US Physics 19 Flashcards
what is demodulation?
Extracting doppler waveform from the carrier frequency
What is the 2 in the doppler equation
represents 2 doppler shifts. One from moving red blood cells and another shift when sound makes it back to transducer.
What is doppler equation
Doppler shift = (2 x velocity of blood x transducer frequency x cos Θ)/propagation speed
What is the cosign of 0
1
what is the cosign of 90
0
What is the cosign of 60
0.5
What is greatest advantage of continuous wave doppler
ability to measure accurately even at high velocities
Primary disadvantage of continuous wave doppler and secondary disadvantage
range ambiguity - CW doppler signals arise form all blood cells in region overlap beams makes difficulty to get exact location of moving blood cells. Lack of time gain compensation. Lower amplitude reflections of deeper blood cells
What don’t cw dopplers use in device
backing material
What does lack of backing material do
improved quality factor, lower bandwidth and greater sensistivity
What is disadvantage of pw dopplers
they use backing material like imaging transducers
What’s the downside to using backing material
worse quality factor, greater bandwidth loss of sensitivity
what’s the nyquist limit
amount over which aliasing occurs. Velocity above that of display causes aliasing
Under what circumstances does aliasing occur?
Pulsed wave only (not continuous), when the sampling rate is too low compared with the measured velocity
What is equation of nyquist limit
PRF/2
How do you avoid aliasing (3 ways)
change the doppler shift or increase the PRF, thus increasing the nyquist limit, lower frequency probe
At what depth are you more likely to get aliasing?
When insonating deep because PRF is lower
Why does lower frequency probe cause less aliasing
Lower amount of doppler shift (think about doppler shift equation)
What is downside to adjusting scale to eliminate aliasing?
When you adjust scale to max you are also adjusting PRF to maximum. Lowers sensitivity to lower velocity.
What does scale adjustment do?
When you adjust scale to max you are also adjusting PRF to maximum.
Downsides to repositioning probe to eliminate aliasing by finding area where area of clinical interest is more shallow
there are no downsides to it
Downsides to selecting lower frequency transducer to eliminate aliasing
with duplex imaging it can produce lower quality image
Downside to using baseline shift to eliminate aliasing?
If signal completely wraps around itself it will be ineffective. Otherwise there are none.
Can you get aliasing with continuous wave doppler? What is downside to CW?
Never. You have range ambiguity.
