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Ultrasound Physics > Artifacts Chapter 8 > Flashcards

Artifacts Chapter 8 Flashcards

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1
Q

What do Artifacts associated with limited detail resolution include?

A 
an inability to correctly visualize dimensions 
or even the presence of structures 
laterally, 
axially, 
and/or elevationally.
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2
Q

What are artifacts?

A

Any perturbation of a signal which distorts a display from “truth”.
(something on the screen that is not true. Something missing, distorted, anything that is not a true reflection of what should be seen)

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3
Q

Give an example of when an artifact is good.

A

The bright white spike which occurs on the doppler spectrum often referred to as a valve click is an artifact caused by circuit saturation.
helps identify timing of the cardiac cycle

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4
Q

Give an example of when a shadow artifact is useful.

A

shadowing that may indicate the presence of a calcification and enhancement may indicat the presence of a fluid.

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5
Q

Give an example of when an artifact not useful.

A

A susceptibility artifact which result from receiving radio frequency signals from outside sources. Such as radio, television, pulse oximeters, Bovie electrosurgical units, ect

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6
Q

What is a susceptibility artifact?

A

result from receiving radio frequency signals from outside sources. Such as radio, television, pulse oximeters signals, Bovie electrosurgical units, ect

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7
Q

When do artifact exist? Give an example.

A

Whenever an assumption of ultrasound is violated.

Such as assumed speed of sound is 1540m/sec, if the speed of sound is not 1540, speed error artifact exists.

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8
Q

What are the image detail resolution related artifacts?

A 
limited
axial resolution
lateral resolution
elevation resolution (slice thickness)
Beam aberration (which affects axial, lateral , and elevation resolution)
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9
Q

What is the forgotten dimension and why?

A

Elevation, since (unless using 3-D imaging) there is no way of directly visualizing the elevation plane which corresponds to the image slice thickness.

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10
Q

Define locational artifacts

A

Artifacts that result in structures appearing in incorrect positions within the image. Makes things appear in the wrong location.

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11
Q

Name the different locational artifacts.

A 
Refraction
Reverberation (Comet tail, Ring Down) 
Multipath
Grating lobes (and side lobes) 
Speed Error
Range Ambiguity
Mirror Image
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12
Q

What do refraction artifacts result in?

A

They result in a lateral displacement of the structure within the image. You’ll often get the structure in the correct location, but you will also get a structure laterally displaced from it as well. due to multiple beams coming down.

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13
Q

What is the result of reverberation artifacts?

A

They result in spurious (extra) structures caused by sound which reverberates, or rings, between two or more surfaces.

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14
Q
  1. When is reverberation more likely to happen?
  2. What is it highly dependent on?
  3. When is it the worse?
  4. What is the affect of reverberation artifact on the structures and tissues?
  5. How often does it occur and is it recognizable?
A
  1. when there is a large acoustic impedance mismatch and relatively specular reflection
  2. highly angularly dependent
  3. worst when the sound is perpendicular to the
    specular reflecting interface.
  4. causes all structures and tissue between the reverberating structures to be replicated as well as the reverberating structure itself.
  5. very common and often not identified by the person scanning or by the person interpreting the scan.
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15
Q

How can you fix lateral displacement caused by refraction?

A

Change where you are coming from. Prove in more than one plane. If you can’t recreate it, it most likely wasn’t real.

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16
Q

How can you fix a reverberation artifact?

A

Change the angle you are approaching the structure.

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17
Q

What is Ring down reverberation?

A

Air filled structures
When sound reverberates within an air sac, the boundaries of the air sac are redrawn repeatedly creating a bright tail-like image below the air sac.
The bright white echoes represent the strong reflection from the large acoustic impedance mismatch with the air.

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18
Q

What is the most likely cause of a reverberation artifact?

A

Specular reflectors in the relative near field. Can occur between any two or more strong specular reflectors.

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19
Q

What happens when reverberation occurs?

A

All tissues between the reverberating structures can. e replicated, not just the specular reverberating structures. Tissue signals can be superimposed over a blood pool giving the appearance of a thrombus or a mass which does not truly exist.

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20
Q

What is a comet tail reverberation?

A

High impedance structures - metallic or calcified
occurs when the sound reverberates within a calcified structure or a metallic structure such as a surgical clip, catheter, or needle tip. There is usually a striated tail of bright reflection below the structure.

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21
Q

When might you see a comet tail reverberation?

A

St. Jude valve, from a calcification in the prostate gland

kidney stones, gall stones, pieces of plaque

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22
Q

What is a Twinkle artifact

A

A special case of a comet tail reverberation that appears when you use color. It result in a “twinkle” color artifact. Caused by stone like structures.

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23
Q

What is the result of a Multi-path artifact?

A

It results in a structure appearing deeper than reality because of the elongated path length (taking longer to get back to the transducer.)

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24
Q

How do Multi-path artifacts occur?

A

They are a result of a specular reflector at an oblique angle (not perpendicular). Since specular reflection is very angle directional and the reflection angle equals the incident angle, the reflection is not directed toward the transducer. (Theoretical, but difficult to find picture of)

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25
Q

What is the difference between grating lobe and side lobe artifacts?

A

Weaker beam artifacts
side lobe - single element transducers (CW)
grating lobes - multi- element transducers (imaging)

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26
Q

What do grating lobes/side lobe artifacts result in?

A

Lateral displacement of structures within an image. As the name suggests, there is energy in regions other than the main beam which cause reflections.
Grating lobes are weaker lobes of energy in directions other than the main beam direction. When they interact with specular reflectors

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27
Q

What causes grating/side lobe artifacts?

A

Due to the complex beam shape there is the existence of lower pressure or weaker beams pointing off-axis.
They exist, in part, because of partial constuctive interference. (never completely out of phase)

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28
Q

How are range ambiguity artifact created?

A

They are a result of reflected data from the previous acoustic transmit adding to the reflection of the current acoustic line.

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29
Q

When are range ambiguity artifacts significant?

A

At shallow depth settings.

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30
Q

What is the result of mirroring artifact?

A

It is an oblique incidence on a specular reflector. It results in an artificial structure symmetric to the actual structure across the “mirroring structure”.
A specular reflector acts as a mirror.

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31
Q

What is a good example of a structure which often acts as an acoustic mirror?

A

The diaphragm

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32
Q

What is the difference between a multi-path and mirror artifact?

A

With mirror artifact there is something acting as a mirror - specular reflector

33
Q

How can you correct for mirror artifact?

How can you tell the difference between the true object and the mirrored object?

A

Not a lot you can do about mirror artifact other than move around the specular reflector. The true object will keep its original shape and size. The mirrored object will change in an obscured way. It will look slightly different

34
Q

How is acoustic shadowing artifact created?

A

Refractive
It is caused by any form of attenuation stronger than the attenuation of the surrounding area.
caused by excessive reflection, absorption, or refraction.

35
Q

What is acoustic shadowing artifact?

A

It is a darker than normal appearance below a structure in an image.
It can be cast on the image below by a strong specular reflector or absorber.

36
Q

How does strong reflection and/or strong absorptions

effect the beam and echoes?

A

It decreases the beam intensity thereby it attenuates the echoes from deeper structures more that usual.

37
Q

What is the primary cause of edge shadowing

A

It is caused by excessive refraction and commonly occurs from the edges of vessels, cystic structures, and bones.

38
Q

How does an Enhancement artifact occur?

A

The bright spot behind something that is weakly attenuating.
If a structure is a weaker reflector than normal or less absorbing than normal, the beam is attenuated less than normal “enhancing” the amplitude of echoes below the weaker reflector or poor absorber.

39
Q

When does enhancement often occur?

Give some examples.

A

when sound propagates through a fluid filled region. Cystic structures, the bladder a small blood pool

40
Q

when does the incident angle occur? When does total internal reflection occur?

A

25 degrees- great than 25

41
Q

What are somethings that can be done to reduce the amount of shadowing and enhancement?

A

compound imaging (taking pictures from multiple different angles and averaging them all together to make one line)

42
Q

How does speckle occur?

A

The scattering from multiple structures occurs simultaneously and adds to varying degrees constructively and destructively creating brighter and darker spots within the image. Lower frequency results in a larger speckle pattern because of the longer wavelength.

43
Q

How does speckle appear on the image?

A

As pseudo tissue texture. Appears homogenous, but is a result of brighter and darker speckles.

44
Q

When do speckle occur?

A

When there are reflecting structures smaller than the resolution of the ultrasound being used.

45
Q

speckle reduction techniques

A

Harmonic imaging
spatial compounding/frame averaging
frequency compounding/fusion. (images created using different transmit frequencies(broadband width).

46
Q

What is the result of using high or low frequency transducers in regards to speckle?

A

Higher frequency results in a smaller speckle pattern because of the shorter wavelength.
Lower frequency results in a larger speckle pattern because of the longer wavelength.

47
Q

Which gives better improvement in the signal to noise and speckle reduction frequency fusion or compound imaging?

A

compound imaging

48
Q

What is beam aberration?

A

Distortion of the beam as a result of differing propagation velocities.
Varying path lengths that the pulses take when they come back to the transducer and it leads to distortion degradation of the image and that there are processing techniques to fix it.

49
Q

What is the model based approach to correct for beam aberration?

A

It makes assumption about the tissue based on the system presets and settings (assume more fat when imaging deeper in larger patients).

50
Q

What is the adaptive approach to correct for beam aberration?

A

It make corrections based on iteratively transmitting beams and performing many calculations on the returned data to determine the best correction factors for the phase delays.

51
Q

Does gain have any impact on aliasing?

A

No, gain is amplitude
aliasing is a violation of the Nyquist effect (to detect a given frequency, you must sample at greater than twice that frequency).
PW - sample size is = to the PRF
Max frequency shift is 1/2 PRF

52
Q

Do wall filter have an impact on aliasing?

A

NO, wall filters are high pass filters. They let high frequencies through. Its a filter, it is not changing anything about the frequency shifts, its just a gate keeper.

53
Q

Is there a relationship between amplitude and frequency?

A

No

54
Q

What is aliasing

A

It is when the Nyquist criterion is violated. When the Doppler frequency shift is greater than one half of the PRF, the Doppler signal wraps around either the spectrum (spectral Doppler) or the color scale (Color Doppler).

55
Q

When is it true spectral doppler aliasing?

A

When it is not possible to determine the actual peak velocity.

56
Q

When is color aliasing is more likely to occur?

A

When it is within the center of a vessel, whenever there is a bend, branch, narrowing, etc., when the angle is closer to 0 or 180 degrees, when the scales are low, and the transmit frequency is high.

57
Q

How to correct color aliasing?

A

lower the baseband,
Increase the scale/ PRF
Use a larger doppler angle (if at 0 degree)
use a lower frequency transducer
find a view that results in a shallower gate depth (pulse faster to it)
use CW
use HPRF doppler (will automatically come on as scale is turned up higher) (used at deeper depths)
Shift the baseline

58
Q

What is range ambiguity with doppler?

A

receiving echoes from previous pulses
If a second transmit pulse is transmitted before all of the echoes from the first are received, the system might actually detect flow from deeper vessels as if at the shallower depth.

59
Q

When is range ambiguity with doppler more likely to occur?

A

In shallow gate depths and also with a fixed focus transducer such as the pencil probes or the older style mechanical transducers.

60
Q

How to fix range ambiguity?

A

Change the scale

61
Q

What does Spectral mirroring results in?

A

Artificial Doppler signal displaying the opposite direction of the true flow.

62
Q

What conditions exacerbate spectral mirroring?

A
  • excessive transmit
  • excessive receive gain
  • superficial Doppler with high frequency transducer
  • Insonification angle close to 90 degrees
  • poor electronic design (poor separation between I and Q channels)
63
Q

How is spectral mirroring caused by angle when the insonification angle is close to 90 degrees?

A

When the insonification angle is close to 90 degrees, elements on one side of the steer line will see flow towards the transducer, whereas elements on the other side of the steer line will see flow away

64
Q

How to correct spectral mirroring

A

Turn down the gain

65
Q

How can you tell what is real and what is artifact with spectral mirroring?

A

Look for symmetry of the highest amplitude signals (brightest) about the baseline.

66
Q

when does spectral broadening occur due to technical errors?

A

sample size too large, too close to the edge

67
Q

What is true spectral broadening a result of?

A

differing blood velocities within a sample volume

68
Q

What is Spectral spread is exacerbated by?

A

– large array transducers (linear arrays)
– superficial gate location
– large insonification angles (especially as the angle gets larger than 60o)
– excessive gain

69
Q

what does spectral spread/broadening artifact do?

A

It smears out the spectrum and causes overestimation of the peak velocity as well as the diminishment of a “spectral window”.

70
Q

What happens when you use a doppler angle over 60 degrees?

A

introduces spectral spread/broadening artifact.

71
Q

what is Blossoming and when does it result in?

A

It occurs when the signal is overgained.
Blossoming results in a higher than true peak velocity as well as the potential loss or decrease of the spectral window (when one exists) in PW Doppler.
It also cause spectral broadening because it starts to fill in the window.

72
Q

what is color blossoming/bleeding?

A

when the doppler signal is over gained, color signals can potentially overlap non flow tissue regions leading to over interpretation of vessel or flow region size.

73
Q

Why does color bleeding happen more on the posterior side of the vessel?

A

The “tail” of the color transmit pulse tends to “stretch” out the color posterior to the flow region. (due to the pulse length)

74
Q

When does wall filter saturation occur?

A

when the dynamic range is not adequately reduced by the wall filters, circuit saturation occur
s.

75
Q

Reasons for spectral doppler signal dropout

A 
Poor doppler angle (close to 90 degrees)
High wall filter setting
transmit frequency too high
transmit power too low
weak signal from insignificant # of rbc's
low receiver gain
improper gray scale setting
refraction artifact
shadowing artifact
76
Q

Reasons for color doppler dropout.

A

Can occur for same reasons as spectral dropout in addition to.
High color scale settings which result in high wall filter settings
low color priority
packet size small when imaging low flow, small, or deeper vessels (related to sensitivity)

77
Q

color and power doppler flash artifact.

A

Appears as a short term flash of color within the color box related to relative motion between the patient and the transducer, and not actual flow

78
Q

What does total internal reflection result in?

A

a loss in spectral signal

79
Q

How is flash artifact corrected?

A

by flash reduction algorithms which has made it less common than in years past. However, flash artifact still occurs, especially when color wall filters and color scales are set too low and when power doppler is being used.

Ultrasound Physics (2 decks)

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