Doppler Flashcards
Cosine for 0, 60, and 90 degree angles
1
.5
0
If Doppler shift is positive then flow:
Is moving towards transducer
If flow is away from transducer the Doppler shift is:
Negative
The machine measures the _____________ but calculates the _____________.
Frequency shift
Velocity
Initial f: 5 MHz
Reflected f: 4.85 MHz
What is Doppler shift?
-150kHz
Advantage of CW Doppler
No upper limits for velocity
No aliasing
Disadvantage of CW
Range ambiguity
What is fast Fourier transform?
Breaks down signal to produce spectral waveform
What is display on horizontal axis of spectral analysis?
Time
What is the vertical axis present?
Velocity
What does the brightness of the display represent?
The amplitude of the signal
Causes of spectral broadening
Tortuous vessels
Turbulent flow
Sample gate too big
Sample gate too close to wall
What is the NyQuist limit? How do we find it?
Level that aliasing begins
1/2PRF
Advantages of PW
Range resolution
Disadvantage of PW
Aliasing doesn’t allow accurate measurements of high velocities
What is a key indicator of severity of disease within a vessel?
Peak velocity
Color flow mapping is a form of pulsed Doppler.
T/F
True
How is a “packet” defined?
of pulses sent per line of color
How big is a packet usually?
Between 3-30 pulses per line
What’s another term for packet size?
Ensemble length
Hue is:
Perceived color
Faster velocities are represented by:
Lighter hues
More saturated means:
Slower flow
Less saturation:
Faster flow
How are velocity maps arranged?
White Lighter color Color Darker color Black Darker color Color Lighter color White
Variance maps add color in a:
Side to side orientation
What are the side colors usually and what are they used for?
Green or yellow
To tag turbulent flow
Color Doppler is not angle dependent.
T/F
False
Advantages of color Doppler
Can see vessels too small to be seen on gray scale image
Helps find PSV in vessel
Documentation of occlusions, severe stenosis, turbulent flow
Disadvantage of color Doppler
Aliasing
Can’t pick up slow/low flow areas
Angle dependent
Fast Fourier transform is faster than autocorrelation.
T/F
False
Temporal resolution is decreased with autocorrelation because:
Large packet size
Lower frame rate
Time domain processing/correlation determines velocity of moving reflector by:
Measuring change in spatial location during known time interval
Time domain processing uses a wide bandwidth to improve:
Spatial info
The Doppler angle MUST be known for time domain correlation.
T/F
True
Power Doppler measures _______________ of flow.
Concentration
Power Doppler is able to:
Detect low flow in smaller vessels on venous flow
Power Doppler is angle dependent.
T/F
False
As the angle between the transducer and the flow decreases, the Doppler shift:
Increases
What is clutter?
Noise in Doppler signal caused by high amplitude, Doppler shifted echoes
Wall filter is a type of:
Reject control
