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Physics > Chapter 19 > Flashcards

Chapter 19 Flashcards

1
Q

Creation of everyday doppler shift

A

The change in frequency as a result of relative motion between the sound source and the receiver

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2
Q

Doppler shifts

A

are created when transmitted sound waves strike moving red blood cells

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3
Q

Positive Doppler Shift

A

When red blood cells move toward the transducer
The reflected ƒ is higher than the transmitted ƒ

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4
Q

Negative Doppler Shift

A

When red blood cells move away from the transducer
The reflected ƒ is lower than the transmitted ƒ

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5
Q

The 2 represents the fact that there are two Doppler shifts during an ultrasound exam

A

First Shift: Receiver (RBC) moves towards or away from the sound source (probe).
Second Doppler Shift: The RBC is hit with the beam and reflects it back towards the probe, thus becoming the sound source while the probe is now the receiver

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6
Q

Nondirectional Doppler systems

A

simply measure the presence of moving blood cells by detecting a Doppler shift

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7
Q

Bidirectional Doppler

A

Bidirectional distinguishes the direction of flow

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8
Q

Doppler spectrum

A

Flow towards is displayed above the baseline
Flow away is displayed below the baseline

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9
Q

Phase quadrature (quadrature detection)

A

is the commonly used signal processing technique used to identify direction in bidirectional Doppler systems

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10
Q

Continuous Wave Doppler

A

Requires two crystals:
One constantly transmits while the other continuously receives the reflected signals from RBC
Advantage: because it is constantly sampling the motion of the red blood cells, CW can accurately measure very high velocities
Disadvantage: Doppler shifts and velocities are arising from the entire length of the beam; therefore, uncertain from where the highest velocity came
A second disadvantage is reflections from deeper RBCs will have lower amplitude

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11
Q

range ambiguity

A

Doppler shifts and velocities are arising from the entire length of the beam

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12
Q

Duplex imaging

A

When there is a 2D reference image and PW or CW Doppler graph/display on the screen

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13
Q

interval display

A

Older systems periodically briefly interrupt the Doppler display (thick black line seen) to update the 2D reference image

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14
Q

simultaneously (display)

A

both the 2D and the Doppler display are live (the reference image is not frozen and no black update lines interrupt the Doppler display)

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15
Q

dedicated CW or “Pedoff” probe

A

Cross shaped wand with a spherical footprint
2 crystals in the shape of a semicircle

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16
Q

Pulsed Wave Doppler

A

One crystal alternates between sending and receiving ultrasound pulses
Advantage: Selecting the exact location of Doppler interrogation called range resolution, range specificity or freedom from range ambiguity artifact
Disadvantage: inaccurate measurement of high velocity signals

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17
Q

Continuous Wave Transducers

A

While performing CW Doppler with a conventional imaging array probe, a select pair of crystals (and associated electronics) are used for the Doppler

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18
Q

Pulsed Wave Doppler Transducers

A

is accomplished by a single crystal in the imaging probe

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19
Q

Aliasing

A

presents as high velocities being cut off and wrapped around our Doppler baseline and displayed as if they are moving in the opposite direction
With audio, aliasing presents as sound arising from the incorrect speaker

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20
Q

the Nyquist Limit

A

when aliasing will occur in PW Doppler

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21
Q

these things will reduce aliasing

A

Imaging shallower (increase PRF)
Lower velocities (decrease DS)
Using a lower frequency probe (decrease DS)

22
Q

Five techniques may be used to avoid aliasing artifact

A

1: Increase the scale to its maximum

#2: Select a new view with a shallower sample volume
#3: Select a lower frequency probe
#4: Shift the baseline so that the entire velocity scale is devoted to one direction
#5: Switch to continuous wave Doppler

23
Q

Color Flow Doppler

A

reports average or mean velocities

24
Q

Color Maps

A

Color Doppler uses a “legend” or “key” to assign the mean velocities at each location a color

25
Q

Color Map: No flow

A

black line and the color black represent no Doppler shift

26
Q

Velocity Mode Color Maps

A

inspect any horizontal line on a velocity map there is no change in color from side to side

27
Q

Variance Mode Color Maps

A

inspect any horizontal line on a variance map there is a change in color from side to side
colors to the left represent laminar flow, colors to the right turbulent flow

28
Q

Color flow does not angle correct

A

only the mean velocity of parallel flow is close to the velocity indicated on the color map

29
Q

Aliasing occurs in color flow imaging

A

When color flow aliases it does so in a wrap around fashion relative to the color bar

30
Q

help to remove aliasing from a color Doppler display

A

-Adjust the color scale (PRF) to its maximum (low velocities will no longer get assigned a color)
-Select a lower frequency transducer
-Try to obtain a shallower color sector
-Baseline shift may suffice to unwrap the signal on the display

31
Q

Transverse Flow

A

(cross sectional flow) requires that we look at the direction of flow relative to scan plane

32
Q

“BART”

A

the standard map where Blue = Away and Red = toward

33
Q

Flow Reversal

A

flow cuts across the color map baseline

34
Q

Doppler Packets or ensemble

A

The number of pulses sent down each scan line in your color sector

35
Q

Larger packets (larger ensemble) have advantages

A

Main: color fill in
More accurate velocity information
More sensitive to low flow

36
Q

The disadvantage to larger packet sizes (sending down more pulses)

A

is decreased frame rate and temporal resolution

37
Q

Color Line Density

A

Choosing a high line density when in color modality, increases the number of lines in the color region of interest

38
Q

Power Doppler

A

A color is assigned based on the amplitude (signal strength) of the returning Doppler Shift
AKA: Color Angio, Energy Mode

39
Q

Advantages to Power Doppler

A

-Increased sensitivity to low or slow flow such as venous or small vessel flow
-Unaffected by Doppler angles unless the angle is exactly 90 degrees
-No aliasing since the velocity information is ignored

40
Q

Disadvantages to Power Doppler

A

-No velocity or direction information
-Low PRF are used to detect slow flow therefore frame rates are lower than conventional color flow, and the lowest of all modalities
-Susceptible to motion of the probe, patient, soft tissue which may result in a burst of color = flash artifact or ghosting

41
Q

Temporal resolution lowest to highest:

A

Power Doppler
Color Doppler
2D
single line frames CW/PW Doppler and M-Mode

42
Q

Crosstalk

A

Appears as identical Doppler spectrum above and below the baseline
A type of mirror artifact

43
Q

clutter

A

small Doppler shifts may be produced by moving anatomy, these low velocity signals appear on the Doppler spectral tracing

44
Q

ghosting artifact

A

small Doppler shifts may be produced by moving anatomy, When these low velocity signals are assigned a color

45
Q

Wall filters

A

-eliminate small Doppler shifts or slow flow around the baseline on the Doppler spectrum
-eliminate color arising from slow velocity reflectors such as tissue

46
Q

Tissue Doppler

A

At times we do determine the Doppler shifts and velocities coming off the walls of the left ventricle

47
Q

Spectral Analysis

A

Reflections arising from a mass of moving blood cells will have many Doppler shifted

identifies the individual Doppler shifts

48
Q

Two methods of digital spectral analysis are currently used

A
  1. Fast Fourier Transform (FFT)
  2. Autocorrelation
49
Q

Fast Fourier Transform

A

-is a digital computer-based technique used to process both PW and CW Doppler signals
-exceedingly accurate
-It displays all individual components (shifts) that make up the complex reflected signal (every shift/velocity gets a brightness dot)

50
Q

Several situations will increase spectral broadening:

A
  1. Turbulent flow
  2. Continuous wave Doppler
  3. Pulsed Doppler with a large sample gate
51
Q

The Doppler shift

A

The difference in frequency between what is transmitted and received

Physics (25 decks)

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