Ch 11 Spectral Doppler (PW + CW)

Question 1

What is spectral doppler?

Answer 1

-Doppler info in quantitative form
-Velocity (y-axis) + time (x-axis)
-Positive shifts are above baseline
-Negative shifts are below baseline

Question 2

Differentiate CW vs PW?

Answer 2

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)

Question 3

Why is spectral doppler used?

Answer 3

-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

Question 4

What is a phase quadrature detector?

Answer 4

It separates forward + reverse doppler shift voltages (in spectral doppler)

Question 5

What is fast fourier transform (FFT)?

Answer 5

-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)

Question 6

What do brighter + weaker spots represent on spectral tracings in regards to FFT?

Answer 6

Brighter:
-greater # of RBCs traveling together at that velocity + direction

Weaker:
-lower # of RBCs traveling at that velocity + direction

Question 7

What is spectral broadening + what produces it?

Answer 7

-Filling in of PW spectral windows
-Indicates a vertical thickening of the spectral trace
-Disturbed or turbulent flow produces it

Question 8

What is the blossoming artifact?

Answer 8

When spectral broadening is artificially produced due to excessive gain or an excessive sample volume length

Question 9

The beam former sends out pulses via the pulser that have pulse lengths of how many cycles?

Answer 9

5-30 cycles of sound

Question 10

Explain the PW operation?

Answer 10

-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

Question 11

What is range gating/sample volume?

Answer 11

-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)

Question 12

What is the m/c artifact encountered in doppler, specifically with PW?

Answer 12

Aliasing

Question 13

How does aliasing appear on a PW tracing?

Answer 13

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)

Question 14

Differentiate true aliasing vs display aliasing?

Answer 14

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

Question 15

List 5 methods for correcting PW aliasing, in order from which should be tried first?

Answer 15

-Shift baseline
-Increase PRF/scale
-Increase doppler angle
-Use lower frequency
-Use CW doppler

Question 16

What is range ambiguity?

Answer 16

-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

Question 17

What do wall filters do?

Answer 17

-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

Question 18

Wall filters reduce signals with what?

Answer 18

Low DSF + high amplitude (known as clutter), it then passes through signals with high DSF + low amplitude (RBCs)

Question 19

What happens if wall filter is set too high?

Answer 19

Can remove slow diastolic flow + flow can be missed

Question 20

List the wall filters for variable clinical applications?

Answer 20

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)

Question 21

Explain CW doppler?

Answer 21

-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)

Question 22

Doppler shifts are detected via what in CW doppler?

Answer 22

Via quadrature phase detector - then presents them as a visual display with audible sounds

Question 23

What is doppler gain?

Answer 23

The brightness (amplification) of the spectral display

Question 24

What is the audible sound we hear in spectral doppler?

Answer 24

-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

Question 25

What info can be presented with a spectral doppler display?

Answer 25

-Presence of flow
-Velocity of RBCs
-Direction of flow
-Amplitude of reflectors (brightness of returning echoes)
-Quality of flow (laminar or turbulent)

Question 26

Why is there aliasing in PW but not in CW?

Answer 26

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)