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PHYS 216 > Artifacts > Flashcards

Artifacts Flashcards

1
Q

What is an artifact?

A

Anything that does not correctly display the structures or functions that are imaged

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2
Q

What 4 things are artifacts related to? (With examples)

A
  1. Malfunctioning equipment
    Ex. Broken crystal causing a shadow
  2. Defective recording device
    Ex. Discrepancy between the machine and PAX system
  3. Improper operation
    Ex. Bad techs
  4. Acoustic properties of tissues and sound propagation
    Ex. Many diff artifacts can occur
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3
Q

What 4 assumptions about the way sound propagates in tissue results in artifacts?

A
  1. All tissues have the same acoustic velocity of 1540 m/s
  2. The sound beam always travels in a straight line
  3. Echo strength only indicates the organs echogenicity
  4. The distance to the reflector equals the round trip time
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4
Q

What determines the velocity in tissue?

A

Compressibility and density

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5
Q

What are the 4 categories of artifacts?

A
  1. Not real
  2. Missing
  3. Improper location
  4. Improper brightness/size/shape
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6
Q

What two groups can artifacts be categorized into?

A
  1. Attenuation group

2. Propagation group

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7
Q

What is the axial resolution artifact?

A

Structures less than 1/2 the SPL apart in the axis of the beam are demonstrated as a single echo

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8
Q

What is the formula for SPL?

A

SPL = λ x RD

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9
Q

How can we correct the axial resolution artifact? (2)

A

SPL = λ x RD

  1. Manufacture the RD smaller in the first place
  2. Decrease the SPL by increasing the frequency which decreases the wavelength (λ)
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10
Q

What is the lateral resolution artifact?

A

A beam width artifact where reflectors appear smeared across the screen in areas where the beam is wide

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11
Q

What accentuates the lateral resolution artifact?

A

Increased gain or TGC

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12
Q

What is lateral resolution artifact also referred to as? (2)

A
  1. Point spreading

2. Image broadening

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13
Q

How do you correct for the lateral resolution artifact? (5)

A
  1. Lower gain
  2. Lower TGC
  3. Focus at correct level
  4. Increase frequency to extend the NZL
  5. Scan in the NZ (better beam width)
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14
Q

What is slice thickness artifact?

A

Cystic structures below the focus level can appear filled in due to echoes from in front and behind the structure (in the z axis) being spread where the beam is wide

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15
Q

What is the slice thickness artifact also referred to as?

A

The volume averaging artifact

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16
Q

What artifact is similar to the lateral resolution artifact?

A

The slice thickness artifact is also a beam width artifact but in the z-axis

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17
Q

Why does the slice thickness artifact occur?

A

Because of the assumption that all echoes return from the centre of the beam

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18
Q

What can fix the slice thickness artifact?

A

Repositioning the probe so that the narrowest portion of the z-axis is at the desired location

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19
Q

What is acoustic speckle?

A

The appearance of bright and dark dots (heterogenous brightness)

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20
Q

Why does acoustic speckle occur?

A

Due to the scatter reflection in the parenchyma due to constructive and deconstructive interference occurring from several off-axis reflections

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21
Q

What corrects acoustic speckle?

A

Persistence averages out the frames to help homogenize the tissue brightness

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22
Q

How does the size of the slice thickness effect the artifact fill in?

A

Greater slice thickness = greater fill in

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23
Q

Where is acoustic speckle predominant?

A

In the near field where constructive and deconstructive interference is the greatest

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24
Q

What is reverberation?

A

The reflection of sound bouncing back and forth between the transducer and a strong interface

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25
Q

Where is reverberation often seen? (2)

A
  1. Probe surface

2. Fluid filled structures

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26
Q

How does reverberation appear?

A

Multiple equally spaced linear lines that get weaker with depth and look like rungs of a ladder

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27
Q

What are the best ways to eliminate reverberation?

A
  1. Change window

2. Use harmonics

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28
Q

What is real and what is not when reverberation is seen?

A

The first echo closest towards the surface is real and the rest aren’t.

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29
Q

What is comet tail?

A

A short-range reverberation artifact between two closely spaced specular reflectors

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30
Q

What things cause comet tail? (6)

A
  • Staples
  • Metal pellets
  • Biopsy needle
  • Metal IUD
  • Calcium
  • Cholesterol crystals
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31
Q

How can comet tail be helpful?

A

In diagnosing calcium in the bladder wall

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32
Q

How can comet-tail be corrected?

A

Change window

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33
Q

What is ring down?

A

A reverberation artifact that looks similar to comet tail but instead appears as a continuous stream of long echoes

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34
Q

What creates the ring down artifact?

A

It is a resonance phenomena associated with gas bubbles, where they vibrate and become a new source of sound

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35
Q

Is ring down useful?

A

Yes, can be useful for diagnosis and also to tell if gas/bowel is present

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36
Q

What is different between the appearance of comet tail and ring down?

A

Ring down has long bright echoes and comet tail is small

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37
Q

What is mirror image?

A

A duplication of an object seen on both sides of a strong reflector due to an odd angle

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38
Q

Why does mirror image occur?

A

The machine assumes the beam is travelling in a straight line but when the beam is reflected by a specular reflector towards the object and then reflected back along the same path the additional travel time places the object deeper than it really is

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39
Q

What is a structure that often causes mirror image?

A

The diaphragm

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40
Q

How is mirror image corrected?

A

It can’t always be so awareness is key

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41
Q

What is a multipath artifact?

A

The beam may take several off-axis vectors before returning to the probe resulting in the incorrect location of the echoes due to the difference in time

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42
Q

What assumption results in multi-path artifacts?

A

That the beam travels in a straight line

43
Q

What is the solution to correct for multipath artifacts?

A

Get perpendicular to the structure of interest to get the sound to come back in a straight line

44
Q

What are refractive artifacts?

A

The beam becomes redirected when an interface it struck at an angle and both media have different velocities

45
Q

What are the types of refractive artifacts? (4)

A
  1. Edge shadowing
  2. Refractive duplication
  3. Refractive malposition
  4. Refractive enhancement
46
Q

What is edge shadowing?

A

A refractive artifact where shadows are seen at the edges of strong curved reflectors due to the beam changing direction with non-perpendicular incidence

47
Q

How is edge shadowing corrected?

A

Changing the window to come at the interface from a different angle

48
Q

What controls can help the appearance of edge shadowing to be smoother and less intense? (2)

A
  1. Persistence

2. Compound imaging/sono CT

49
Q

In order to diagnose stones or calcifications what control would you use?

A

Take off Sono CT/Compound imaging in order to make the edge shadowing more intense

50
Q

What is refractive duplication?

A

A lateral duplication of the echo interface on either side of it’s true location

51
Q

Why does refractive duplication occur?

A

Due to the refraction of the beam by a strong interface, as the beam is swept the sound bends as it travels through structures and returns to the probe.

52
Q

What is refractive malposition?

A

When the echo interface is falsely placed lateral from it’s true position

53
Q

Why does refractive malposition occur?

A

Due to the bending of the beam generating an echo which is falsely positioned along the beam axis

54
Q

What is the solution to refractive malposition?

A

Get perpendicular to the structure

55
Q

What is responsible for grating lobes?

A

Length and width vibration of the array probe “cross talking”

56
Q

What is a propagation speed error?

A

Based on the machines assumption that sound travels at 1540 m/s in all tissue reflectors can be placed in the wrong location proximal or distal to it’s true location

57
Q

What is the speed of sound in fat?

A

1460 m/s

58
Q

How do reflectors behind fat appear?

A

Deeper than they actually are because sound slows down as it travels through fat (takes longer to get back to probe).

59
Q

How do you correct for speed propagation error?

A

You can’t

60
Q

What is a “broken diaphragm” and indication of?

A

Fatty liver (propagation speed error causes this artifact).

61
Q

What is range ambiguity?

A

It is an artifact that occurs when a pulse is released before all echoes have made it back to the probe

62
Q

What setting is too high when the range ambiguity artifact occurs?

A

PRF is too high thereby exceeding it’s limit set by the depth

63
Q

How does range ambiguity appear?

A

Echoes appear closer than they actually are

64
Q

Where is range ambiguity seen?

A

Not in 2D but in doppler

65
Q

How do you correct range ambiguity? (2)

A
  1. Reduce frame rate or PRF

2. Switch to CW (but no image will be available)

66
Q

What artifact is responsible for a sample volume outside the vessel still picking up a waveform due to a very high PRF?

A

Range ambiguity

67
Q

What are the 4 attenuation artifacts?

A
  1. Shadowing
  2. Enhancement
  3. Focal enhancement
  4. Edge shadowing
68
Q

What is shadowing?

A

A lack of echoes underneath a highly attenuating structure due to absorption, reflection or scattering of sound

69
Q

How would you attempt to see structures blocked by shadowing?

A

Scan from different window

70
Q

Is shadowing helpful or hurtful?

A

Both.

Helpful = Helps with diagnosis (stones)

Hurtful = blacks out anatomy

71
Q

How is shadowing enhanced to help with diagnosis? (5)

A
  1. Increase frequency
  2. Harmonics
  3. Focus at shadow
  4. Reduce 2D gain
  5. Turn off compound/Sono CT
72
Q

How does increasing the frequency help to enhance shadowing for diagnosis?

A

Increasing the frequency will increase absorption therefore increasing attenuation (attenuation = shadowing)

73
Q

What is enhancement?

A

Posterior brightness behind structures that attenuate less than surrounding structures

74
Q

What types of structures can have enhancement?

A

Both cystic and solid

75
Q

What is a very rare reason for enhancement?

A

A very homogenous medium causes enhancement

76
Q

Is enhancement helpful?

A

Yes, with diagnosis

77
Q

How is enhancement hurtful?

A

Brighter structures can appear thicker on ultrasound resulting in incorrect caliper placement

78
Q

What is focal enhancement?

A

A false brightening to a particular area of the image at the focal zone

79
Q

What is focal enhancement also known as?

A

Focal banding

80
Q

Why does focal enhancement occur?

A

The intensity of the beam is highest at the focal zone (narrowest part of the beam) so all echoes that come back from this region are brighter.

81
Q

How can you compensate for the focal enhancement artifact?

A

Bump down TGC’s

82
Q

What is aliasing?

A

When the Niquest limit is exceeded (1/2 PRF) the velocities travel so fast that they wrap around the baseline

83
Q

How is aliasing helpful?

A

When assessing colour flow is can be useful in identifying areas of higher velocities or turbulance

84
Q

How do you correct for aliasing? (5)

A
  1. Increase PRF (scale/velocity range)
  2. Move baseline
  3. Switch to CW
  4. Increase the angle
  5. Lower the operating frequency
85
Q

Why is an angle of 60 used in doppler?

A

Acceptable margin of error and reproducible

86
Q

Why does doppler require a lower frequency?

A

Because of the increased attenuation due to rayleigh scatter (RBC’s = weak echoes)

87
Q

What is the doppler slice thickness artifact?

A

Velocities from multiple flows (Diff shades of white and grey) are seen in one envelope due to the beam width phenomenon

88
Q

What is the doppler reverberation artifact?

A

Similar to 2D, it is seen as a buzzing or equidistant colour lines

89
Q

What is the twinkle artifact?

A

A doppler reverberation artifact helpful in diagnosing calcium

90
Q

How does the twinkle artifact appear?

A

Reverberation between the front and back wall of the stone will produce a mosaic of colour deep into and behind the stone

91
Q

What is another structure where the doppler reverb artifact will be seen?

A

Prosthetic valve

92
Q

How is the doppler mirror image seen?

A

Colour = false vessel deep to the real one

Spectral = Doppler flow on both sides of baseline

93
Q

How do you correct for doppler mirror image on spectral and colour?

A

Specral:

  • Reduce PW gain (shouldn’t spill [over)
  • Angle of incidence too too perpendicular (don’t want to be 90 degrees)

Colour:
- Turn down gain

94
Q

What is doppler flash or clutter?

A

Low level echoes in the colour flow seen in tissue due to tissue movement

95
Q

What structures show doppler flash/clutter?

A

Heart wall, valves, vessel wall motion

96
Q

How is doppler flash/clutter corrected?

A

Wall Filters within the machine eliminate this motion as the velocities are usually slower than blood (increase wall filter)

97
Q

What is the risk of the machine’s doppler flash/clutter filters being set too high?

A

Eliminating real low flow information

98
Q

What is doppler noise?

A

Occurs when doppler gains are too high causing blossoming of colour outside vessel or within cysts with no flow

99
Q

How can doppler noise effect spectral?

A

Falsely increase peak systolic velocities

100
Q

What is the halo effect?

A

Doppler noise on the spectral trace causing over estimation of a peak velocity

101
Q

What are other reasons you may get artifacts while scanning related to the malfunction of equipment? (4)

A
  1. Electronic interference
  2. Faulty scan converter
  3. Faulty software
  4. Faulty equipment
102
Q

What is electronic interference?

A

An echo pattern with high/low intensity that is random/geometric resulting from radio frequency signals generated by other equipment

103
Q

What is the 60 Hz bandwith artifact?

A

A low level echo found below the spectral trace due to electronic interference

104
Q

What are some faulty equipment that may change the appearance of the image? (4)

A
  1. Malfunctioning crystal
  2. Cracked damping material
  3. Defective recording device
  4. Improper transducer shielding

PHYS 216 (11 decks)

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