Ch 11 Spectral Doppler (PW + CW) Flashcards
What is spectral doppler?
-Doppler info in quantitative form
-Velocity (y-axis) + time (x-axis)
-Positive shifts are above baseline
-Negative shifts are below baseline
Differentiate CW vs PW?
CW:
-detects the doppler shift within the region of overlap b/w the beams of transmitting + receiving probe elements
-unable to detect velocities at a specific location
-uses entire cursor line
PW:
-emits u/s pulses + receives echoes using a single element probe/array
-ability to get info from a particular depth (range gating, sample volume)
Why is spectral doppler used?
-Used to quantify blood flow in heart + blood vessels
-CW + PW is combined with grayscale u/s (duplex scanning)
-CW + PW present doppler shift info in an audible + visual form
What is a phase quadrature detector?
It separates forward + reverse doppler shift voltages (in spectral doppler)
What is fast fourier transform (FFT)?
-A math technique for separating a spectrum of doppler shifts into individual frequency bins
-It converts a signal into individual spectral components and thereby provides frequency information about the signal
-Each bin corresponds to a narrow range of frequency shifts, which gets converted into a velocity
-FFT determines the energy in each bin (the # of RBCs traveling at that particular velocity)
What do brighter + weaker spots represent on spectral tracings in regards to FFT?
Brighter:
-greater # of RBCs traveling together at that velocity + direction
Weaker:
-lower # of RBCs traveling at that velocity + direction
What is spectral broadening + what produces it?
-Filling in of PW spectral windows
-Indicates a vertical thickening of the spectral trace
-Disturbed or turbulent flow produces it
What is the blossoming artifact?
When spectral broadening is artificially produced due to excessive gain or an excessive sample volume length
The beam former sends out pulses via the pulser that have pulse lengths of how many cycles?
5-30 cycles of sound
Explain the PW operation?
-Beam former sends out pulses via the pulser
-Returning echoes are processed in the detector, where their voltages are amplified + compared with the pulser frequency to determine the DS value
-DS are then sent out to loud speakers for audible display
What is range gating/sample volume?
-Ability to determine where echoes are coming from at a given depth
-The sonographer has the control of the location + length of the sample volume
(only in PW)
What is the m/c artifact encountered in doppler, specifically with PW?
Aliasing
How does aliasing appear on a PW tracing?
Appears as a wrapping of the signal, where the missing peaks of the signal appear on the opposite side of the baseline
(must increase PRF/scale or shift baseline to fix)
Differentiate true aliasing vs display aliasing?
True:
-it is NOT possible to increase the scale to “unwrap” + fix the velocity
-due to the DSF being greater than 1/2 the PRF
Display:
-the signal peaks are “wrapped” + can be simply “unwrapped” by shifting the baseline
List 5 methods for correcting PW aliasing, in order from which should be tried first?
-Shift baseline
-Increase PRF/scale
-Increase doppler angle
-Use lower frequency
-Use CW doppler
What is range ambiguity?
-To do with PW doppler b/c can detect specific area + depth due to sample volume box
-It occurs when we increase the PRF
-In PW, not only echoes from the sample volume are received but deeper echoes from previously transmitted events are received as well
What do wall filters do?
-Reject frequencies below an adjustable value
-Eliminates clutter (low velocity, high amplitude specular reflectors) due to motion
-Wall filters achieve this by reducing the dynamic range (echo strengths) by decreasing signals with frequency shifts below the wall filter setting
Wall filters reduce signals with what?
Low DSF + high amplitude (known as clutter), it then passes through signals with high DSF + low amplitude (RBCs)
What happens if wall filter is set too high?
Can remove slow diastolic flow + flow can be missed
List the wall filters for variable clinical applications?
Venous: <50 Hz
Arterial: 50-100 Hz
Adult Echo: 200-600 Hz
Pediatric Echo: 600-800 Hz
(typical ranges for wall filters are 10-1600 Hz)
Explain CW doppler?
-Detects flow that occurs anywhere along the cursor line/beam
-Can NOT sample at specific locations b/c we are continuously transmitting + receiving
-Can detect higher velocities b/c no aliasing
(useful for detecting vascular or valve stenosis)
Doppler shifts are detected via what in CW doppler?
Via quadrature phase detector - then presents them as a visual display with audible sounds
What is doppler gain?
The brightness (amplification) of the spectral display
What is the audible sound we hear in spectral doppler?
-We hear doppler shifts
-Sound is influenced by the velocity of the reflector, angle b/w the reflector + propagating sound beam, as well as boundary motion
What info can be presented with a spectral doppler display?
-Presence of flow
-Velocity of RBCs
-Direction of flow
-Amplitude of reflectors (brightness of returning echoes)
-Quality of flow (laminar or turbulent)
Why is there aliasing in PW but not in CW?
B/c PW is a pulsed modality, whereas CW is constantly transmitting + receiving echoes which means there are no pulses of sound per scan line (PRF)
