Module 7- Imaging Features & Artifacts Flashcards
What are artifacts?
Structures and features (echoes) that appear on the image but do not have a one-to-one correspondence to an anatomical structure
What are the assumptions of ultrasound imaging systems?
(1) Sound travels in a straight line
(2) Reflections are produced by structures along the main axis of the sound beam
(3) Intensity of an echo corresponds to a reflector’s scattering strength
(4) Imaging plane is thin
(5) Sound travels directly to and from a reflector
(6) Sound always travels at 1.54 mm/μsec in soft tissue
What can cause non-structural echo signals to be seen on the display?
Interference from outside sources can add noise to the image
What can cause the image to be missing real structural echoes on the display?
- Strongly reflecting objects create shadowing which appears as black where normal anatomy should be seen
- An improper operating crystal can produce black lines on the image which obscure the normal anatomy
What can cause the displacement of echo signals on the display?
Scanning a medium with oblique incidence (can cause lateral displacement of structures)
What can cause a distortion of the echo signal on the display?
An increase or decrease of the amplitude of the signal can be caused by wave interference
TRUE/FALSE: Scanning a medium at oblique incidence can cause axial displacement.
FALSE (it can cause LATERAL displacement)
TRUE/FALSE: When wave interference causes an echo signal to be distorted, it is either increasing or decreasing the frequency of the signal.
FALSE (wave interference can cause an increase or decrease in the AMPLITUDE of the signal)
What are the mechanisms of artifact production?
- Ultrasound equipment assumptions
- Viewer assumptions and/or errors
- Equipment malfunction
- Operator error
- Improper use of equipment
How can axial resolution be improved?
Shorten SPL
Can be accomplished by:
- Use a thin element transducer (higher frequency)
- Increase the frequency (because it will have a shorter wavelength)
- Shorter wavelength
- Reduce PD (by reducing the # of cycles per pulse)
- Reduce the # of cycles per pulse (reduced by adding damping)
How can lateral resolution be improved?
- Changing the focus (this makes the beam width narrowest in the area of interest)
- Decreasing the depth
- Higher frequency
Is it better to have a high or low value with spatial resolution?
Low
TRUE/FALSE: Axial resolution is the ability to distinguish structures that are in close proximity to each other from front to back.
TRUE (axial resolution is anterior to posterior (on one top of the other on the screen)
What is artifact of axial resolution?
- Occurs when multiple structures are along the main axis of the beam and appear only as one reflector on the image
- Contributes to incorrect representation of size and shape of interfaces and to missing interfaces
What is artifact of axial resolution?
- Occurs when multiple structures are along the main axis of the beam and appear only as one reflector on the image
- Contributes to incorrect representation of size and shape of interfaces and to missing interfaces
What is artifact of lateral resolution?
- Occurs when side-by-side structures are not producing two distinct echoes, but one
- Contributes to incorrect representation of size and shape of interfaces and to missing interfaces
Why is acoustic speckle considered to be an artifact?
Because a one-to-one correspondence between image brightness and scanned objects does not exist
What is acoustic speckle?
- The granular appearance of images and spectral displays
- Caused by constructive and deconstructive interference of scattered sound waves
- Dominates the near face of the transducer
- Interferes with the ability of the system to detect low- contrast objects
What interferes with the ability of the system to detect low-level contrast objects?
Acoustic speckle
What is section thickness?
Thickness of the scanned tissue volume perpendicular to the scan plane
What are other names form section thickness?
- Slice thickness
- Out-of-plane focusing
- Elevational resolution
- Width focusing
- Z plane
How can section thickness cause artifact?
May cause misdiagnosis when echo fill-in occurs in a cystic structure (ex. debris appearance in the gallbladder that is not real)
What is temporal resolution?
The ability to precisely position a moving structure
What is an artifact of temporal resolution?
Low frame rates result in less accurate positioning of reflectors in motion
(Depth, # of Focus, # of Scan Lines) x Frame rate = <77,000
What are the names for axial resolution?
-Longitudinal
-Axial
-Range
-Radial
-Depth
(LARRD)
What are the names for lateral resolution?
-Lateral
-Angular
-Transverse
-Azimuthal
(LATA)
What transducer component reduces the “ringing” of the crystal, improves axial resolution and reduces spatial pulse length?
Backing material
What are the artifacts associated with propagation?
- Reverberation artifacts (comet tail/ring down)
- Mirror-image artifacts
- Multipath
- Refraction (shadowing, ghost image)
- Side lobes / grating lobes
- Range ambiguity
- Proagation speed errors (speed of sound)
What is reverberation artifact?
- Appears as more interfaces than actually exist
- Multiple, equally spaced reflections decreasing in intensity
What are some forms of reverberation artifacts?
- Comet-tail artifact
- Ring-down artifact
What is comet-tail artifact?
- Type of reverberation artifact
- Caused by metal objects within the body (surgical clips, prosthetic valves, bullet)
- Appears as multiple small bands
What is ring-down artifact?
- Type of reverberation artifact
- Caused by small gas bubbles, such as air, in the biliary tree
- Appears as a single, long, strong, echo behind the reflector
How can reverberation artifacts be overcome?
Harmonics
What is mirror-image artifact?
- Reflector and object (true and false image) are equidistant from the strong reflector
- Mirror image duplicate (false image) always appear DEEPER than the true anatomic structure
How can you try to overcome mirror-image artifacts?
Try to come from a different location/angle
What is multipath artifact?
- Relates to paths to and from a reflector that are not the same
- The beam strikes an interface at an angle and is reflected from a second or third interface before being reflected to the transducer
How does multipath artifact appear?
- Results in incorrect axial location of an interface due to the increased time the signal takes to return to the receiver
- Improper brightness
How can you try to overcome multipath artifact?
Come through a different structure
The diaphragm appearing as 3 lines instead of one is an example of what kind of artifact?
Multipath artifact
How do artifacts due to refraction appear?
- Echoes received after refraction appear on the screen in improper locations
- Produces shadowing at the edges of structures that are curved
What is ghost image artifact?
- Type of refraction
- Rectus abdominus muscle can cause the beam to be refracted
- Causes a duplication of structures that appear side-by-side (ex. two celiac axis)
The celiac axis appearing as two structures is an example of what kind of artifact?
Ghost image artifact (refraction)
How can you overcome refraction artifacts?
Scan from a different angle
What is side lobe/grating lobe artifact?
- Acoustic energy is emitted in a direction different than the main axis of the sound beam
- Energy off the main axis of a single crystal transducer (for side lobes) or arrays (for grating lobes) can introduce positioning artifacts
How do side lobe/grating lobe artifacts appear?
- Reflection appears at the correct depth, but is lateral from the true anatomy
- Reflected structure appears weaker than the real structure is
Why are array transducers not commonly seen?
- Due to corrective processes
- Sub-dicing and Apodization (which is performed during the transducer manufacturing process)
Why is range ambiguity an artifact?
-The misplacement of an interface when the assumption that each echo is derived from the most recent pulse is violated
What is range ambiguity artifact?
Depth placement error that occurs because the time between the transmitted pulse and the detected echo is not measured correctly
What is the cause of range ambiguity artifact?
PRF is set too high
How can you overcome range ambiguity artifact?
- Decrease the PRF
- Come from a different angle
What is another name for propagation speed errors?
Speed of sound artifact
What is speed of sound artifact?
- Causes improper location of interfaces
- Reflectors appear in the correct number but at improper depths
If the speed of sound is _____ than 1540 m/s within the mass, the reflector will be placed too far from the transducer.
Less
If the speed of sound is ______ than 1540 m/s within the mass, the reflector will be placed too close to the transducer.
greater
Greater
The diaphragm located posterior to a fatty tumor in the liver appears displaced too far from the transducer. This is an example of what type of artifact?
Speed of sound artifact
What are the artifacts associated with attenuation?
- Acoustic shadowing
- Acoustic enhancement
- Focal banding
What is acoustic shadowing?
- The absence or reduced intensity of echo information distal to the reflector
- Caused by the sound beam intersecting a highly attenuating (highly reflective) structure
TRUE/FALSE: Both shadowing and enhancement are useful artifacts.
TRUE
What is acoustic enhancement?
- The increase in echo brightness posterior to the structure
- Occurs when the sound beam intersects a very low-attenuating (weak) reflector (such as fluid)
What is focal banding?
- Focusing characteristics of the transducer creates a region of increased brightness
- Caused by the greater intensity in the focal zone or incorrect TGC settings
How can you overcome focal banding?
Smoothing out the TGC curve
What is near field artifact?
- Caused by reverberation
- Seen in anterior walls of organs very close to the skin surface
How can you overcome near field artifact?
Turning on Harmonics
What are some operator caused artifacts?
- Excessive gain or TGCs
- Excessive reject
What is the result of excessive gain or TGCs?
- Alteration of the normal echogenicity of structures present on the image
- Contrast resolution is diminished
What is the result of excessive reject?
- Boundaries and bright reflectors are seen but the low level echoes (parenchyma of organs) are eliminated
- Alteration of the normal echogenicity of structures present on the image
- Contrast resolution is diminished
What is something that can cause electrical interference?
Other equipment in the room such as patient monitors
How does electrical interference appear?
An arc of vibrating bands across the monitor when the image is live (whether the operator is scanning or not)
What is the result of electrical interference?
Degradation of contrast resolution
TRUE/FALSE: Aliasing is an artifact with both Pulse Wave Doppler and Color Doppler.
TRUE
What are some artifacts of Doppler?
- Aliasing
- Mirror imaging or ghosting (color)
- mirror imaging or crosstalk (spectral)
- Color registration (ghosting or flash, blossoming, color noise/clutter)
- Incident beam angle
What is aliasing?
- A wrap-around of the color display resulting gin an apparent reversal of the flow direction
- Occurs when the Doppler shift frequency exceeds half of the PRF (Nyquist limit)
How does Mirror imaging or ghosting with color Doppler appear?
Color Doppler image shows the true vessel closest to the transducer with the duplicate (artifact) vessel below it
How does Mirror imaging or crosstalk with spectral Doppler appear?
Spectral Doppler shows waveform above and below the baseline
What causes mirror imaging or crosstalk with spectral Doppler?
- The Doppler gain is set too high
- The Doppler angle is close to 90 degrees
What is ghosting or flash artifact with color Doppler?
- Color is improperly assigned to stationary structures
- May be caused by any movement of the transducer, structure, or patient (breathing or talking)
How does ghosting or flash artifact with color Doppler appear?
A sudden burst of color that encompasses the frame
How is ghosting or flash artifact with color Doppler suppressed?
- Increasing the color filter
- Decreasing the persistence
- Reducing the width of the color field of view
- Having the patient hold their breathe or stop talking
What is Color bleed also called?
Blossoming
What is blossoming?
The extension of color beyond the region of flow to the adjacent tissue
How can you overcome blossoming?
Decreases the transmit power and color gain
What is color noise also called?
Clutter
What is color noise?
A random variation in signal detection which causes areas with no color flow to be encoded
How can you overcome color noise?
Filter or threshold may eliminate the low level echoes
How does artifact caused by the incident beam angle appear?
Improper Doppler angle will result in little or no waveform on the spectral tracing, and no color or black in the vessel
What can interfere with the reconstruction process for some 3-D and 4-D applications?
Clutter or noise on the 2-D slices
A feature that appears on the image but does not correspond to an anatomical structure is called:
Artifact
The artifact that interferes with the ability to detect a low-contrast object is known as:
Acoustic speckle
The artifact that occurs from the thickness of the scanned tissue volume perpendicular to the scan plane is called:
Elevational resolution (section thickness)
Multiple, equally-space reflections on the image decreasing in intensity is called:
Reverberation
A type of reverberation artifact due to metal within the body is called:
Comet-tail
Acoustic energy emitted in a direction different than from the main axis of the sound beam is called:
Side lobes
If the speed of sound is greater than 1540 m/s, the echo will be placed:
Too close to the transducer
The absence or reduced intensity of echo information distal to the reflector is called:
Shadowing
An example of an artifact helpful in diagnosis is:
Shadowing (also enhancement)
The extension of color beyond the region of flow to the adjacent tissue is called:
Blossoming (color bleeding)
Low-level echo image artifact caused by other electrical equipment is called:
Banding
Harmonics can be used to eliminate:
Reverberation
When the Doppler gain is set too high which artifact can appear on the image?
Crosstalk
