Artifact Flashcards
What are common causes of artifacts?
When imaging system assumptions are violated or not true.
When the equipment defects and causes interference
What are the six basic assumptions of the imaging system?
- Sound travels only in a straight line
- Sound travels directly to and from the reflector
- Sound travels in soft tissue at exactly 1540 m/s
- Reflections arise only from the structures positioned in the main axis of the beam
- The imaging plane is very thin
- Reflection strength is related to characteristics of the tissue causing reflection
Are artifacts visible in all views of the imaged anatomy?
No, they are present in some and absent in others
When should instrument malfunction be considered?
When the obvious artifacts are still visible after the sonographer has taken corrective measures
What is reverberation?
Multiple, equally spaced echoes that are located at increasing depths
Violation of second assumption
What causes reverberation?
A sound wave bouncing between two reflectors that are strong and parallel to the beam
What are the four primary characteristics of reverberation?
Appear in multiples
Equal distance apart
Located parallel to sound beam main axis
Located at everincreasing depths
What is comet-tail/ring down?
A solid hyperechoic line that is directed down
Reverberation without the spaces
What causes comet-tail?
The merging of closely spaced reverberations
More likely to occur when reflecting surfaces are in a medium with very high propagation speed
What are the two main characteristics of comet-tail?
Appears as a single hyperechoic line
Located parallel to sound beam main axis
What is shadowing?
Hyperechoic region in an image that is found extending below a structure with high attenuation
Same color as the background
What causes shadowing?
Attenuation that is higher in the tissue above the shadow than in surrounding tissue
What are the four primary characteristics of shadowing?
Hypo/anechoic area that extends down
Results from too much attenuation
Found under a structure with very high attenuation
Prevents true anatomy visualization
Is shadowing ever useful?
Yes, it can provide diagnostic information when characterizing tissue
What is edge shadow?
A special form of shadowing that is a hypoechoic region extending downwards from a curved reflector’s edge
What causes edge shadow?
It is caused by a decrease in intensity. The sound beam refracts and diverges, causing intensity drop.
What are the four main characteristics of edge shadowing?
Hypo/anechoic region
Results when beam strikes reflector and spreads
Extends down from curved edge, parallel to beam
Prevents visualization of true anatomy
What is enhancement?
A hyperechoic region that is brighter and the same color as the foreground of the image. Found beneath a tissue tissues with very low attenuation.
What causes enhancement?
It is caused by abnormally low attenuating structures (cysts)
What are the three primary characteristics of enhancement?
Hyperechoic (same color as foreground)
Results from too low attenuation
Located underneath a structure with very low attenuation
Is enhancement ever useful?
Yes, when characterizing tissue
What is focal enhancement?
It is a special kind of enhancement that occurs when a side-to-side region in an image appears brighter than the tissues at other depths
What is another name for focal enhancement?
Another term is focal banding
What are the two primary characteristics of focal enhancement?
A hyperechoic region that runs side-to-side and is the same brightness as the foreground
Results from increased intensity at the focus of the image
What is mirror image?
It is when an object appears with a mirror image replica deeper and on the other side of a strong reflector.
What causes mirror image?
It is caused by sound reflecting off a strong reflector and redirecting toward a different structure, making a replica incorrectly appear
What assumptions does mirror image violate?
It violates assumptions one and two
What are the four major characteristics of mirror image?
Second copy of true reflector/object
Artifact replica always found deeper than true reflector
The mirror lies on straight line between transducer and artifact
Replica and true reflector are found at equal distances from mirror
What is cross-talk?
Mirror image artifact on spectral doppler
It shows identical doppler spectrums above and below baseline, appearing as bidirectional flow
What is speed error?
Artifact created when a sound wave moves through a medium at a speed other than soft tissue (1540 m/s)
It makes the reflectors appear at incorrect depths
What are the three main characteristics of speed error?
A correct number of reflectors
Reflectors appear at incorrect depth
It appears as a step-off with the object looking like a split or cut
Where is the reflector displayed when the speed is slower than soft tissue?
When speed is slower than 1540 m/s, the reflectors are seen too deep on the image
Where is the image displayed when the speed is faster than soft tissue?
When speed is faster than 1540 m/s, the reflectors are seen too shallow on the image
What is refraction?
Artifacts that create a replica of the true reflector at the same level as the true reflector
Created when the direction of a sound pulse changes during transmission
What causes refraction?
It is caused by a sound wave striking a boundary obliquely when the media on opposite sides of the boundary do not have the same propagation speed
What assumption is violated to create refraction?
It violates assumption one
What are the two primary characteristics of refraction?
A replica of the true reflector
Replica at the same depth or side by side with true reflector
How does refraction affect resolution?
It degrades lateral resolution
What are lobe artifacts?
Lobe artifacts are shown when the sound energy is transmitted in a direction other than the main axis of the sound beam
What are side lobes?
Side lobes are lobe artifacts that are caused by mechanical transducers w/ single transducers
What are grating lobes?
Grating lobes are lobe artifacts that are caused by array transducers
What is subdicing?
It is a process in which the PZT element is divided into small pieces
What does subdicing achieve?
Reduce lobe artifact
What is adopization?
It is when the subdiced elements are excited with different voltages
Those in the center with higher voltages, and those in the outermost region with lower voltages
What is slice thickness artifact?
It is when reflections from structures above or below the imaging plane can be seen in the image
What causes slice thickness artifact?
It is caused by an incorrect assumption that the imaging plane is thin and uniform, when it is really three dimensional and neither thin nor of a uniform thickness
What are other terms for slice thickness artifact?
Section thickness
Partial volume artifact
What type of resolution is determined by slice thickness?
Elevational resolution
How is slice thickness reduced?
With a thinner imaging plane, created by 1.5 dimensional array
What is noise?
Echoes of small amplitude that is more likely to affect low-level hypoechoic regions
Results from electrical interference, signal processing, and spurious reflections
What is speckle and how does it appear on ultrasound?
It is noise with tissue texture that is speckled or grainy in appearance, especially seen in the shallow area of the image
The image will have fake detail, like grainy tissue
What causes speckle?
It is caused by constructive and destructive interference of wavelets that are small and round in shape
What is clutter?
Another form of noise that is the presence of false signals of echo that arise from locations no in the main sound beam
What is range ambiguity?
It is when a very deep reflection does not arrive at the transducer until the next pulse has been transmitted, making it be interpreted as a very shallow structured reflected in the second pulse
What causes range ambiguity?
It is caused by a reflecting structure that is found deeper than the imaging depth of that image
How are structures incorrectly placed in an image with range ambiguity?
Structures are incorrectly placed because when it takes too long for the reflection to come back to the transducer, the machine can interpret it as part of the next pulse, and, therefore, a shallow structure
How can you eliminate range ambiguity?
It can be eliminated by increasing the PRP
Longer PRP means deeper imaging and decreased PRF
What is lateral resolution artifact?
It is when one reflection is created by two reflectors
What causes lateral resolution artifact?
It is caused by a pair of side-by-side reflectors that are closer than the sound beam width and are perpendicular to the sound beam
How might lateral resolution artifact appear in an image?
It can appear as two objects seen in one reflection or as a small reflector shown as a wide line instead of small dot
Where is lateral resolution artifact most and least likely to occur?
It is most likely to occur where the sound beam is the widest
It is the least likely to occur where the sound beam is the narrowest (focus)
What is axial resolution artifact?
It is an artifact that creates one reflection from two closely spaced reflectors
What causes axial resolution?
It is caused by a long pulse striking two structures that are closely spaced (less than ½ SPL) and one after the other. Structures are parallel to the sound beam
How could axial resolution artifact appear on an image?
It will appear as one reflection that represents multiple closely spaced small structures
How can axial resolution artifact be minimized?
It can be minimized by transducers with short pulses, high frequency, and less ringing
What is multipath artifact?
Artifact created when sound wave pulses go off a secondary structure as they go to or from the true reflector
It shows subtle, nonspecific changes, and it may not be identified
What is temporal resolution artifact?
The inaccurate positioning of moving reflectors caused by low frame rates (and temporal resolution)
What is spatial resolution artifact?
Artifact that is caused by lower line density, more spaced sound pulses that exhibit less detail
It may not show all of the necessary information of the image because the decreased line density gives less information
How are pixel density and spatial resolution related?
They are directly related
When pixel density is low, the pixels are large, degrading spatial resolution
When pixel density is high, spatial resolution is improved
