MEDS12001 Lecture Notes - Lecture 9: Visual Artifact, Mirror Image, Turbulence

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25 May 2018
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ARTIFACTS
Artifacts in sonography occur as apparent
structures that are one of the following:
o Not real
o Missing
o Misplaced
o Of incorrect brightness, shape, or
size
ASSUMPTIONS OF ULTRASOUND SYSTEM
Sound travels in a straight line
Echoes originate only from objects located
on the beam axis
The amplitude of the returning echoes is
related directly to the reflecting or
scattering properties of distant objects
The distance to reflecting or scattering
objects is proportional to the round-trip
travel time
PROPAGATION ARTIFACTS
Slice thickness
Third dimension
Beam width perpendicular
to the scan plane
Possible to resolve by
using tissue harmonic
imaging
Speckle
Granular
appearance of
images that is
the result of
interference of
echoes from
the distribution of scatterers in tissue
Echoes can combine constructively or
destructively
Reverberation
Equally spaced
reflections of
diminishing amplitude
with increased imaging
depth
Two or more strong
reflectors are
encountered in the sound path; multiple
reflections will occur
Mirror image
Duplication of a
structure on the
opposite side of a
strong reflector
Form of
reverberation
Common around the
pleura and diaphragm
Refraction
Change of direction of
the sound beam from
one medium to the
next
Displaces structures
laterally from their
correct locations
Grafting lobes
Additional weaker
beams emitted
from an array
transducer
Duplicate
structures laterally
to the true ones
Normally do not
produce displayed
echoes
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2 | P a g e
Speed error
Occurs when the speed of sound of soft
tissue is faster or slower than the assumed
1.54 mm/µs
Slower speeds
place echoes
deeper
(position 2)
Faster speeds
place echoes
closer
(position 3)
Range ambiguity
All echoes are not
received before the
next pulse is emitted
Places structures
much closer to the
surface than they
should be
ATTENUATION ARTIFACTS
Shadowing
Weakening of echoes
distal to a strongly
attenuating or reflecting
structure or from the
edges of a refracting
structure
Enhancement
Strengthening of
echoes distal to a
weakly attenuating
structure
Increased
brightness behind
a weakly attenuating structure
DOPPLER ARTIFACTS
Nyquist limit
The highest frequency in a sampled signal
represented unambiguously
Equal to one half the pulse repetition
frequency
The minimum number of samples required
to avoid aliasing
Aliasing
Under sampling of the
Doppler shifts in a
pulsed Doppler system
Appearance of Doppler
information (spectral or
colour) on the wrong
side of the baseline
Range ambiguity
Pulse emitted before all the echoes from
the previous pulse
have been received
Multiple sample
volumes will
appear as a result
Mirror image
Duplication of a vessel or Doppler shift on
the opposite side of a strong reflector
Mirror vessel will demonstrate colour and
spectral flow
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3 | P a g e
Flash artefact
Sudden burst of colour Doppler
Typically caused by tissue or transducer in
motion
Demonstrates an extension of colour
beyond the region of blood flow
SPECTRAL BROADENING
Seen in turbulent flow conditions
However incorrect settings or sampling
can mimic spectral broadening
EQUIPMENT CONTROLS
Overall gain
TGC
Dynamic range
Movement/blur
Banding (multiple focal zones)
ARTIFACTS
ARTIFACTS
Any structure visible on an image that does
not accurately represent the true position or
presence or reflectivity of a corresponding
structure in tissue.
WHY DO ARTIFACTS OCCUR:
1. Ultrasound unit assumptions
2. Equipment failure/faulty
3. Equipment settings are inappropriate
4. Electrical interference
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Document Summary

Reverberation: artifacts in sonography occur as apparent, equally spaced structures that are one of the following, not real, missing, misplaced, of incorrect brightness, shape, or size. Slice thickness: third dimension, beam width perpendicular to the scan plane, possible to resolve by using tissue harmonic imaging. Mirror image: duplication of a structure on the opposite side of a strong reflector, form of reverberation, common around the pleura and diaphragm. Refraction: change of direction of the sound beam from one medium to the next, displaces structures laterally from their correct locations. Grafting lobes: additional weaker beams emitted from an array transducer, duplicate structures laterally to the true ones, normally do not produce displayed echoes. Speed error: occurs when the speed of sound of soft tissue is faster or slower than the assumed. 1. 54 mm/ s: slower speeds place echoes deeper (position 2, faster speeds place echoes closer (position 3)

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