Artifacts Flashcards
What is an artifact?
Anything that does not correctly display the structures or functions that are imaged
What 4 things are artifacts related to? (With examples)
- Malfunctioning equipment
Ex. Broken crystal causing a shadow - Defective recording device
Ex. Discrepancy between the machine and PAX system - Improper operation
Ex. Bad techs - Acoustic properties of tissues and sound propagation
Ex. Many diff artifacts can occur
What 4 assumptions about the way sound propagates in tissue results in artifacts?
- All tissues have the same acoustic velocity of 1540 m/s
- The sound beam always travels in a straight line
- Echo strength only indicates the organs echogenicity
- The distance to the reflector equals the round trip time
What determines the velocity in tissue?
Compressibility and density
What are the 4 categories of artifacts?
- Not real
- Missing
- Improper location
- Improper brightness/size/shape
What two groups can artifacts be categorized into?
- Attenuation group
2. Propagation group
What is the axial resolution artifact?
Structures less than 1/2 the SPL apart in the axis of the beam are demonstrated as a single echo
What is the formula for SPL?
SPL = λ x RD
How can we correct the axial resolution artifact? (2)
SPL = λ x RD
- Manufacture the RD smaller in the first place
- Decrease the SPL by increasing the frequency which decreases the wavelength (λ)
What is the lateral resolution artifact?
A beam width artifact where reflectors appear smeared across the screen in areas where the beam is wide
What accentuates the lateral resolution artifact?
Increased gain or TGC
What is lateral resolution artifact also referred to as? (2)
- Point spreading
2. Image broadening
How do you correct for the lateral resolution artifact? (5)
- Lower gain
- Lower TGC
- Focus at correct level
- Increase frequency to extend the NZL
- Scan in the NZ (better beam width)
What is slice thickness artifact?
Cystic structures below the focus level can appear filled in due to echoes from in front and behind the structure (in the z axis) being spread where the beam is wide
What is the slice thickness artifact also referred to as?
The volume averaging artifact
What artifact is similar to the lateral resolution artifact?
The slice thickness artifact is also a beam width artifact but in the z-axis
Why does the slice thickness artifact occur?
Because of the assumption that all echoes return from the centre of the beam
What can fix the slice thickness artifact?
Repositioning the probe so that the narrowest portion of the z-axis is at the desired location
What is acoustic speckle?
The appearance of bright and dark dots (heterogenous brightness)
Why does acoustic speckle occur?
Due to the scatter reflection in the parenchyma due to constructive and deconstructive interference occurring from several off-axis reflections
What corrects acoustic speckle?
Persistence averages out the frames to help homogenize the tissue brightness
How does the size of the slice thickness effect the artifact fill in?
Greater slice thickness = greater fill in
Where is acoustic speckle predominant?
In the near field where constructive and deconstructive interference is the greatest
What is reverberation?
The reflection of sound bouncing back and forth between the transducer and a strong interface
Where is reverberation often seen? (2)
- Probe surface
2. Fluid filled structures
How does reverberation appear?
Multiple equally spaced linear lines that get weaker with depth and look like rungs of a ladder
What are the best ways to eliminate reverberation?
- Change window
2. Use harmonics
What is real and what is not when reverberation is seen?
The first echo closest towards the surface is real and the rest aren’t.
What is comet tail?
A short-range reverberation artifact between two closely spaced specular reflectors
What things cause comet tail? (6)
- Staples
- Metal pellets
- Biopsy needle
- Metal IUD
- Calcium
- Cholesterol crystals
How can comet tail be helpful?
In diagnosing calcium in the bladder wall
How can comet-tail be corrected?
Change window
What is ring down?
A reverberation artifact that looks similar to comet tail but instead appears as a continuous stream of long echoes
What creates the ring down artifact?
It is a resonance phenomena associated with gas bubbles, where they vibrate and become a new source of sound
Is ring down useful?
Yes, can be useful for diagnosis and also to tell if gas/bowel is present
What is different between the appearance of comet tail and ring down?
Ring down has long bright echoes and comet tail is small
What is mirror image?
A duplication of an object seen on both sides of a strong reflector due to an odd angle
Why does mirror image occur?
The machine assumes the beam is travelling in a straight line but when the beam is reflected by a specular reflector towards the object and then reflected back along the same path the additional travel time places the object deeper than it really is
What is a structure that often causes mirror image?
The diaphragm
How is mirror image corrected?
It can’t always be so awareness is key
What is a multipath artifact?
The beam may take several off-axis vectors before returning to the probe resulting in the incorrect location of the echoes due to the difference in time
What assumption results in multi-path artifacts?
That the beam travels in a straight line
What is the solution to correct for multipath artifacts?
Get perpendicular to the structure of interest to get the sound to come back in a straight line
What are refractive artifacts?
The beam becomes redirected when an interface it struck at an angle and both media have different velocities
What are the types of refractive artifacts? (4)
- Edge shadowing
- Refractive duplication
- Refractive malposition
- Refractive enhancement
What is edge shadowing?
A refractive artifact where shadows are seen at the edges of strong curved reflectors due to the beam changing direction with non-perpendicular incidence
How is edge shadowing corrected?
Changing the window to come at the interface from a different angle
What controls can help the appearance of edge shadowing to be smoother and less intense? (2)
- Persistence
2. Compound imaging/sono CT
In order to diagnose stones or calcifications what control would you use?
Take off Sono CT/Compound imaging in order to make the edge shadowing more intense
What is refractive duplication?
A lateral duplication of the echo interface on either side of it’s true location
Why does refractive duplication occur?
Due to the refraction of the beam by a strong interface, as the beam is swept the sound bends as it travels through structures and returns to the probe.
What is refractive malposition?
When the echo interface is falsely placed lateral from it’s true position
Why does refractive malposition occur?
Due to the bending of the beam generating an echo which is falsely positioned along the beam axis
What is the solution to refractive malposition?
Get perpendicular to the structure
What is responsible for grating lobes?
Length and width vibration of the array probe “cross talking”
What is a propagation speed error?
Based on the machines assumption that sound travels at 1540 m/s in all tissue reflectors can be placed in the wrong location proximal or distal to it’s true location
What is the speed of sound in fat?
1460 m/s
How do reflectors behind fat appear?
Deeper than they actually are because sound slows down as it travels through fat (takes longer to get back to probe).
How do you correct for speed propagation error?
You can’t
What is a “broken diaphragm” and indication of?
Fatty liver (propagation speed error causes this artifact).
What is range ambiguity?
It is an artifact that occurs when a pulse is released before all echoes have made it back to the probe
What setting is too high when the range ambiguity artifact occurs?
PRF is too high thereby exceeding it’s limit set by the depth
How does range ambiguity appear?
Echoes appear closer than they actually are
Where is range ambiguity seen?
Not in 2D but in doppler
How do you correct range ambiguity? (2)
- Reduce frame rate or PRF
2. Switch to CW (but no image will be available)
What artifact is responsible for a sample volume outside the vessel still picking up a waveform due to a very high PRF?
Range ambiguity
What are the 4 attenuation artifacts?
- Shadowing
- Enhancement
- Focal enhancement
- Edge shadowing
What is shadowing?
A lack of echoes underneath a highly attenuating structure due to absorption, reflection or scattering of sound
How would you attempt to see structures blocked by shadowing?
Scan from different window
Is shadowing helpful or hurtful?
Both.
Helpful = Helps with diagnosis (stones)
Hurtful = blacks out anatomy
How is shadowing enhanced to help with diagnosis? (5)
- Increase frequency
- Harmonics
- Focus at shadow
- Reduce 2D gain
- Turn off compound/Sono CT
How does increasing the frequency help to enhance shadowing for diagnosis?
Increasing the frequency will increase absorption therefore increasing attenuation (attenuation = shadowing)
What is enhancement?
Posterior brightness behind structures that attenuate less than surrounding structures
What types of structures can have enhancement?
Both cystic and solid
What is a very rare reason for enhancement?
A very homogenous medium causes enhancement
Is enhancement helpful?
Yes, with diagnosis
How is enhancement hurtful?
Brighter structures can appear thicker on ultrasound resulting in incorrect caliper placement
What is focal enhancement?
A false brightening to a particular area of the image at the focal zone
What is focal enhancement also known as?
Focal banding
Why does focal enhancement occur?
The intensity of the beam is highest at the focal zone (narrowest part of the beam) so all echoes that come back from this region are brighter.
How can you compensate for the focal enhancement artifact?
Bump down TGC’s
What is aliasing?
When the Niquest limit is exceeded (1/2 PRF) the velocities travel so fast that they wrap around the baseline
How is aliasing helpful?
When assessing colour flow is can be useful in identifying areas of higher velocities or turbulance
How do you correct for aliasing? (5)
- Increase PRF (scale/velocity range)
- Move baseline
- Switch to CW
- Increase the angle
- Lower the operating frequency
Why is an angle of 60 used in doppler?
Acceptable margin of error and reproducible
Why does doppler require a lower frequency?
Because of the increased attenuation due to rayleigh scatter (RBC’s = weak echoes)
What is the doppler slice thickness artifact?
Velocities from multiple flows (Diff shades of white and grey) are seen in one envelope due to the beam width phenomenon
What is the doppler reverberation artifact?
Similar to 2D, it is seen as a buzzing or equidistant colour lines
What is the twinkle artifact?
A doppler reverberation artifact helpful in diagnosing calcium
How does the twinkle artifact appear?
Reverberation between the front and back wall of the stone will produce a mosaic of colour deep into and behind the stone
What is another structure where the doppler reverb artifact will be seen?
Prosthetic valve
How is the doppler mirror image seen?
Colour = false vessel deep to the real one
Spectral = Doppler flow on both sides of baseline
How do you correct for doppler mirror image on spectral and colour?
Specral:
- Reduce PW gain (shouldn’t spill [over)
- Angle of incidence too too perpendicular (don’t want to be 90 degrees)
Colour:
- Turn down gain
What is doppler flash or clutter?
Low level echoes in the colour flow seen in tissue due to tissue movement
What structures show doppler flash/clutter?
Heart wall, valves, vessel wall motion
How is doppler flash/clutter corrected?
Wall Filters within the machine eliminate this motion as the velocities are usually slower than blood (increase wall filter)
What is the risk of the machine’s doppler flash/clutter filters being set too high?
Eliminating real low flow information
What is doppler noise?
Occurs when doppler gains are too high causing blossoming of colour outside vessel or within cysts with no flow
How can doppler noise effect spectral?
Falsely increase peak systolic velocities
What is the halo effect?
Doppler noise on the spectral trace causing over estimation of a peak velocity
What are other reasons you may get artifacts while scanning related to the malfunction of equipment? (4)
- Electronic interference
- Faulty scan converter
- Faulty software
- Faulty equipment
What is electronic interference?
An echo pattern with high/low intensity that is random/geometric resulting from radio frequency signals generated by other equipment
What is the 60 Hz bandwith artifact?
A low level echo found below the spectral trace due to electronic interference
What are some faulty equipment that may change the appearance of the image? (4)
- Malfunctioning crystal
- Cracked damping material
- Defective recording device
- Improper transducer shielding
