Artifacts Flashcards
(34 cards)
1
Q
Artifacts appear as structures that are…
A
- not real
- missing
- misplaced
- of incorrect brightness, shape, or size
2
Q
Assumptions of the Ultrasound System
A
- sound travels in a straight line
- echoes originate only from objects located on beam axis
- amplitude of returning echoes is related directly to reflecting or scattering properties of distant objects
- distance to reflecting or scattering objects is proportional to round-trip travel time
3
Q
4 categories of artifacts
A
- Propagation
- Attenuation
- Spectral Doppler
- Color Doppler
4
Q
Propagation Artifacts
A
- comet tail
- grating lobe
- mirror image
- range ambiguity
- refraction
- reverberation
- ring-down
- slice (sectional) thickness
- speckle
- speed error
5
Q
Attenuation Artifacts
A
- enhancement
- focal enhancement
- refraction (edge) shadowing
- shadowing
6
Q
Spectral Doppler Artifacts
A
- aliasing
- nyquist limit
- range ambiguity
- mirror image
7
Q
Color Doppler Artifacts
A
- aliasing
- mirror image
- shadowing
- refraction
- clutter
- noise
8
Q
Slice thickness Artifact definition
A
echoes from structures or tissues in the slice thickness plane get mapped into objects visualized in 2D
9
Q
Other names for slice thickness artifact
A
- section-thickness artifact
- partial-volume artifact
10
Q
What causes slice thickness artifact?
A
- non-zero beam width perpendicular to the scan plane causes slice-thickness artifact*
- third dimension, beam width & direction perpendicular to scan plane
- possible to resolve by using tissue harmonic imaging to narrow beam
11
Q
Speckle definition
A
- granular appearance of images
- echoes can combine constructively (brighter than should be) or destructively (loss of signal)
12
Q
What causes speckle artifact?
A
- result of interference of echoes from distribution of tiny scatterers in tissue
- scatterers = smaller than wavelength of propagating pulse
13
Q
Reverberation definition
A
- multiple reflections occurring btwn. transducer & a strong reflector OR btwn. 2 strong reflectors (or w/in a structure)
- equally spaced reflections of diminishing amplitude w. increased imaging depth
14
Q
Comet tail
A
- reverberation btw. closely spaced surfaces
- echoes typically closely spaced but can appear separated
15
Q
Mirror Image definition
A
- duplication of a structure on the opposite side of a strong reflector
- form of reverberation
- common around pleura & diaphragm
- can be from bowel gas
16
Q
Refraction definition
A
- change in direction of sound beam from one medium to the next
- displaces structures laterally from their correct locations
17
Q
What can cause refraction?
A
- sometimes due to change of propagation speed
- sometimes associated w/ angle of incidence w/ respect to the boundary
18
Q
What does refraction look like?
A
- one real structure is imaged as 2 artifactual objects b/c of the refracting structure close to the transducer
- if unrefracted pulses can propagate to real structure, a triple presentation (1 correct, 2 artifactual) will result
19
Q
What issues can refraction cause?
A
results in improper positioning of a reflector on the display
20
Q
Grating Lobes definition
A
- additional weaker beams emitted from an array transducer (off axis energy)
- duplicates structures laterally to true ones
- originates at source of beam, not at a refractor
21
Q
What do grating lobes look like?
A
- off axis energy: side lobes primarily associated w/ single element transducers, grating lobes associated w/ array transducers
- energy is weaker than primary beam: generally do not create echoes from normal tissues, strong reflectors can generate echoes from off axis energy
22
Q
Grating lobe vs. mirror image
A
- mirror image has apparent echogenic mirror to reflect
- mirror image is horizontally reversed
23
Q
Exception to grating lobe
A
artifact is normally weaker but may not be the case w/ very strong reflectors such as metallic needles
24
Q
Speed error definition
A
- displaces structures axially
- occurs when speed of sound in soft tissue is faster or slower than assumed 1.54 mm/us
- slower speeds place echoes deeper
- faster speeds place echoes closer
25
Roundtrip rule
13 us/cm
26
Speed error equation
distance = 1/2 x (1.54mm/us) x 130 us = 10cm
27
Broken needle artifact
- type of speed error artifact
- speed of sound in cyst is slower than in tissue placing that part of needle deeper than the rest of the needle
28
Speed of sound in silicone
- b/c speed of sound in silicone is slower than tissue, speed errors can occur in breast implant imaging
- chest wall can appear deeper than it should
29
What does the gel standoff pad do in breast imaging?
gel standoff pad is placed btwn. transducer & breast, moving beam focus closer to cyst
30
Range Ambiguity definition
- all echoes are not received before next pulse is emitted
- places structures much closer to surface than they should be
31
What can low attenuation in a cyst cause?
enhancement artifact
32
How can range ambiguity be avoided?
PRF automatically is reduced in deeper imaging to avoid range ambiguity, also causes reduction in frame rate
33
Shadowing artifact definition
weakening of echoes distal to a strongly attenuating OR reflecting structure or from edges of a refracting structure
34
Shadowing w/ a circular region
- sound beam enters circular region of higher propagation speed, it is refracted, refraction occurs again while it leaves
- causes spreading of beam w/ decreased intensity
