"Now you see it, now you don`t!" - A pictorial review of ultrasound artifacts

44 Pages·2014·2.84 MB·English

"Now you see it, now you don`t!" - A pictorial review of
ultrasound artifacts
Poster No.: C-0543
Congress: ECR 2014
Type: Educational Exhibit
Authors: 1 2 1 2
A. M. Calin , M. Calin ; Cluj-Napoca/RO, Cluj Napoca/RO
Keywords: Education and training, Artifacts, Technical aspects, Physics,
Education, Ultrasound-Colour Doppler, Ultrasound, Ultrasound
physics
DOI: 10.1594/ecr2014/C-0543
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Learning objectives
To enlist the assumptions of the ultrasound beam
To depict the most commonly encountered artifacts
To explain the physical principles that lead to development of artifacts
Background
An artifact represents an acoustic image that does not correspond with the analyzed
anatomical structure.
Ultrasound techniques are prone to plentiful artifacts, which are commonplace in clinical
practice and may induce confusion to the radiologist.
The main sources of artifacts are improper scanning technique and physical limits of the
modality.
Some artifacts are undesired, while others help to identify certain structures.
The key to understanding artifacts is the comprehension of ultrasound production,
propagation and interactions, supported by a good knowledge of image generation.
Findings and procedure details
In order to determine the exact location and intensity of each echo and to produce an
ultrasound image, the equipment - despite its continuous development - still relies on the
following basic physical assumptions:
1. The pulse of ultrasound and its echo travel in a straight line.
2. The resulted echo returns to the transducer after a single reflection.
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3. The time taken for an ultrasound to return to the transducer as an echo, time of flight,
is directly related to the depth of an object.
4. The speed of sound in human tissue is constant and set at 1540 m/sec.
5. The returning echoes are presumed to originate from the center of the ultrasound beam
and are thus displayed within the central vector, representing the original beam.
6. The acoustic energy is uniformly attenuated and the intensity of the resulted echo
depends only on the acoustic properties and the interface size.
Often, the basic assumptions cannot be maintained and the machine is not able to
differentiate deviations from these principles. Therefore, echoes that do not correspond
to the position or intensity of the original interface may be erroneously displayed and
perceived as artifacts.
In grey-scale imaging, artifacts may arise due to unavoidable errors related to the
ultrasound beam characteristics, the presence of multiple echo paths, velocity errors and
attenuation errors. The color Doppler technique is also prone to artifacts, which can be
even more troublesome than those on grey-scale imaging.
Beam-width artifact
This artifact is identifiable after the complete understanding of the shape of the ultrasound
beam.
At the exit from the transducer, the main ultrasound beam is approximately as wide as
the transducer, then it narrows at the approach of the focal zone and widens again after
it has passed the focal zone. The distal widening may exceed the actual width of the
transducer and a highly reflective object located within this widened beam may produce
echoes. The machine assumes that these echoes arise from within the narrowed, focal
zone and displays them as such.
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Fig. 12: Beam-width artifact - The ultrasound machine assumes that echoes return
only from the plane indicated by the dotted lines. In reality, the ultrasound beam
narrows towards and widens distal to the focal zone. If an object lies within the widened
beam, in the peripheral area (purple circle), the echoes generated by it are falsely
displayed as overlapping the object of interest (green square).
References: Adriana Calin, Cluj-Napoca
Recognition: Clinically, this artifact should be taken into consideration when a structure
that should have an anechoic appearance contains peripheral echoes.
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Fig. 1: Transverse view of the urinary bladder - multiple echoes inside the expected
anechoic structure (blue arrow).
References: - Cluj-Napoca/RO
Significance: These low-level echoes within anechoic or cystic structures can cause the
wall to appear thickened and indistinct. They can be mistaken for debris, sludge, gravel
or clotted blood.
Improvement: By repositioning the patient, adjusting the focus to the level of interest and
placing the transducer at the center of the examined object, the generated echoes will
diminish.
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Fig. 13: Correction of the beam-width artifact - By placing the object of interest (green
square) within the center of the focal zone, the falsely displayed echoes generated by
the peripheral object (purple circle) will disappear.
References: Adriana Calin, Cluj-Napoca
Side-lobe artifact
Side-lobes represent multiple beams of low amplitude that arise radially from the main
ultrasound beam axis.
They are generated by the radial expansion of the piezoelectric crystals and are mainly
produced by linear-array transducers.
If a strong reflector is located in the path of these low-energy, off-axis pulses, echoes
may result, but they will be interpreted as having originated from within the main beam
and displayed as such.
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Fig. 14: Side-lobe artifact - When the multiple off-axis beams of ultrasound, also
known as side-lobes (grey ellipses), encounter and object (purple square), the
machine assumes that the corresponding returning echoes come from the main beam.
Therefore, on the display, the object appears misplaced and duplicated.
References: Adriana Calin, Cluj-Napoca
Recognition: Curved, superfluous echoes within anechoic structures.
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Fig. 2: Longitudinal view - multiple falsely displayed echoes within a cystic lesion (blue
arrow).
References: - Cluj-Napoca/RO
Significance: Additional, unnecessary echoes may be mistaken for septa or sediment
Improvement: By angling the transducer or changing the scan plane, the false echoes
will disappear.
Reverberation artifact
This artifact derives from the erroneous assumption that the echo returns to the
transducer after a single reflection and that the depth of an interface is related to the time
necessary for this round trip.
When the ultrasound beam encounters two highly reflective surfaces, the echoes may
be relentlessly reflected back and forth before returning to the transducer for detection
and display. This will result in multiple echoes, which will be recorded and displayed.
The only echo that will be displayed in the proper location is the echo that returns after
a single reflection. The ensuing echoes will take longer to return to the transducer and
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the processor will incorrectly assume that the delay is the result of an increased distance
from the transducer.
Fig. 15: Reverberation artifact - Between two highly reflective interfaces, ultrasound
echoes (grey arrows) will be repeatedly reflected.
References: Adriana Calin, Cluj-Napoca
Recognition: On the image, multiple equidistantly spaced linear reflections will appear,
known as the reverberation artifact. It is generated when the ultrasound beam is
perpendicular to a strong reflector, to a soft tissue-air interface or to the abdominal wall
with a considerable depth of subcutaneous tissue.
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Fig. 3: Transverse abdominal scan - reverberations (blue arrow).
References: - Cluj-Napoca/RO
Significance: This troublesome artifact is consistently present in anechoic organs, but
can also appear in solid organs, can mimic highly echoic structures and also obscure
superficial metastases or cysts.
Improvement:
Besides changing the direction of the beam, tissue harmonic imaging (THI) can be very
useful in the presence of reverberations. Tissue harmonic imaging consists in sound
waves formed within the body by the interaction of the fundamental pulse and the tissue.
Low amplitude, high frequency waves will be generated, while the fundamental wave
will be suppressed by various means. It helps in clearing the image, especially that of
anechoic structures or cystic lesions.
Comet tail artifact
The comet tail artifact represents a special form of reverberation. It occurs when the
two highly reflective interfaces are closely spaced. In this case, the resulting sequential
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