Clinical Significance of US Artifacts
Summary author: Nadiv Hossain
Summary editor: Zonia Ghumman, M.D.
Original publication details
Authors: Michael Baad, Zheng Feng Lu , Ingrid Reiser, David Paushter
DOI: https://doi.org/10.1148/rg.2017160175
Reference: Baad, M., Lu, Z. F., Reiser, I., & Paushter, D. (2017). Clinical significance of US artifacts. RadioGraphics, 37(5), 1408–1423.
Artifacts due to beam and resolution
Beam-width artifact
Occurs due to poor lateral resolution
Lateral blurring of point target occurring as echoes from same target are insonated at adjacent beam positions
Improved by dynamic beam focusing, multiple transmit focal zones, increasing frequency, focal zone placement, or local speed of sound selection
Section thickness artifact
Occurs due to poor elevational resolution
Low level echoes in an anechoic structure
Mimics debris in anechoic structures (e.g. mimics debris in bladder or sludge in gallbladder); causes aberrant internal echoes in small cysts and vessels
Improved by positioning the area of interest in focal zone with standoff pad or acoustic standoff of gel or using 1.5D or 2D transducer arrays
Axial resolution
Ability to discriminate two adjacent points along the axial direction
Objects spaced less than one-half of the spatial pulse length will not be able to be resolved as separate
Improve resolution by choosing a higher frequency
Speckle noise
Granular appearance of an US image is caused by constructive and deconstructive interferences of ultrasound waves interacting with microstructures in tissues
Degrades spatial and contrast resolution. Reduces image contrast and lesion detectability. However, it also gives tissues their characteristic image texture
Can be reduced by speckle reduction imaging, spatial compound imaging, or tissue harmonic imaging
Secondary lobe artifact (Side lobes and Grating lobes)
Arises from reflections of unwanted ultrasound energy directed off-axis from the main beam
Side lobe:
Sound energy emitted at angles outside of the primary beam
Grating lobe:
Far off-axis grating lobe results in error in position the returning echoes
Compared to side lobes, grating lobes occur at more oblique angles (up to 90 degrees)
Mimics debris in anechoic structures
Side lobe artifacts can be reduced by repositioning patients, changing transducer angle, or reducing gain. Tissue harmonic imaging reduces grating lobes artifacts.
Reverberation
Reflections between highly reflective surfaces in parallel
Appears as multiple bright parallel lines at uniform intervals that decrease in intensity with increasing depths
Mimics debris within cystic structures
May be useful in identifying air in abnormal locations such as pneumatosis, pneumoperitoneum, or pneumobilia
Improved by using tissue harmonic imaging, decreasing gain, changing angle of insonation, and using multiple windows
Artifacts due to location (path or speed)
Comet Tail
Subtype of reverberations
Reverberations from so closely spaced highly reflective interfaces that individual echoes are not discernible
Appears as tapering echogenic triangle or cone distal to a strongly reflective structure
Helpful to identify adenomyomatosis, colloid nodules, calcifications, metal objects (foreign bodies, surgical clips, intrauterine devices)
May be more visible by using tissue harmonic imaging
Becomes less visible by using spatial compound imaging
Ring Down
Subtype of reverberations
Resonant vibrations within air bubbles
Helpful to identify: abnormal air such as abscesses, pneumoperitoneum, portal venvous gas, emphysematous infections, pneumobilia; normal air in bowel loops, appendix; B-lines on lung US for diagnosis of interstitial conditions
Improved by using spatial compound imaging
Mirror Image
Reflections off a strong specular reflector pro- duce a mirror image of an object
A portion of the beam is reflected from the target back along transmitted course, again off the specular reflector, and back to the transducer
Mimics abnormal pathologic conditions such as lesions adjacent to the diaphragm appearing as equidistant on opposite sides of the diaphragm, pseudothickened bowel wall or consolidated lung
Improved by decreasing gain, changing angle of insonation, or using multiple imaging windows
Multipath artifact
Occurs due to small off-axis reflections on path to or from primary reflector
These artifacts are relatively minor and cause general image degradation rather than appearing as discrete artifacts
May be altered by varying the angle of insonation or imaging window
Refraction
Refraction at oblique interfaces causes altered location of objects, duplication of underlying structures, and/or shadowing at edges
Most pronounced at highly oblique interfaces, such as the lateral aspect of a curved surface, and with interfaces between substances such as fat and muscle with large differences in speeds of sound
This principle underlies misregistration, ghosting, edge shadowing
Misregistration: Edges of structures and relationships of objects may actually slightly different than they appear
Ghosting: Deep structure may appear in duplicate or triplicate
Edge shadowing: Edge of a large curved boundary appearing as hypoechoic parallel lines projecting distal to the edges of the structure
False position of lesions during biopsy, can mimic duplication of structures (gestational sac, spinal cord, aorta), cyst characterization by using edge shadowing
Improved by varying angle of insonation or imaging window
Speed of sound artifacts
A constant speed of sound is used to calculate depth in soft tissues, but there actually is variability among different tissues
If speed of sound in tissue is less, it will take longer for the echo to be received and will be displayed as if it originated from a more distant target
If speed of sound in tissue is greater, the echo will appear closer
Composed of boundary distortion, misregistration of targets, errors in size, and phase aberration
Inaccurate locations and measurements of structures in axial dimension, loss of lateral resolution, bayonet artifact
Modern US units allow manual speed of sound correction
Range ambiguity
Echoes deep to imaging range are depicted within range on the next pulse
Mimics debris in large fluid-filled structures
Improved by decreasing pulse repetition frequency, reducing the number of focal zones, increasing the image depth
Increased through transmission
Increased intensity of echoes distal to a low-attenuating structure
Distal tissues appear echogenic
Helps to differentiate cystic from solid structures
May be increased by using tissue harmonic imaging
Artifacts due to attenuation characteristics
Acoustic shadowing
Reduction in echo strength distal to a highly attenuating or reflective object
Seen with calcifications, bone, and gas
Clean shadowing seen with large calculi and bone
Partial shadowing seen with fat or smaller calculi
Dirty shadowing seen with gas
Increases with increased frequency and tissue harmonic imaging; decreases with inappropriate focal zone placement, excessive beam width, and spatial compounding imaging
Citation
Baad, M., Lu, Z. F., Reiser, I., & Paushter, D. (2017). Clinical significance of US artifacts. RadioGraphics, 37(5), 1408–1423.