chapter 21: artifacts

artifact

an error in imaging

artifacts include reflections that are described as:

• not real

• not seen on the image

• incorrect shape or size

• incorrect position

• incorrect brightness

causes of artifacts:

• violation of assumptions

• equipment malfunction or poor design

• the physics of ultrasound

• operator error

hyperechoic

portions of an image that are brighter than surrounding tissues or tissues that appear brighter than normal

hypoechoic

portions of an image that are not as bright as surrounding tissues or tissues that appear less bright than normal

anechoic

an extreme form of hypoechoic, meaning entirely without echoes

isoechoic

describes structures with equal echo brightness

homogeneous

a portion of tissue or an image that has similar echo characteristics throughout

heterogeneous

a portion of tissue or an image that has differing echo characteristics throughout

6 assumptions of imaging systems:

• sound travels in a straight line

• sound travels directly to a reflector and back

• sound travels in soft tissue at exactly 1,540 m/s

• reflections arise only from structures positioned in the beam’s main axis

• the imaging plane is very thin

• the strength of a reflection is related to the characteristics of the tissue creating the reflection

reverberation artifact

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 ultrasound beam

characteristics of reverberation:

• resembles a ladder or venetian blind

• appear in multiples

• appear equally spaced

• are located parallel to the sound beam’s main axis

• are located at ever-increasing depths

comet tail artifact

• reverberation with the spaces squeezed out

• appears as a solid hyperechoic line directed downward

• AKA ring down artifact

how is comet tail artifact created?

it is created when closely spaced reverberations merge

comet tail artifact is more likely to appear when:

the reflecting surfaces are located in a medium with a very high propagation speed

characteristics of comet tail artifact:

• appears as a single long hyperechoic echo

• is located parallel to the sound beam’s main axis

shadow artifact

appears as a hypoechoic or anechoic region extending downward from a highly attenuating structure

shadow appears when:

the attenuation is higher in the tissue above the shadow than in the surrounding tissue

characteristics of shadowing:

• hypo- or anechoic

• the result of too much attenuation

• located beneath the structure with abnormally high attenuation

• prevents visualization of true anatomy on the scan

edge shadow artifact

• a special form of shadowing that appears as a hypoechoic region extending down from the edge of a curved reflector

• AKA shadowing by refraction

edge shadow is created when:

the sound beam refracts at the edge of a curved reflector and diverges resulting in a drop in intensity

characteristics of edge shadow:

• hypo- or anechoic

• results when the beam spreads after striking a curved reflector

• extends downward from the curved reflector’s edge, parallel to the beam

• prevents visualization of true anatomy on the scan

enhancement artifact

appears as a hyperechoic region beneath tissues with abnormally low attenuation

characteristics of enhancement:

• hyperechoic

• the result of too little attenuation

• located beneath a structure with abnormally low attenuation

focal enhancement artifact

• AKA focal banding

• a special form of enhancement in which a side-to-side region of an image appears brighter than tissues at other depths

• banding is most prominent at the focus

characteristics of focal enhancement:

• a hyperechoic side-to-side region

• results from increased intensity at the focus

mirror image artifact

created when sound reflects off a strong reflector (mirror) and is redirected toward a second structure causing a second copy of the structure to incorrectly appear on the image

in a mirror artifact, the artifact is located:

deeper than the real structure and the mirror is always located along a straight line between the transducer and the artifact

characteristics of mirror image:

• a second copy of a true reflector

• the artifact appears deeper than the true reflector

• a bright reflector, the mirror, lies on a straight line between the artifact and the transducer

• the true reflector and artifact are equal distances from the mirror

speed error artifact

• AKA propagation speed error or range error artifact

• created when a sound wave propagates through a medium at a speed other than that of soft tissue (1,540 m/s)

• the correct number of reflectors are displayed but at incorrect depths

• appear as a “step-off” as if structures are split or cut

in speed error artifact, when a medium’s speed is slower than that in soft tissue:

reflectors are placed too deep on the image and distances are overestimated

in speed error artifact, when a medium’s speed is faster than that in soft tissue:

reflectors are located too shallow on the image and distances are underestimated

characteristics of speed error artifact:

• correct number of reflectors

• improper depth

• appears as a step-off

lobe artifact

• appears when sound energy is transmitted in a direction other than along the beam’s main axis

• degrades lateral resolution

• a reflection may be created when a strong reflector is in the path of the lobe

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

side lobe artifact

lobes created by a single crystal transducer such as a mechanical probe

grating lobe artifact

lobes created by array transducers

what are the 2 ways to reduce lobe artifacts?

• subdicing

• apodization

subdicing

the PZT element is divided into small pieces

apodization

the subdiced elements are excited with different voltages

refraction artifact

• created when a sound pulse changes direction during transmission

• degrades lateral resolution

characteristics of refraction artifact:

• a second copy of the reflector

• the copy is side by side or at the same depth as the true reflector

slice thickness artifact

• AKA section thickness artifact or partial volume artifact

• related to the dimension of the beam that is perpendicular to the imaging plane

• fills in hollow structures

lateral resolution artifact

• occurs when a pair of side-by-side reflectors are closer than the width of the sound beam

• two objects appear as one reflection on the image

• may display a small reflector as a wide line rather than a narrow dot

• AKA point spread artifact

where is lateral resolution artifact least likely to occur?

at the focus

where is lateral resolution artifact most likely to occur?

where the beam is wide

axial resolution artifact

• created when a long pulse strikes two closely spaced structures where one is in front of the other

• only one reflection will appear on the image if the structures are closer together than ½ the spatial pulse length

multipath artifact

• created when sound pulses glance off a second structure on the way to or from the primary reflector

• often results in subtle, nonspecific changes that cannot be explicitly identified on the image

curved and oblique reflector artifact

when a sound beam strikes a curved or oblique reflector, some of the reflected sound may be directed away from the transducer

characteristics of curved and oblique reflector artifact:

• reflectors are absent on the image

• reflectors appear too weak on the image

• reflectors appear different from other similar reflecting boundaries

range ambiguity artifact

occurs when a reflecting structure is located deeper than the imaging depth of the image and this reflector is placed at a shallow location on the image

noise

appears as small amplitude echoes and results from many sources including electrical interference, signal processing and spurious reflections

speckle

results from the constructive and destructive interference of small sound wavelets

clutter

• another form of noise

• the presence of false echo signals arising from locations outside of the main sound beam