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ULTRASOUND PHYSICS > Artifacts > Flashcards

Artifacts Flashcards

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1
Q

definition of artifacts

A

artifacts are structures and features (echoes) that appear on the image but do not have a one-to-one correspondence to an anatomical structure.

they can be useful when assisting in diagnosing, may need to be ignored or eliminated.

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2
Q

What do we assume of the ultrasound machine?

A

1) sound travels in a straight line
2) sound travels directly to and from the reflector
3) the imaging plane is thin
4) sound always travels at 1.54 mm/µsec
5) reflections are produced by structures along the main axis of the sound beam
6) the intensity of an echo corresponds to a reflector’s scattering strength

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3
Q

definition of axial resolution

A

ability to distinguish two structures that are in close proximity to each other - front to back - anterior to posterior

(on top of each others on ultrasound image)

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4
Q

the backing layer does what?

A

stops the ringing in the probe - which allows it to have shorter pulse lengths - which increases the axial resolution

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5
Q

definition of lateral resolution

A

minimum distance between two side-by-side structures that can be separated and still produce two distinct echoes

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6
Q

What are some causes of artifacts?

A
  • faulty equipment
  • improper equipment operation
  • improper settings of controls (ex. gain settings)
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7
Q

what is acoustic speckle? what does it look like? what causes it?

A

a one-to-one correspondence between image brightness and scanned objects does not exist.

a granular appearance of images and spectral display.

caused by: interference of echoes from the distribution of scatters in tissue

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8
Q

where does acoustic speckle occur?

A

near the face of the transducer

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9
Q

What is section thickness? and what else is it called?

A
  • is the thickness of scanned tissue volume perpendicular to the scan plane
  • slice thickness, out-of-plane focusing, elevational resolution or width focusing
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10
Q

What is temporal resolution?

A

the ability to precisely position a moving structure

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11
Q

what does low frame rates result in?

A

less accurate positioning of reflectors

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12
Q

reverberation artifact

A
  • harmonics (double the frequency)

- appear as multiple, equally spaces reflections on the image, decreasing in intensity

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13
Q

comet-tail artifact

A
  • form of reverberation artifact
  • caused by metal object(s)
  • surgical clips, prosthetic valves, bullet etc
  • appears as multiple small bands
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14
Q

ring-down artifact

A
  • type of reverberation
  • produced by small gas bubbles
  • appears as a long, single, strong echo behind the reflector
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15
Q

mirror-image artifact

A

-occurs when a structure is located in front of a highly reflective surface causing the anatomy to be reproducr=ed or duplicated on the other side of the interface

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16
Q

where does the duplicated or mirrored image appear at?

A

ALWAYS deeper than the true anatomical structure

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17
Q

examples of strong reflectors

A

diaphragm, pleura and bowel

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18
Q

multipath artifact

A
  • path to and from a reflector that are not the same

- results in: incorrect axial location of an interface due to an increase in time and improper brightness

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19
Q

refraction artifact

A
  • produces shadowing at the edge of structures that are curved
  • heart, fetal head, or a cross-section of a vessel
  • rectus abdominal muscles (duplicated structures, ghost image artifact)
  • ex: two gestational sacs, two CAs, two AO valves
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20
Q

side lobes

A
  • single crystal transducer
  • energy off the main axis
  • the reflection is at the correct depth - but LATERAL from true anatomy
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21
Q

grating lobes

A
  • array transducers
  • reflection at the correct depth - but LATERAL from true anatomy
  • **corrected by sub-dicing and apodization - during manufacturing process
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22
Q

what is range ambiguity

A

it is the misplacement of an interface when the assumption that each echo is derived from the most recent pulse is violated

-the time between the transmitted pulse and the detected echo are not measured correctly

23
Q

propagation speed errors

A

-causes the improper location of interfaces

24
Q

If the mass has a speed of sound GREATER than 1540 m/sec - what will happen to the reflector?

A

the reflector will be placed closer to the transducer - it information got back faster

25
Q

If the mass has a speed of sound LESS than 1540 m/sec - what will happen to the reflector?

A

the reflector will be placed deeper and further from the transducer - it took longer for the information to get back

  • a posterior displacement*
    ex: a fatty tumor (propagation speed of fat is slower than ST)
26
Q

What is acoustic shadowing?

A

the absence or reduced intensity of echo information, distal to a reflector

27
Q

what causes acoustic shadowing?

A

what the sound beam intersects a highly attenuating (highly reflective) structure

ex: calcium stone / bone
- most or the entire sound beam is redirected to the transducer = black space behind the strong echo structure

28
Q

What is a useful artifact?

A

***acoustic shadowing

***enhancement

29
Q

What is enhancement?

A

an artifact that occurs when the sound beam intersects a very low attenuation (weak reflector) structure i.e., fluid

30
Q

what is focal banding?

A

an artifact that occurs when focusing characteristics of the transducer, creating a region of increased brightness.

  • caused by greater intensity in the focal zone
  • improper TGC settings
31
Q

What does reverberation cause? how can you correct it?

A
  • *near field artifact
  • seen in the anterior walls of organs that are close to the skin surface
  • correct by using harmonics
32
Q

What causes an image to appear too bright or too dark?

A

improper gain or TGCs - operator dependent

33
Q

what happens with excessive reject?

A

all the low-level echoes are eliminated - operator dependent

34
Q

examples of electrical interference

A

patient noise - snoring, talking, coughing etc

patient rooms - machines and monitors

35
Q

what is aliasing and how does it occur?

A
  • it is a color artifact - in PW and color Doppler

- occurs when the Doppler shift frequency exceeds half the PRF - pulse repetition frequency

36
Q

What is the Nyquist limit?

A

it is the limit of aliasing where the Doppler shift frequency exceeds 1/2PRF

37
Q

What does aliasing look like? how do you correct it?

A
  • when using color Doppler, aliasing is a wrap-around of the display = apparent reversal of flow direction
  • increase the PRF
38
Q

with mirrow imaging or ghosting: what is the relationship between the true anatomical structure vs artifact?

A

the true, anatomincal vessel, is closer to the trasnducer. The duplicated, artificat, vessel is below it.

39
Q

What artifact happens when the Doppler gain is set too high or the Doppler angle is close to 90 degrees? What does it look like?

A
  • mirror imaging or crosstalk

- the spectral Doppler shows waveform above and below the baseline

40
Q

What is color ghosting or flash?
What causes it?
How can it be corrected?

A
  • is is when color is being improperly assigned. The sudden burst of color that encompasses the frame.
  • any movenment causes it: pt breathing, talking etc
  • can correct it by increasing the color filter, decreasing the persistence, and reducing the width of the color field of view
41
Q

what is color bleeding or blossoming?

how do you eliminate it?

A
  • it is the extension of color beyond the region of flow to the adjacent tissue
  • decrease the overall color gain or the transmit power (ALARA, which we will not do)
42
Q

What is color noise/clutter?

how can it be eliminated?

A
  • a random variation in signal detection which causes area with no flow to be color encoded
  • by using filter, threshold, wall filter to eliminate low level echoes
43
Q

What ultrasound Doppler is angle dependent?

A
  • color Doppler

- pulse wave Doppler

44
Q

a feature that appears on the image but does not correspond to an anatomical structure is called ____________?

A

an artifact

45
Q

which artifact interferes with the ability to detect a low-contrast object?

A

acoutstic speckle

46
Q

what artifact occurs from the thickness of the scanned tissue volume, perpendicular to the scan plane?

A

elevational resolution

47
Q

definition: multiple, equally-spaced reflections on the image, decreasing in intensitiy is called ____________

A

reverberations

48
Q

what is it called when the acoustic energy is emitted in a direction different than from the main axis of the soud beam?

A

side lobes

49
Q

the extension of color beyond the regoin of flow, to the adjacent tissue is called:

A

blossoming

50
Q

when dealing with acoustic speckle - what do the dots/pixel represent?

A

each dpt or pixel represents a collection of scatterers

51
Q

name some types of artifacts

A
  • not real
  • missing
  • improperly located
  • improper brightness
  • improper shape and size
52
Q
  • define interference

- what factors are at play?

A

-a group of very closely spaced reflectors are positioned in the path of the beam - the average signal obtained from these reflectors depends on many factors

  • **1.acoustic impedance differences between scatteres and surrounding medium
    2. frequency of insonation
    3. scatter size
    4. number of scatters per unit volume
53
Q

What is referred to as the “dead zone”

A

the area close to the trasnducer face - where is there poorest resolution

54
Q

a shallower depth has what relationship to PRF

A

an increased PRF (not far into the body)

ULTRASOUND PHYSICS (2 decks)

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