Ch 21 Artifacts

Artifacts

• An artifact is an error in imaging, characterized as:

  • Not real

  • Not seen on the image

  • Incorrect shape or size

  • Incorrect position

  • Incorrect brightness

• Reflections are a type of artifact.

Causes of Artifacts

• Violation of assumptions

• Equipment malfunction or poor design

• The physics of ultrasound

• Operator error

• Most artifacts are explained by identifying violated assumptions during image creation.

Image Characteristics

• Terms used to describe ultrasonic image features:

  • Hyperechoic

  • Hypoechoic

  • Anechoic

  • Isoechoic

  • Homogeneous

  • Heterogeneous

Hyperechoic

• Portions of an image that are brighter than surrounding tissues or appear brighter than normal.

Hypoechoic

• Portions of an image that are not as bright as surrounding tissues or appear less bright than normal.

Anechoic

• An extreme form of hypoechoic, meaning entirely without echoes.

• Echo-free.

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 different echo characteristics throughout.

Basic 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 axis.

• The image plane is very thin.

• The strength of the reflector is related to the characteristics of the tissue creating the reflection.

General Artifact Behavior

• Artifacts appear in some views but not others.

• Actual anatomy remains visible.

• Corrective measures can eliminate artifacts.

• Instrument malfunction should be considered when obvious artifacts remain visible even after corrective measures are taken.

Reverberation Artifact

• Multiple, equally spaced reflections appear on the display.

• Caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the beam.

• Relates to the assumption that sound travels directly to a reflector and back, which is invalid.

Characteristics

• Appear in multiples.

• Appear equally spaced.

• Located parallel to the sound beam’s main axis.

• Located at ever-increasing depths.

• Reverberation places too many relations on the image.

• The 1st and 2nd reflections are real, but the remaining echoes that appear deeper do not correctly correspond to true anatomy.

• Resemble a ladder or Venetian blind.

Comet Tail Artifact

• Described as “reverberation with spaces squeezed out.”

• Appears as a solid hyperechoic line directed downward.

• Also known as a ring-down artifact.

• Relates to the assumption that sound travels directly to a reflector and back, which is invalid.

Characteristics

• Created when closely spaced reverberations merge together.

• More likely to appear when reflecting surfaces are located in a medium with a very high propagation speed, such as a mechanical heart valve.

• Can arise from the resonance or vibration of small structures, such as gas bubbles, after they have been bombarded by a sound pulse.

• Appears as a single long hyperechoic echo.

• Located parallel to the sound beam's main axis.

Shadow Artifact

• Appears as a hypoechoic or anechoic region extending downward from a very strong attenuating medium.

• Shadowing can be used to help with a diagnosis.

• Shadowing is entirely unrelated to the speed of sound in a medium.

• Relates to the assumption that the strength of a reflection is related to the characteristics of the tissue creating the reflection, which is invalid.

Characteristics

• Hypo- or anechoic (background color).

• The result of too much attenuation.

• Located beneath a structure with abnormally high attenuation.

• Prevents visualization of true anatomy on the scan.

Edge Shadow Artifact

• A special form of shadowing extending along the edge of a curved reflector.

• Created as sound beams refract and diverge along the edge of a curved structure.

• Hypoechoic region extending down from the edge of a curved reflector.

• Prevents the display of true anatomic structures that are positioned within the extended hypoechoic region.

• The hypoechoic region is the same color as the background.

• Also known as shadowing by refraction.

• Relates to the assumption that the intensity of a reflection is related to the characteristics of the tissue creating the reflection, which is invalid.

Characteristics

• Hypo- or anechoic (background color).

• 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.

Difference Between Shadow and Edge Shadow

• Edge shadowing is caused by the shape, not attenuation.

Enhancement Artifact

• Appears as a hyperechoic region beneath tissue with abnormally low attenuation.

• The opposite of shadowing.

• Can help with clinical diagnosis and provide valuable diagnostic information to help characterize tissue.

• Entirely unrelated to the speed at which sound travels through a medium.

• Relates to the assumption that the intensity of a reflection is related to the characteristics of the tissue, which is invalid.

Characteristics

• Hyperechoic (appears the same as the foreground tissue).

• The result of too little attenuation.

• Located beneath a structure with abnormally low attenuation.

Focal Enhancement Artifact

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

• Also known as focal banding.

• Has the appearance of an incorrect TGC.

• Most prominent at the region of the focus.

• Structures at the focus appear brighter than those at other depths.

• An entire horizontal band of tissues appears hyperechoic.

• Relates to the assumption that the intensity of a reflection is related to the characteristics of the tissue, which is invalid.

Characteristics

• A hyperechoic side-to-side region (appears the same as the foreground color).

• Results from increased intensity at the focus.

Mirror Image Artifact

• Created when sound reflects off of a strong reflector (mirror) and is redirected towards a second structure.

• Redirection causes a replica or second copy of the structure to incorrectly appear on the image.

• The artifact is placed deeper than the real structure.

• Mirror artifact may be seen with color Doppler; the shallower vessel is real, and the deeper one is an artifact.

• Relates to the assumptions that sound travels in a straight line and sound travels directly to a reflector and back, which are invalid.

Characteristics

• 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.

• Cross-talk artifact: A type of mirror image artifact that appears on spectral Doppler display.

Speed Error Artifact

• Created when a sound wave propagates through a medium at a speed other than that of soft tissue ($1.54 km/s$ or $1,540 m/s$).

• The correct number of reflectors are displayed, but they appear at incorrect depths.

• Also known as range error artifact or range ambiguity artifact.

• Relates to the assumption that sound travels through soft tissue at exactly $1,540 m/s$, which is invalid.

When the Medium’s Speed is Slower than Soft Tissue

• Sound travels slower than the ultrasound system expects.

• Pulses return slowly.

• Go-return time is too long.

• The system assumes reflectors are far from the transducer.

• Reflectors are placed too deep on the image.

• Distances are overestimated (reported number is too large).

When the Medium’s Speed is Faster than Soft Tissue

• Sound travels faster than the ultrasound system expects.

• Pulses return quickly.

• Go-return time is too short.

• The system assumes the reflectors are close to the transducer.

• Reflectors are located too shallow on the image.

• Distances are underestimated (reported number is too small).

Characteristics

• Propagation speed errors are called range error artifacts.

• Correct number of reflectors.

• Improper depth.

• Appears as a step-off.

• Example: If two pins are $100mm$ apart but show up $90mm$ apart on the display, then if they were $50mm$ apart in the test object, they would appear $45mm$ apart due to the 10% underestimation. This means sound is traveling faster than soft tissue's speed.

Lobe Artifacts

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

• Lobes degrade lateral resolution.

  • Side lobes: Created by a single crystal transducer.

  • Grating lobes: Created by an array transducer.

Characteristics

• Generally weaker than the main beam and typically do not generate reflections.

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

• Impossible to distinguish the true reflector and artifact from a single image.

• Real anatomy will appear in all views, whereas an artifact will not.

• Related to the assumption that reflections arise from structures located along the beam’s main axis, which is invalid.

• A second copy of the true reflector.

• The artifact and the true reflector are located side-by-side at the same depth.

Reducing Grating Lobes

• Subdicing: Dividing each PZT element into smaller pieces.

• Apodization: Exciting the subdiced elements with different voltages (higher voltages for elements closer to the center and lower voltages for outermost elements).

Refraction Artifact

• Occurs when a sound pulse changes direction during transmission (transmission with a bend).

• Occurs only when a sound beam strikes a boundary obliquely, and the media on either side of the boundary have different propagation speeds.

• Degrades lateral resolution.

• Relates to the assumption that sound travels in a straight line, which is invalid.

Characteristics

• A second copy of the reflector is seen.

• The copy is side-by-side, or at the same depth as the true reflector.

Slice Thickness Artifact

• The 2-D imaging plane is not thin and uniform; in reality, it is 3-D.

• Reflections from structures above or below the assumed imaging plane may appear in the image.

• The beam flares out like a bell of a trumpet.

• Slice thickness is related to the dimension of the beam perpendicular to the imaging plane.

• Elevation resolution is determined by the thickness of the imaging plane.

• Appears when the beam dimension is greater than the reflector size.

• Also called section thickness artifact or partial volume artifact.

• The thickness of the imaging plane varies; thicker portions have inferior elevation resolution compared to thinner portions.

• The true reflector lies either above or below the assumed imaging plane but is displayed within the image.

• Usually fills in hollow structures such as a cyst.

• Can be reduced with thinner imaging planes, such as those created with 1.5-dimensional array transducers.

• Relates to the assumption that the imaging plane is extremely thin, which is invalid.

Lateral Resolution Artifact

• Also known as point spread artifact.

• Occurs when a beam is wider than the distance between two reflectors that are side to side.

• Two objects appear as one.

• Displays a small reflector as a wide line.

• Focusing improves lateral resolution.

• When two structures are closer together than the beam's diameter, only one reflection is seen on the image.

Axial Resolution Artifact

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

• Only one reflector will appear on the image if the structures are closer together than one-half spatial pulse length.

• Higher frequency transducers have better axial resolution (shorter SPL).

Other Artifacts

Multipath Artifact

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

• Results in the transmit path differing from the return path.

• Results in subtle, nonspecific changes that can be identified.

• Proves invalid the assumption that a pulse travels directly to a structure and back.

Curved & Oblique Reflectors Artifact

• Occurs when a sound beam strikes a curved or oblique reflector; some of the sound may be directed away from the transducer.

• Reflection is weaker, causing the reflector to appear absent, weak, or different from similar reflectors.

Temporal Resolution Artifact

• Temporal Resolution (T.R.) is the ability to precisely position a moving structure.

• Determined by frame rate (higher frame rate = better temporal resolution = better ultrasound movie).

• Poor T.R. caused by low frame rates results in less accurate positioning of reflectors in motion.

Spatial Resolution Artifact

• Related to the overall detail in an image.

• Determined by sound beam’s spacing or line density.

• Higher line density (closely packed pulses) on the image shows great detail and superior spatial resolution.

• Lower line density (wider gaps between pulses) on the image shows less detail and the spatial resolution is inferior.

Digital Format

• When the display information is in digital format, spatial resolution is related to pixel density.

• Improves with a large number of pixels in the image of a fixed size.

• Each pixel is small, resulting in more detail.

• Degrades with low pixel density.

• Each pixel is large.

• Spatial Resolution Artifact occurs when the pixel size is greater than the size of the reflector.

Range Ambiguity Artifact

• Assumes that all reflections are received by the transducer before the next pulse is transmitted.

• Occurs when a reflection from a deep structure is received after the next pulse has been transmitted.

• This reflector will be interpreted as coming from the most recent pulse and will appear as a very shallow reflector.

• Occurs when a reflecting structure is located deeper than the imaging depth of the image, and this reflector is placed at a shallow level.

• Can be eliminated by increasing the PRP (longer PRP means deeper imaging and decreased PRF).

Noise Artifact

• Appears as small amplitude echoes and results from many sources:

  • Electrical interference

  • Signal processing

  • Spurious reflections

• More likely to affect low-level hypoechoic regions rather than a bright echogenic area.

Other Forms of Noise

• Speckle

• Clutter

Speckle Artifact

• Grainy area not directly related to the actual biologic tissue.

• Noise resulting from the constructive and destructive interference of small sound wavelets.

  • Constructive: in-phase

  • Deconstructive: out-of-phase

• The image will contain fake detail, such as the grainy tissue-like texture.

• Can be reduced by Spatial Compounding (pg. 254).

Clutter Artifact

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

• Side lobes, grating lobes, and thickness artifacts are sources of clutter.

• With Doppler, it is the contamination of valid Doppler signals with high-amplitude reflections from vessel walls or heart muscle.

• These high-amplitude reflections overpower the true Doppler signals created by moving blood.

Harmonic Imaging

• Using Harmonics reduces the image’s noise content.

• Goal is to selectively distinguish meaningful reflections from noise, resulting in an increased signal-to-noise ratio (Refer to Ch 17).

Artifact Summary

• Anatomic reflectors absent on image

  • Shadowing

  • Shadowing by refraction

  • Lateral resolution

  • Axial resolution

• Anatomic reflector appears multiple times on image. Artifact positioned deeper than the true anatomy.

  • Comet tail

  • Ring down

  • Reverberations

  • Mirror image

• Anatomic reflector appear multiple times on image. Artifact displaced to the side of the true anatomy

  • Refraction

  • Side lobe

  • Grating lobe

• Anatomic reflectors appear with abnormal brightness

  • Enhancement (hyperechoic)

  • Banding (hyperechoic)

  • Shadowing (hypoechoic)

  • Shadowing by refraction (hypoechoic)

• Anatomic structures appear at incorrect depth

  • Speed errors

  • Range ambiguity artifact

• Anatomic structures appear in the incorrect imaging plane

  • Slice or section thickness

• Anatomic structures do not correspond to echoes on the image

  • Acoustic speckle

  • Multipath