Artifacts Flashcards
What are the broad categories of artifact? Briefly explain
- Acoustic properties - these occur as a result of u/s beam interaction with tissue. They are unavoidable
- Instrument related - such as miscalibration, dead crystals, faulty monitors or defective recorders
- Technique related - Improper operation, inexperienced sonographers, poor controls operation e.g. TGC/gain/other
Why do artifacts occur?
Because ultrasound machines operate on the basis of several assumptions in relation to beam/tissue interaction. If these assumptions are incorrect artifacts will occur
What are the top 5 assumptions of ultrasound machines?
- There is a constant rate of attenuation through soft tissue (1db/cm/MHz)
- All echoes arise from the centre of a razor thin ultrasound beam
- The propagation velocity of ultrasound through soft tissue is 1540 m/s
- The roundtrip time of a given echo is directly related to the depth of the reflector from the transducer
- An ultrasound beam travels in a straight line and reflects just once
What are the categories of artifact in ultrasound (in violation of the 5 assumptions of the machine)
- Attenuation artifacts
- Beam dimension artifacts
- Depth of origin artifacts
- Beam path artifacts
What is attenuation artifact? What are the two primary types, and subtypes?
When the ultrasound beam encounters a structure that has a different rate of attenuation than what the machine assumes for soft tissue (1db/cm/MHz), attenuation artifacts occur. This affects the image by displaying echoes too bright or too dark, or by omitting genuine echoes all together.
- Acoustic Enhancement
- Acoustic Shadowing
- Reflective shadowing
- Attenuation shadowing
- Edge shadows
What category artifact does acoustic shadowing belong?
An attenuation artifact
What is reflective shadowing
If a structure along the scan line is highly reflective, insufficient sound is transmitted beyond the structure. Thus, there is little or no reflection from behind the reflective shadow. Examples include air/gas interfaces or bone interface.
What is attenuation shadowing
If a structure along the scan line is highly attenuating, insufficient sound is transmitted beyond this structure. Thus, there is little or no reflection from structured behind the attenuating structure - causing a shadow.
What are edge shadows?
Results from a combination of reflection and refraction at the edges of a rounded structure when the propagation velocity is different from that in surrounding tissue. After passing through the structure, beams from multiple scan lines diverge/converge depending on the velocity.
How is shadowing artifact avoided?
- Scan through different plane - avoid shadowing structure
- Alter relative anatomy (i.e. breathing, patient position etc)
- Replace air with fluid
- Try increased probe pressure (to remove air gaps)
What are the positives/negatives of shadow artifact
Positives - can help diagnose calcification
Negatives - can remove real echoes from image. May obstruct information. May mistake as gaseous medium for real masses, e.g. tumour
What is acoustic enhancement artifact?
Refers to increased brightness or “enhancement” beyond a structure in an image. This artifact occurs due to increase transmission of ultrasound through a less attenuative medium (like fluid) resulting in increased signal density in deeper tissues.
What are the positives and negatives of acoustic enhancement artifact? How can we avoid?
Positives.
- Mostly positive, as we can use fluid filled structures as the basis of most acoustic windows
- Aids in clinical diagnosis and differentiating solid/fluid leisons
Negatives
- May saturate display
- Distal info may be obliterated
To avoid this artifact?
- Adjust TGC to LOWER gain below the fluid filled structure (to even out echoes)
- Scan from different plan
- Otherwise, we mostly utilise!
Beam dimension artefacts are violating which U/S machine assumption?
All echoes detected originated from the centre of a razor thin ultrasound beam
What are the common beam dimension artifacts?
- Beamwidth artifacts
- Slice thickness artifacts
- Grating lobe (side lobe) artifacts
There are three types of beamwidth artifacts. Explain.
- Two seperate reflectors lying side by side within the U/B beam appear as one
- A narrow reflector within the ultrasound beam appears wider than it really is
- A strong reflector to the side of an echo-free cavity appears within this cavity
Can beamwidth artifacts be of use?
No. They are never of use and these artifactual echoes may be misinterpreted as a “mass” when there is not.
How can beamwidth artifacts be reduced?
Reducing the beam width of the ultrasound beam. This can be done by narrowing the focus at the focal point.
Explain how slice thickness artifacts occur
The ultrasound beam is 3D however presents as a 2D (width and depth) image. Echoes positioned within the slice thickness of the ultrasound beam will generate echoes, and are assumed by the machine to have risen from the 2D imaging plane, and appear amongst the image. Slice thickness echoes have reduced amplitude as the edges of the ultrasound beam are of lower intensity than the central axis.
Provide a clinical example of what a slice thickness artifact may do
Hollow structure such as a cyst may appear filled in
or
Cysts that have a smaller dimension than the slice thickness may not be visible at all
What is the difference between slice thickness and beam width artifact
Slice thickness artifacts are different because the structure causing the artifact is NOT seen in the image display, because it never interrogated the main axis of the beam. This is why they are more difficult to recognise.
In beam width artifacts, the “offening” structure is always seen to the side of the artifact.
Can slice thickness artifacts be of use
No. They cause misinterpretation.
How do we avoid slice thickness artifact. Explain the use of gain.
It is difficult to avoid. Try a different imaging place.
Reducing gain WILL NOT help. Artifacts and real echoes will both disappear if the gain is reduced.
Describe side lobe / grating lobe artifacts appear
Side lobes are secondary beams of energy that radiate/diverge away from the main beam - these have the capacity of transmitting u/s energy just like the main beam.
They can appear
- As echoes in the wrong place
- As if they have generated from the
centre of the beam
Due to the lower intensity echo of side lobes, they are not very obvious unless they occur in echo free areas (like fluid) or are produced by a very bright reflector.
What are grating lobe artifacts
Similar to side lobe artifacts, but are caused by diffraction from electronic array transducers. They have the same undesirable effect (misplaced echoes), and tend to be more prominent than side lobe artifacts.
How do we avoid side lobe / grating lobe artifacts?
- Adopisation in manufacturing
decreases presence of grating lobes
in array transducers - Harmonic imaging
What is the main issues with how depth of origin artifacts present? What are the two types?
Displacing echoes - showing echoes in the wrong position
Propagation velocity artifacts
Range ambiguity artifacts
Explain how propagation velocity artifacts occur and how they will present on the image
The U/M machine assumes the velocity of all soft tissue is 1540m/s, and the distance of placement of an echo is determined by how long it takes for the echo to return to the transducer (using range equation).
Propagation velocity errors occur when the assumed value for velocity in the range equation (1540) is incorrect.
When the velocity is slower than assume - echoes are placed deeper than they should be.
When the velocity is faster than assumed, the echoes are place shallower/closer than they should be.
How do we avoid propagation velocity artifacts?
We can’t. The machine will always assume 1540m/s.
Can propagation velocity artifacts be of use?
Mostly no. They can cause significant measurement errors (distance of reflector may be incorrect, or the measurement of reflector may be incorrect)
One useful thing may be differentiating between silocine and saline implants
Explain how range ambiguity artifacts occur
Considering the pulsed nature of ultrasound, it is assumed that for each pulse, all echoes are received before the next pulse is emitted.
In order for a pulse to be ambiguously detected, the PRP must be longer than the time taken for the echo to return to the transducer using the range equation.
If an echo from pulse 1 is received after pulse 2, the ultrasound machine will assume it was generated from pulse 2 and the structure will place it twice
1. too close to the transducer
2. in its real position
What is the violated assumption for range ambiguity artifact
The roundtrip time of a given echo is directly related to the depth of the reflector from the transducer
How do we avoid range ambiguity artifacts?
The PRF must decrease as the distance of the structure from the transducer increases. The PRF is automatically controlled and will vary depending on FOV depth. Usually the PRF is automatically reduced with increased depth.
What are the common types of beam path artifact?
- Reverberation
- Refraction
- Mirror
- Multipath
What is reverberation artifact
When there are two strong reflectors in close proximity, reverberation artifacts can be caused by multiple reflections between the two. If there is a large acoustic mismatch, and the interface is oriented perpendicular to the direction of propagation, reverberations can occur. The ultrasound beam “ping pongs” back and fourth.
What are three examples of when reverberation artifacts occur?
Can occur between
- The transducer and a strong reflector (e.g. tranducer - skin interface)
- Two strongly reflective anatomical surfaces (e.g. soft tissue - gas interface)
- Two strongly reflective artificial surfaces
What is the violated assumption for reberation artifact
The ultrasound beam travels in a straight line and reflects just once.
What are two other types of reverberation artifact
Ring down
Comet tail
What is ring down artifact
This type of reverberation artifact is causes be a resonance phenomenon associated with the presence of a collection of gas bubbles. This results in a continuous streak of ultrasound.
What is a comet tail artifact
This is a type of reverberation artifact when there are two highly reflective, closely spaced interfaces such as a surgical clip/ bullet/ prosthetic valve. Multiple reverberations merge together to form a streak of ultrasound that rapidly diminishes in intensity over depth.
How can reverberation artifacts be avoided?
- Reposition transducer so it is not perpendicular to the involved “offending” interfaces
- Try a different transducer frequency
- Scan from different angles
- Adjust TGC - decrease initial gain or use delay to decrease effect at orgin
NOTE - cannot simply turn gain right down, as might miss real echoes
Can reverberation artifacts be of use?
Sometimes - can help identify tips of needles in biopsies
No - generally, they are problematic because the mask anatomical structures of lesions
What are refraction artifacts
We know refraction occurs when there is a change in direction of the U/S when it strikes an interface at an angle. Refraction will occur when:
1. There is an oblique angle of incidence between beam and interface
2. There are different propagation velocities on either side of the interface
What is the violated assumption of refraction artifact
The ultrasound beam travels in a straight line and reflects just once
Explain how refraction artifacts appear / interfere with the image
- Echoes in the wrong place (may cause reflector to be misplaced)
- Added echoes in the wrong place (may produce duplicated structures)
- Improper brightness of missing (may cause shadowing at edges of structures that are large/curved)
- Can cause duplication of a single structure (e.g. two gestational sacs / fetuses - mistake for twins)
How do we avoid refraction artifacts?
To avoid duplication of a single structure, try changing the transducer position (e.g. so that the beams travel through one muscle only)
Avoid perpendicular angle of incidence
Try another acoustic window
Can refraction artifacts be of use?
No
Can be very misleading (e.g. twin pregnancy misdiagnosed)
What is a mirror artifact
Similar to reverberation. When there is a high acoustic impedance mismatch between an interface and a specular reflector within the scan plane, the specular reflector acts like a mirror and near total reflection occurs.
What is the violated assumption of mirror artefact?
The ultrasound beam travels in a straight line and travels just once
What appearances can be caused by mirror artifact
Added echoes
Two images of the same structure are displayed on the image (duplicated)
- The structure that acts as the mirror always lies between the two duplicated structures
- The mirror image always appears deeper than the image from the real structure (placed on the “other side” of the mirror)
What are some clinical examples of mirror artifact
Pleura - lung interface
Diaphragm - lung interface
Pericardial - myocardial interface
Other strong reflecting interfaces
How are mirror artifacts avoided?
- Change the angle or interrogation so the specular reflector is not perpendicular to the transducer
- Angle away from the mirror
Can mirror artfiacts be useful. What are some clues to decipher mirror artifact
Never useful. Very misleading. Hard to differentiate.
Clues
- If the pathology is real, the specular interface will not be bright (no longer a high acoustic impedance mismatch)
- If echoes are artifacts, they will have the EXACT same appearance as the organ they have arisen frome.
What are multipath artifacts
An artifact that occurs when the ultrasound beam strikes multiple interfaces before returning to the transducer. They are produced when multiple surfaces or curved interfaces are encountered by the u/s beam at an angle, then the beam is reflected from a second or even third interface before returning. It therefore takes a longer amount of time to return, and the echo is placed at an incorrect depth and is weaker (than if it travelled directly from the interfaces)
What is the violated assumption of multipath artifact
The ultrasound beam travels in a straight line and reflects just once
What appearance errors occur with mirror artifacts. How can they be avoided?
Echoes in the wrong place with improper brightness
Change the orientation of the ultrasound beam
What are some miscellaneous artifacts
- “Main bang” efffect (dead zone)
- Acoustic speckle
- YOU
What is main bang artifact. How can it be reduced?
This effect is a problem with the mechanical probes and annular arrays in which the near field is obscured due to an initial large pulse from the transducer
To reduce
- try higher frequency
- try using a stand-off
- try decreasing initial gain on tcg
What is acoustic speckle artifact?
be aware, that the detailed echoe pattern is often not directly related to scattering properties of tissue. Acoustic speckle may occur within the imaging FOC, occurring in a region of tissue where scatterers are spaced less than axial resolving distance. This results in an interface pattern due to constructive and destructive interferance of echoes returning simulataneously. It may result in the loss of small or low contrast objects.
How can you, the operator be an artifact cause itself
- Noise created by excess gain settings
- If gain settings are too low, weak echoes may be lost
- Reducing the dynamic range to improve contrast between low intensity (blood) and high intensity (myocardium) structures may eliminate important low intensity structures
What are 2 general rules for understanding artifact (conclusions)
- Artifacts are not seen in 2 planes
- Real structures are seen in more than 1 plane
SO, check to see if you can reproduce it in another plane!
