminate aliasing
The doppler freq at which aliasing occurs
Nyquist limit (nyquist freq)
PRF/2
Nyquist limit (kHz)
Aliasing is more likely to occur with higher or lower frquencies?
Higher
Five ways to eliminate aliasing
- adjust scale
- select transducers with low freq
- select new view with shallower sample volume
- use CW doppler
- baseline shift
PRF and Nyquist limit are lower with shallower or deeper sample volumes?
deeper
______ sampler volumes (gates) create Doppler spectra with larger, cleaner envelopes
smaller
Larger sample volumes create doppler sepectra with smaller envelopes known as what?
Spectral broadening
The horizontial axis of a doppler spectrum is?
time
The vertical axis of a Doppler spectrum is?
doppler shift or velocity
relationship between transducer frequency and doppler shift
when freq is halved, doppler shift is half
Gray shades on a Doppler spectrum are related to:
Amplitude of the reflected signal or the number of RBC’s creating the reflection
pulsed doppler
- Min of one crystal
- range resolution
- limit on max velocity
- damped, low Q, wide bandwidth
Continuous Doppler
- min of two crystals
- range ambiguity, region of overlap
- unlimited max velocity
- undamped, high Q, narrow bandwidth
imaging transducers
- normal incidence
- higher freq- improved resolution
- pulsed wave only
- at least 1 crystal
Doppler transducers
- 0 to 180 degrees
- lower freq
- pulsed or CW
- 1 (PW) or 2 (CW) crystals
color doppler reports
average velocities
variance mode
color map that varies side to side. left side: laminar right side: turbulent
Which setting has the greatest effect on jet size?
Color doppler gain
Multiple ultrasound pulses are needed to accurately determine RBC velocities by doppler?
A packet or ensemble length
Advantages of more pulses in the packet
greater accuracy of the velocity measurement sensitivity to low flow
disadvantages of more pulses in the packet?
More time is required to acquire info
frame rate is reduced
temporal resolution is decreased
Why does color doppler measure mean velocity?
the packet size must balance between accurate velocity measurements and temporal resolution
Synonyms for power doppler
energy mode or color angio
Advantages of power doppler
- increased sensitivity to low flow
- not effected by doppler angle unless 90 degrees
- no aliasing
limitations of power doppler
- no measurement of velocity
- slower frame rates
- susceptible to motion (flash artifact)
performed to extract or identify the individual freq making up a complex signal?
spectral analysis
Spectral analysis method for pulsed or CW doppler
Fast Fourier transform
spectral analysis for color flow doppler
autocorrelation or correlation function
FFT and autocorrelation are what type of techniques
digital
used due to the large amount of Doppler information that requires processing?
Autocorrelation
Slightly less accurate, but faster than FFT
Can a successful Doppler exam be preformed regardless of a patients hematocrit?
yes
Ghosting
Low velocity motion from pulsating vessel walls can also produce small doppler shifts that bleed into surrounding anatomy
High pass filter
Eliminates low magnitude Doppler shifts that are created from moving anatomy rather than red blood cells
Wall filters are used to reject?
Clutter
color flash is also called?
ghosting
Cross talk is in?
doppler only
Special form of mirror image where the Doppler spectrum appears above and below the baseline?
Cross talk
Cause of crosstalk
Doppler gain set too high
Incident angle near 90 degrees when flow is at focus
