Artifacts Flashcards
error in imaging
artifact
artifacts include reflections that are described as…
not real, not seen on image, incorrect shape/size/position/brightness
causes of artifacts
violation of assumption
equipment malfunction / poor design
the physics of ultrasound
operator error
portions of an image that are brighter than surrounding tissues, or tissues that appear brighter than normal
hyperechoic
portions of an image that are not as bright as surrounding tissues, or tissues that appear less bright than normal
hypoechoic
extreme form of hypoechoic, meaning entirely without echoes
black
anechoic
structures with equal echo brightness
isoechoic
a portion of tissue or an image that has similar echo characteristics throughout
hoomgenous
a portion of tissue or an image that has differing echo characteristics throughout
heterogeneous
6 assumptions used in imaging systems
- sound travels in a straight line
- sound travels directly to a reflector and back
- sound travels in soft tissue at exactly 1540 m/s
- reflections only arise from structures positioned in the beams main axis
- the imaging plane is very thin
- the strength of a reflection is related to the characteristics of the tissue creating the reflection
appear on the display as multiple equally spaced echoes caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the US beam
reverberation
characteristics of reverberation
appear in multiples
appear equally spaced
are located parallel to the sound beams main axis
are located at ever increasing depths
with reverberation, do shallow or deep echoes correspond to true anatomic structures
shallow - real
deep - artifact
reverberation with the spaces squeezed out
appears as a solid hyper echoic line directed downward
also known as ring down artifact
comet tail
how is comet tail artifact created
when closely spaced reverberations merge
do comet tails appear when reflecting surfaces are located in an area of high/low propagation speed
high speed (such as a mechanical heart valve)
comet tail can also arise from…
resonance of small gas bubbles
characteristics of a comet tail
appears as a single long hyper echoic echo (bright white line)
located parallel to the sound beams main axis
this type of artifact appears as hypo echoic or anechoic region extending downward from a highly attenuating structure
(they are the same color as the image background)
shadows
what causes shadowing
when too much attenuation occurs, deep reflectors do not appear on the image
(attenuation is higher in the tissue above the shadow than in the surrounding tissue)
characteristics of shadowing
hypo or anechoic
result of too much attenuation
located beneath the structure with abnormally high attenuation
prevents visualization of true anatomy on the scan
how is shadowing related to the speed of sound in a medium
UNRELATED
type of artifact that is a special form of shadowing that appears as a hypo echoic region extending down from the edge of a curved reflector
edge shadow
what happens during edge shadow artifact
the sound beam refracts (transmits with a bend) at the edge of a curved reflector, the beam diverges and results in a drop in intensity that causes edge shadowing
edge shadow is also called
shadowing by refraction
characteristics of edge shadow
hypo or anechoic
results when the beam spreads after striking a curved reflector
extends downward from the curved reflectors edge, parallel to the beam
prevents visualization of true anatomy on the scan
artifact that appears as a hyper echoic region beneath tissues with abnormally low attenuation
enhancement
enhancement artifact is the opposite of…
shadowing
how is enhancement artifact related to speed of sound
UNRELATED
characteristics of enhancement
hyper echoic
result of too little attenuation
located beneath a structure with abnormally low attenuation
a special form of enhancement in which a side-to-side region of an image appears brighter than tissues at other depths
focal enhancement
focal enhancement is also called
focal banding
has the same appearance as an incorrect TGC setting
focal enhancement
in focal enhancement, an entire horizontal region of tissue appears… hyper echoic/hypoechoic
hyper echoic
characteristics of focal enhancement
hyper echoic side-to-side region
results from increased intensity at the focus
this artifact is created when sound reflects off a strong reflector and is redirected toward a second structure
the redirection causes a replica of the structure to appear on the image
mirror image
is mirror image artifact located more superficially or deeper than the real sttructure
deeper
characteristics of mirror image
a second copy of a true reflector
appears deeper than the true reflector
the bright reflector/mirror lies on a straight line between the artifact and the transducer
true reflector and artifact are equal distances from the mirror
this artifact is a mirror image artifact that appears on a spectral doppler display
crosstalk
this artifact is created when a sound wave propagates through a medium at a speed other than that of soft tissue
(correct number of reflectors are displayed but at incorrect depths)
speed error
propagation speed error are also called
range error artifact
characteristics of speed error
correct number of reflectors at improper depth
appears as a step-off
this artifact appears when sound energy is transmitted in a direction other than along the beams main axis
lobes
lobe artifact degrades _____ resolution
lateral
characteristics of lobe artifact
a second copy of the true reflector
the artifact and the true reflector are located side-by-side at the same depth
other names for lobe artifact
side lobes
grating lobes
lobes created by a single crystal transducer, such as a mechanical probe, are called
side lobes
lobes created by array transducers are called
grating lobes
how to reduce grating lobes
apodization - process of differential excitation
dividing each PZT element into small pieces (called subdicing)
artifact that is created when a sound pulse changes direction during transmission
refraction
refraction artifact degrades _____ resolution
lateral
characteristics of refraction
second copy of reflector
copy is side-by-side or at the same depth as the true reflector
what is slice thickness
the beam flares out like the bell of a trumpet, therefore reflections from structures above or below the assumed imaging plane may appear in the image
slice thickness is also called
section thickness artifact
partial volume artifact
slice thickness is related to the dimension of the beam that is parallel/perpendicular to the imaging plane
perpendicular
_____ resolution is determined by slice thickness
elevational
slice thickness is reduced by these transducers
1.5 array
occurs when a pair of side-by-side reflectors are closer than the width of the sound beam
these reflectors are positioned perpendicular to the beam
two objects appear as one reflection on the image
lateral resolution artifact
lateral resolution artifact is also called
point spread artifact (it can display a small reflector as a wide line)
lateral resolution artifact is most likely to occur where the beam is wide/narrow
wide
artifact that is created when a long pulse strikes two closely spaced structures and only one reflection appears on the image
axial resolution artifact
transducers that create short/long pulses will minimize axial resolution artifact
short
artifact that is created when sound pulses glance off a second structure on the way to or from the primary reflector
multipath
artifact that occurs when a sound beam strikes a curved or oblique reflector and some of the reflected sound may be directed away from the transducer
curved and oblique reflectors
characteristics of curved/oblique reflectors
absent on image
appear weak on image
appear different from other similar reflecting boundaries
artifact with poor temporal resolution caused by low frame rates that results in less accurate positioning of moving reflectors
temporal resolution artifact
image with low line density that exhibits less detail, and the spatial resolution is poor
spatial resolution artifact
how does the systems display affect spatial resolution?
spatial resolution is affected by the number of horizontal scan lines per frame
(or pixel density for digital displays)
occurs when a reflecting structure is located deeper than the imaging depth of the image. this reflector is placed at a shallow location on the image
range ambiguity artifact
range ambiguity artifact can be eliminated by
increasing PRP
appears as small amplitude echoes and results from many sources including electrical interference, signal processing, and spurious reflections
more likely to affect low level hypo echoic regions
noise
appears as tissue texture, especially within the shallow parts of an ultrasound image
speckled grainy appearance that is not directly related to the actual biologic tissue
speckle
why does acoustic speckle arise
constructive and destructive interference of small sound wavelets
another form of noise
the presence of false echo signals arising from locations outside of the main sound beam
clutter
reduces an images noise content
harmonic imaging
