Week 10 Flashcards
Define Range Ambiguity
the location of the flow cannot be determined due to no image
A wall filter filters out _________ ƒ’s, and leaves ________ƒ’s from ________ on the display.
low, high, display
______ velocities with spectral broadening often need ____________ ____________ to see waveforms.
high, increased gain
What does demodulation do?
calculates the ƒ shift
What does phase quadrature do?
determines positive or negative ƒ shifts
PW doppler uses ________ ________ of the returning sound waves rather than continuously monitoring
frequent samples
Another term for sampling frequency is what?
pulse repetition frequency
what does PRF stand for?
pulse repetition frequency
In Doppler, the more frequently the returning echo wave is sampled, the more __________ the display.
accurate
The PRF for Doppler is usually what?
4,000 - 12,000 / sec
The result of less than two samples per cycle is what?
Aliasing
Aliasing is due to what?
too low PRF for the highest frequencies
The PRF that will give you less than 2 samples per cycle is what?
the Nyquist limit
You usually want the PRF to be at least ______ the highest f shift or PSV
2x
What are the 6 ways to eliminate Aliasing?
- Increase scale (increases PRF)
- Decrease baseline
- Use a lower frequency transducer
- Find a higher angle (always <60º)
- Change to a CW Doppler (No velocity)
- Use high PRF mode (possible range ambiguity)
High PRF mode is used to measure what?
high velocities
Lower ƒ transducers are better to detect _________ velocities due to __________ ƒ shifts.
higher, lower
Because of the Doppler equation: a __________ transducer yields lower ƒ shifts
lower
If blood flows __________ the Doppler beam it creates a higher frequency. ( ________ shift)
toward, positive
If blood flows away from the Doppler beam it creates a ________ frequency ( _______ shift)
lower, negative
Flow toward the Doppler beam is displayed _______ the baseline or ________.
above, positive
When is the flow toward the beam displayed below the baseline?
When the waveform is inverted
What two things tell you whether the waveform has been inverted or not?
The INVERTED notation
Negative numbers on the scale sample
Flow going toward the left side of the screen is always going toward the _______.
Head
Flow going towards the feet will always be going to the ______ side of the screen.
right
If a Doppler Beam is steered vertically, flow away from the beam is in the ________ direction.
downward
If flow is going downward, it is going _________ the skin.
away from the skin
If a peak Doppler frequency shift for an echo is 6 kHz, at which of the following PRF’s will aliasing occur?
a. 6 kHz
b. 12 kHz
c. 18 kHz
d. 24 kHz
a. 6 kHz
For a given Doppler signal, the Doppler angle is 0º and the highest Doppler shift is 6 kHz. The PRF is 9 kHz. Will aliasing occur?
Yes. 6 x 2 = 12. the PRF is 9. 9<12.
