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.
What is range ambiguity and what do you see it with?
You don’t know the depth you are working with because doppler information continuously reflects back to source before and after the region of interest.
Summarize the strategy employed of the different ways to eliminate aliasing?
- Shift baseline
- Move to shallower region
- Increase scale
- Use continuous wave
- Reduce probe frequency
- Display more visually appealing only
- Increased PRF
- Increased PRF
- Never aliases, but range ambiguity
- Decrease the doppler shift
Compare pulsed doppler to CW doppler transducer (5) qualities
Pulsed/CW at least 1 crystal/always 2 dampened PZT/No dampening low Q factor/high q factor wide bandwidth/low bandwidth lower sensitivity/highwe sensitivity
Effect of frequency on imaging probe vs a doppler probe
high frequency - improved resolution with imaging
Lower frequency - avoids aliasing
What is difference in what velocity is measured with color vs spectral (pulsed and CW) doppler
Pulsed and CW - peak velocity
Color doppler - mean velocity
Are color doppler subject to aliasing?
yes - it uses pulsed wave doppler
What are two types of color mode doppler
Variance mode (indicates laminar and turbulent flow in addition to velocity) and velocity mode (indicates just velocity).
What is clue to which side is laminar vs turbulent with variance maps
laminar = L or left side
How are color doppler images created?
With packets containing multiple pulses
What are upsides/downsides to creating color doppler with packets?
Packets composed of large # pulses = more accurate velocity measurement, inc sensitivity to low flow. More time needed to acquire data, lower frame rate and poorer temporal resolution
What is power color flow doppler?
It displays only power and not speed or direction.
3 upsides and downsides to power color flow doppler?
3 + inc sensitivity to low flow, unaffected by angles (unless exactly 90 degrees), no aliasing
3- no measurement of velocity or direction, lower frame rates than color flow, susceptible to motion of transducer leading to sudden burst of color
what is clutter/ghosting artifact with doppler and what caused by?
low frequency doppler shifts due to slow moving stuff like heart
When do you see clutter vs ghosting?
Clutter - seen with spectral displays
Ghosting - Seen with color flow
How do you get rid of clutter/ghosting
Wall filter to reject low frequency shifts
Another name for wall filter
high pass filter
What does crosstalk look like and what is cause?
Wave above and below baseline appears the same. It reflects either gain set too high or incident angle near 90 degrees.
Two ways to perform spectral analysis of doppler waveform
autocorrelation and FFT
Benefits of FFT over autocorrelation
FFT very accurate, but not as fast as autocorrelation and so given large amounts of data autocorrelation used with color flow doppler
Typical doppler shift in diagnostic imaging applications
between 20Hz AND 20kHz (audible)
