Artifacts Flashcards
Main categories of artifacts (4)
Resolution
Attenuation
Doppler
Propagation/location
Refraction and redirection or reverberation of sound waves is what type of artifact
Propagation
Degrade lateral resolution of artifact - which type of artifact
Propagation
With multipath artifact where is the second copy of the reflector placed?
At a DEEPER location
Which artifact occurs when it hits a rounded surface?
Multipath
How do you fix a double aorta
Change the window
Artifacts caused by air (2)
Ring down and comet tail
Adenomyomatosis will have this artifact
Comet tail
Emphysematous or pneumobilia will have this artifact
Ring down
Sound reflecting off a strong reflector causes what artifact? How does this occur
Mirror image
Interrogation at 90 degrees
Side lobes vs grating lobes
Side lobes are single element probes
Grating lobes are transducer arrys
How to fix grating lobes (3) explain
Apodization - decrease voltage and thus amplitudes on outer elements
Subdicing - each element is divided into subelements and wired together as single element
Dynamic aperture - use only few elements narrow the beam info returned
What is vertical misregistration
Propagation speed error
If the prop speed is faster than 1540 m/s, where will the reflector be placed?
Reflector will be closer
With propagation speed error, the reflector will be placed deeper when the prop speed is (faster or slower than 1540)
When it is slower, it will be placed deeper
How to fix section thickness artifact
THI
What is the assumption with section thickness artifact
The beam is razor thin
Assumptions with US (6)
- Sound travels in a straight line and at a constant speed in soft tissue
- Sound travels to and from a transducer
- Echoes only originate from the central sound beam
- Intensity of the echo corresponds to the strength of a reflector
- The imaging plane (elevation plane) is thin
- Distance to the reflector is proportional to the time it takes an echo to return
What artifacts are caused by attenuation (3)
Posterior enhancement
Posterior shadowing
Edge shadow
6 assumptions of US
- Sound always travels in a straight line (path)
- Sound always travels directly to and from a reflector (path)
- Sound travels exactly 1540 m/s in soft tissue (speed)
- Reflectors are always positioned in the central axis of the beam (reflector position)
- Strength/intensity of reflection is related only to the characteristics of the object causing the reflection (other reflectors)
- Imaging plane (elevation) infinitely thin
Resolution artifacts are related to
The ability of the US system
Resolution artifacts are related to
Ability to distinguish objects in the 3 imaging dimensions
Resolution artifact assumptions (2)
- Reflectors are always positioned in the central axis of the main beam
- The imaging plane is infinitely thin
Location/propagation speed artifacts create
creating an image of a differently shaped sized object
attenuation artifacts are the group of artifacts that create (2)
creating an object in an incorrect position
creating an object of incorrect brightess
Attenuation artifact assumption
The assumption that the strength / intensity of the reflection is related only to the characteristics of the object causing the reflection - what artifact?
spectral doppler artifact relates to
velocity measurement error
colour doppler error
presence/absence of a signal error
elevation resolution is determined by the thickness of the crystal and increases with distance away from the focal zone
true
location/prop speed artifacts include (6)
- multipath
- mirror image
- refraction
- reverberation
- side lobes/grating lobes
- speed error/range ambiguity
MMRRSS
attenuation artifacts (3)
- acoustic shadowing
- enhancement
- speckle
double aorta is an example of what artifact
refraction artifact example
Gas in colon is an example of what artifact
reverberation
comet tail and ring down artifacts are examples of what artifact
reverberation
ring down artifact occurs when
an ultrasound beam passes through a group of small air bubbles - reverb occurs between the reflective interfaces between the bubbles
ring down distinguished by the appearance of
a posterior tail that does not taper in width or decrease in brightness
comet tail is when there is reverberation of
high amplitude echoes within a small object (ilke a surgical clip or small metallic foreign body)
multipath assumption
sound pulse always travels directly to the structure and back to the transducer
where is the echo placed in multipath
deeper than the actual structure (travel time is longer than expected if the pulse strikes a second structure on the way out or way back)
side lobes/grating lobes assumption
all reflectors lie in the central axis of the main beam
what happens with side/grating lobes
low intensity off-axis sound beams produce a weak reflected signal. objects that reflect echoes arising from these lobes are misplaced as though they are lying in the main axis
shadowing occurs when
sound waves strike a medium with extremely high attenuation
enhancement occurs when
sound waves strike a medium with extremely low attenuation
speckle is the result of
constructive/destructive interference and resultant scattering that creates bright and dark spots not related to a true anatomic structure
anterior reverberation AKA
main bang
lateral position errors are caused by
refraction
range ambiguity artifacts occurs when
2nd pulse sent out before all echoes returned from the 1st pulse
range ambiguity places structures closer or further?
closer
if sound travels in a medium faster than 1.54, the echo will be placed ___
too close
if sound travels in a medium slower than 1.54, the echo will be placed
too deep
how to reduce spectral mirror image (AKA cross talk)
decrease gains/power output decrease angle (near 90degrees will show flow on either side of BL)
acoustic shadowing is what type of artifact
attenuation
shadowing is caused by
- increased attenuation
2. increased reflection (IRC)
aliasing occurs when
the doppler shift exceeds the nyquist limit
how do you fix aliasing (5)
- shift baseline down
- lower the operating frequency
- increase PRF (possiblity of range ambiguity)
- increase doppler angle (possibility of error)
- use continuous wave
flash artifact is caused by
___ caused by motion - anything thats moving may cause a doppler shift
Grating lobes duplicates structures ____ to the real structure
Lateral
propagation speed error displaces objects ______
Axially
What 2 artifacts displaces structres laterally
Grating lobes
Refraction (2)
What creates slice thickness artifact?
What can the artifact look like?
The beam width - echoes received from the centre of the beam are averaged in with echoes received from off centre, creates a “fill-in” effect.
false echoes (debris) in an echo free area
What is speckle?
What does the artifact look like?
How is it reduced? (2)
Produced by constructive/destructive interference patterns caused by scatterers in tissue
Looks like granular appearance
Reduced by compounding, persistence
What causes reverberation?
What does it look like?
Multiple reflections between strong reflectors
Display of additional reflectors (not real) beneath the real reflector with equal separation intervals
What is comet tail?
What does it look like?
A form of reverb; series of closely spaced discrete echoes
A comet, brightest closest to transducer, fading away from it
How does ring down differ from comet tail artifact?
Ring down caused by the resonating or vibrating gas bubble after it is bombarded by US pulse.
Comet tail is produced as sound ping pongs between two reflectors
What creates mirror image artifact?
What does mirror image look like?
A strong reflector creates mirror image
There will be a duplicate structure on the other side of the reflector
What creates refraction artifact?
What does it look like?
Different propagation speeds between different tissues
Structures are displaced laterally from the correct locations and can be doubled
What is side lobe?
What is grating lobe?
What do side lobe/grating lobe artifacts look like?
How is this artifact reduced?
Side lobe- beams that propagate from a single element outside the primary beam
Grating lobe - similar to side lobes but for multi-element transducers
Look like - a duplicate of the true reflector appears on the image as its correct depth, but is positioned laterally from the true anatomy
Reduce - apodization, subdicing (gating lobes), THI
What creates propagation speed error?
What does it look like?
When the speed through the tissue isnt 1.54 mm/us
The structure can have the incorrect shape and/or be positioned too close or too far
What causes range ambiguity
What does it look like
What corrects this artifact
When echoes are received from the previous pulse
Structures shown too close to the surface
Decrease PRF
What reduces enhancement and shadowing artifacts
Spatial compounding
What is nyquist limit (not formula)
Upper limit of the detectable doppler shift
