Doppler Spectral Analysis Flashcards
Spectral broadening
A single frequency Doppler shift for a reflector moving at constant velocity is obtained only for a very large plane target isonated by a large acoustic field
In Spectral broadening, the reflector only contributes to ?
the backscattered signal as it moves across a finite width beam
What causes the beat frequency to vary in amplitude?
The fluctuation of the detected signal as RBC’s move in and out of the sampling volume
Transit time broadening, also know as?
broadening of the spectrum
Transit time broadening, overall effect?
to smear the magnitude of a frequency component over a wider range of frequencies
Transit time broadening, created difficulties in?
spectral interpretation
Spectral broadening, what accentuates the broadening effect?
By narrowing the Narrowing the beam width and shortening the receiver gate in PW mode
Spectral broadening,
what is necessary are necessary to assess high velocity jets accurately?
Longer pulse lengths with a corresponding loss in positional information
Spectral broadening, transmitted pulse consists of?
multiple frequencies which introduces variations into the received frequency and contributes to spectral broadening
Spectral broadening, the physical size of the beam aperture creates a range of?
angles that intercept the vessel
Distribution of flow power spectra, for a vessel with a nearly uniform velocity distribution?
- For a vessel with a nearly uniform velocity distribution the spectral display shows a characteristic window appearance
- The window is an area between the high velocity components and the baseline that is relatively signal free
- Doppler shifts are confined to a relatively narrow velocity range
Distribution of flow power spectra, a nearly blocked lumen?
- Arterial regions with eddy flow exhibit time dependent behavior in which the velocity distribution reverts to laminar flow during periods of reduced pressure
- The spectral display changes shape to reflect a loss in plasticity
Distribution of flow power spectra, a nearly blocked lumen?
creates high speed jets with rotating flow elements
Distribution of flow power spectra, partial blockage of a vessel?
causes flow disturbance which extends the velocity distribution over a wider range
And…
•The increased variation in Doppler shifts is evidenced by a broadening of the spectral display
Four features of disturbed flow immediately distal to the obstruction include?
–Increased peak velocity
–Spectral broadening
–Altered flow direction
–Less pulsatile shape
What is used as an indicator of severity of the stenosis?
The peak velocity in particular, increases as luminal size decreases
What are the descriptors of the power spectrum that help characterize vascular Doppler signals?
- Maximum frequency
- Mean frequency,
- Median frequency
- Mode frequency
Power spectrum descriptors
maximum frequency corresponds to?
to the fastest moving RBC’s within the sample volume
What is analyzed for the maximum frequency shift?
Each FFT segment
–Presented as a time varying trace on the display
Why is a cut off limit applied?
to prevent maximum Doppler shift from being associated with high frequency noise
What is a good approximation of the true maximum frequency?
If the power spectrum demonstrates a relatively steep falloff in magnitude for the high frequency components, fmax
Often the maximum Doppler shift in each time segment is converted to?
Using?
- Velocity
2. Using transducer frequency and Doppler angle
