Chapter 20 Recognizing

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Chapter 20Recognizing Understanding
Ultrasound Artifacts
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Artifacts What are they?

• Echoes appearing on the sonographic image which
  do not correspond in location, intensity or to
  actual interfaces in the patient.

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Artifacts Things to know

• Artifacts are often present in multiples.
• Occur due to
• Equipment malfunction or design
• Operator error
• Violation of assumptions

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Assumptions

• The transmitted wave travels along a straight
  line path from the transducer to the object and
  back to the transducer.
• The attenuation of sound in tissue is equal along
  the path.
• Beam dimensions are small in both section
  thickness (elevational) and lateral directions.
• All detected echoes originate from the axis of
  the main beam only.

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More Assumptions

• All received echoes are derived from the most
  recently transmitted pulse.
• The ultrasound wave travels in soft tissue at a
  constant rate of 1540 m/s in tissue.
• Each reflector contributes a single echo when
  interrogated along a single scan line.
• The amplitude of the echo is related to the
  characteristics of the object scanned and is
  directly related to the reflective properties of
  the object.

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Categories

• Image detail resolution related
• Locational artifacts
• Attenuation artifacts
• Doppler artifacts

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Detail Resolution Related

• Limited axial resolution
• Longitudinal, radial, range, depth
• Limited lateral resolution
• Angular, transverse, azimuthal
• Limited elevational resolution
• Slice thickness, partial volume
• Order of resolution
• Axial is best
• Lateral is second best
• Elevational is worst (in relative terms)

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Resolution
(Depth, Range)
Axial
Lateral
(Beam Width)
Elevational (Beam Thickness)
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Axial Resolution

• The ability to display two reflectors as two
  distinct reflectors when lying front to back,
  parallel to the sound beams main axis.
• Equal to the spatial pulse length (SPL) / 2.
• SPL (mm) of cycles in the pulse x the
  wavelength.
• If two reflectors are closer than the SPL/2, they
  appear as one reflector.
• Higher frequency sound or less cycles in the
  pulse ? better axial resolution.

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Axial Resolution


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Lateral Resolution

• The ability to display two reflectors as two
  distinct reflectors when lying side-by-side,
  perpendicular to the sound beams main axis.
• Related to the width of the sound beam.
• If two reflectors lying side-by-side are
  insonated at the same time due to the width of
  the sound beam, they will appear as one
  reflector.

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Lateral Resolution
Image
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Axial / Lateral Resolution
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Axial / Lateral Resolution
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Axial / Lateral Resolution
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Elevational Resolution

• Determined by the thickness of the imaging plane.
• The 3rd plane
• The nonimaging plane
• Measured in a direction perpendicular to the
  imaging plane.
• True reflector lies outside, above to below, the
  imaging plane.
• Fills in anechoic structures, e.g. bladder and
  cysts.

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Elevational Resolution

• A 1D transducer
• Elevational plane focusing is by utilization of
  acoustic lenses with a fixed focal length.
• B 1.5D transducer
• Elevational thickness is controlled
  electronically.

A
B
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Elevational Resolution

• Debris at the base of the bladder

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Locational Artifacts

• Refraction
• Reverberation
• Comet tail
• Ringdown
• Multipath
• Lobes
• Side lobes
• Grating lobes
• Speed error
• Range ambiguity
• Mirror image

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Refraction

• Predicted by Snells Law
• 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
• Requires
• Oblique incidence
• Different propagation speeds in the two media

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Refraction Types

• Misregistration
• Improper placement
• Distortion of size or shape
• Defocusing
• Loss of beam coherence
• Shadowing at the edge of large curved structures
• Ghost image
• Altered sound beam path as it encounters the
  rectus muscles

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Refraction
Misregistration
Defocusing
Real object
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Refraction
Ghost Image
Wavefront
Real object
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Refraction - Misregistration

• Inappropriate assignment of the superior pole of
  the kidney due to bending of the sound beam at
  the fat layer surrounding the liver.
• Velocity of sound is lower in fat than in soft
  tissue.
• Image from another plane where the kidney is
  totally covered by, or totally uncovered from,
  the liver.

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Refraction - Defocusing

• Fetal head with edge shadowing

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Refraction Ghost Image

• Single gestational sac duplicated
• Second copy of the reflector, which is
  side-by-side at the same depth as the true
  reflector.

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Reverberation

• Additional echoes from an interface which are
  recorded on the image.
• Appearance
• Series of bright bands
• Parallel to sound beams main axis
• Decreasing in intensity
• Equidistant from each other
• Echoes can appear between the transducer and a
  strong reflector or between two strong reflectors
  located within the medium.
• Echoes may also be the result of defective
  equipment or improper technique.

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Reverberation
Echoes are separated equally in time, resembling
a ladder.
Echoes decrease in strength over time.
Strong Reflector
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Reverberation

• Reverberations due to bowel gas

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Reverberation
Reverberation seen within the carotid artery due
to strong reflectors superficial to the
artery
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Comet Tail

• A series of echoes created by multiple
  reflections within a small but highly reflective
  object.
• Occur due to acoustic mismatch
• The greater the mismatch the greater the
  likelihood of comet tail formation.
• Characteristics
• Single long hyperechoic echo
• Parallel to the sound beams main axis

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Comet Tail

• May arise from the near wall of the gallbladder
  when crystalline deposits are present.
• Surgical clips, staples, sutures and mechanical
  heart valves are sources for comet tail
  artifact.

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Comet Tail
Internal reflections give rise to multiple echoes
from an object.
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Comet Tail
St. Jude valve in open position.
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Ringdown (Resonance)

• Similar to comet tail artifact.
• Occurs due to the resonance (vibration) of gas
  bubbles after being bombarded with ultrasound.

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Ringdown
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Multipath

• Results from insonating a specular reflector at
  an oblique angle.
• Reflection angle equals the incident angle.
• The sound wave encounters a second reflector
  which then redirects the sound wave back to the
  transducer.
• Based on a longer time of flight, a second copy
  of the reflector is placed artifactually deeper
  in the image.

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Multipath
Real object
Artifact
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Side Lobes Grating Lobes

• Side lobes weak, off axis lobes associated with
  a single piezoelectric element.
• Grating lobes weak off axis lobes associated
  with an array of piezoelectric elements.
• When these weak off-axis lobes encounter a strong
  specular reflector, the reflected energy may be
  added to the energy of the main beam, creating an
  overlying structure or causing clutter through
  the main axis of the sound beam.
• A displaced structure will be displaced laterally
  from the real structure.
• The real reflector will generally not be seen on
  image.
• Clutter may mask the ability to recognize weak
  echoes that are truly within the main sound
  beams axis.

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Side Lobes Grating Lobes
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Side Lobes Grating Lobes
Artifact
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Reducing Lobe Artifacts

• Apodization varying the strength of the
  voltage exciting the piezoelectric elements
  across the aperture.
• Elements closer to the center of the aperture are
  excited with more intense voltage than elements
  at the periphery of the aperture.

Pulser
Delay lines
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Reducing Lobe Artifacts

• Subdicing array piezoelectric elements are
  divided into smaller subelements.
• Reduces the center-to-center distance of the
  elements to lt1 wavelength.
• Subelements are wired together to form the
  original sized element.

44
Speed Error

• Sonographic equipment presumes a propagation
  velocity of 1540 m/s.
• (13µs rule)
• Reflectors will be inappropriately positioned if
  the propagation velocity is different than the
  presumed.
• Propagation velocity gt1540 m/s
• Go-return time short
• Reflector will be more shallow than actual
• Propagation velocity lt1540 m/s
• Go-return time long
• Reflector will be deeper than actual

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Speed Error

• Characteristics
• Correct number of reflectors
• Improper depth
• Appears as a step-off

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Speed Error
Speed through the liver tumor (T) is lt1540 m/s
causing the diaphragm to look discontinuous
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Speed Error

• Hepatic Lipoma

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Range Ambiguity

• Shallow depth settings have short go-return times
  (high PRFs).
• At shallow reflectors some of the sound energy is
  reflected and some is transmitted.
• The reflected sound energy arrives at the
  transducer and a second sound pulse is generated.
• The transmitted sound wave continues to
  propagate, interacting with tissue (reflectors)
  until all of its energy is lost.
• Echoes from the first pulse arrive at the
  transducer after the transmit of the new pulse.
• The system interprets the echoes returning from
  depth as being associated with the second pulse
  and places artifact in the near field of the
  image.

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Range Ambiguity
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Mirror Image

• Created as sound reflects off of a strong
  reflector and is redirected toward a second
  structure.
• Appears as a second copy of the structure which
  is placed deeper on the image.
• Mirror is always present along a straight line
  between the transducer and the artifact.
• True reflector and the artifact are equal
  distances from the mirror.

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Mirror Image
Strong reflector
Object
False Image
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Mirror Image
Hepatic cyst is mirrored on the opposite side of
the diaphragm.
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Mirror Image

• Temporal artery aneurysm mirrored by the skull

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Mirror Image

• Subclavian artery mirrored by the pleura of the
  lung.

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Mirror Image
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Attenuation Artifacts

• Acoustic shadowing
• Enhancement
• Reverberation
• Comet tail
• Ring down
• Refraction
• Speckle

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Acoustic Shadowing

• Anechoic or hypoechoic region seen deep to a
  highly attenuating medium.
• Prevents visualization of true anatomy.
• Considered a beneficial artifact.
• May be classified as
• Clean
• Posterior to calcification or bone due to high
  percentage of absorption reflection with no
  transmission.
• Dirty
• Posterior to air filled structures due to high
  percentage of reflection small percentage
  of transmission.

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Acoustic Shadowing
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Acoustic Shadowing
Intermittent acoustic shadowing due to supporting
rings of a reinforced gortex graft
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Acoustic Shadowing
Calcified plaque
Bone - Vertebral Body
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Acoustic Shadowing

• Dirty Shadow

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Acoustic Shadowing
Edge shadow due to defocusing of the sound beam.
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Edge Shadowing
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Edge Shadow

• Characteristics
• Hypo- or anechoic
• Spreading (defocusing) of the sound beam after
  striking a curved reflector
• Extends downward from the curved reflectors edge
• Parallel to the sound beam
• Prevents visualization of true anatomy

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Enhancement

• Hyperechoic region extending beneath an
  abnormally low attenuating structure.
• Considered a beneficial artifact.

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Posterior Enhancement

• Enhancement seen posterior to a Bakers cyst
  in the popliteal fossa

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Posterior Enhancement

• Enhancement deep to the bladder

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Focal Enhancement

• Aka Focal Banding
• Side-to-side region of increased intensity at the
  focus of an image, which appears brighter than
  tissues at other depths.
• Has appearance of incorrect TGC setting.
• Especially prominent with linear phased array
  transducers.

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Enhancement Focal Banding
Increased intensity at the focus due
to a strongly focused sound beam.
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Speckle

• The result of scattering due to the rough surface
  characteristics.
• Due to scattering partial constructive and
  destructive interference occurs resulting in a
  speckle appearance of the tissue.
• Typically associated with
• Liver parenchyma
• Thyroid
• Heart muscle
• Skeletal muscle
• Spleen
• Kidney
• Higher frequency transducers create finer speckle
  patterns than do lower frequency transducers.

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Speckle
Liver
Spleen
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Speckle
Testicle
Thyroid
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Three-In-One or MORE!

• Arrow
• Ring down
• E
• Enhancement
• S
• Acoustic shadow

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Resources

• Understanding Ultrasound Physics, Third Edition
• Sidney K. Edelman, Ph.D.
• Ultrasound Physics and Instrumentation,
  Fourth Edition
• Hedrick, Hykes, Starchman Elsevier / Mosby
• Ultrasound Physics Instrumentation, 4th Edition
  
• Frank R. Miele, MSEE
• Diagnostic Ultrasound - Principles and
  Instruments, Seventh Ed.
• Frederick W. Kremkau Elsevier / Saunders