Artifacts Flashcards
definition of artifacts
artifacts are structures and features (echoes) that appear on the image but do not have a one-to-one correspondence to an anatomical structure.
they can be useful when assisting in diagnosing, may need to be ignored or eliminated.
What do we assume of the ultrasound machine?
1) sound travels in a straight line
2) sound travels directly to and from the reflector
3) the imaging plane is thin
4) sound always travels at 1.54 mm/µsec
5) reflections are produced by structures along the main axis of the sound beam
6) the intensity of an echo corresponds to a reflector’s scattering strength
definition of axial resolution
ability to distinguish two structures that are in close proximity to each other - front to back - anterior to posterior
(on top of each others on ultrasound image)
the backing layer does what?
stops the ringing in the probe - which allows it to have shorter pulse lengths - which increases the axial resolution
definition of lateral resolution
minimum distance between two side-by-side structures that can be separated and still produce two distinct echoes
What are some causes of artifacts?
- faulty equipment
- improper equipment operation
- improper settings of controls (ex. gain settings)
what is acoustic speckle? what does it look like? what causes it?
a one-to-one correspondence between image brightness and scanned objects does not exist.
a granular appearance of images and spectral display.
caused by: interference of echoes from the distribution of scatters in tissue
where does acoustic speckle occur?
near the face of the transducer
What is section thickness? and what else is it called?
- is the thickness of scanned tissue volume perpendicular to the scan plane
- slice thickness, out-of-plane focusing, elevational resolution or width focusing
What is temporal resolution?
the ability to precisely position a moving structure
what does low frame rates result in?
less accurate positioning of reflectors
reverberation artifact
- harmonics (double the frequency)
- appear as multiple, equally spaces reflections on the image, decreasing in intensity
comet-tail artifact
- form of reverberation artifact
- caused by metal object(s)
- surgical clips, prosthetic valves, bullet etc
- appears as multiple small bands
ring-down artifact
- type of reverberation
- produced by small gas bubbles
- appears as a long, single, strong echo behind the reflector
mirror-image artifact
-occurs when a structure is located in front of a highly reflective surface causing the anatomy to be reproducr=ed or duplicated on the other side of the interface
where does the duplicated or mirrored image appear at?
ALWAYS deeper than the true anatomical structure
examples of strong reflectors
diaphragm, pleura and bowel
multipath artifact
- path to and from a reflector that are not the same
- results in: incorrect axial location of an interface due to an increase in time and improper brightness
refraction artifact
- produces shadowing at the edge of structures that are curved
- heart, fetal head, or a cross-section of a vessel
- rectus abdominal muscles (duplicated structures, ghost image artifact)
- ex: two gestational sacs, two CAs, two AO valves
side lobes
- single crystal transducer
- energy off the main axis
- the reflection is at the correct depth - but LATERAL from true anatomy
grating lobes
- array transducers
- reflection at the correct depth - but LATERAL from true anatomy
- **corrected by sub-dicing and apodization - during manufacturing process
what is range ambiguity
it is the misplacement of an interface when the assumption that each echo is derived from the most recent pulse is violated
-the time between the transmitted pulse and the detected echo are not measured correctly
propagation speed errors
-causes the improper location of interfaces
If the mass has a speed of sound GREATER than 1540 m/sec - what will happen to the reflector?
the reflector will be placed closer to the transducer - it information got back faster
If the mass has a speed of sound LESS than 1540 m/sec - what will happen to the reflector?
the reflector will be placed deeper and further from the transducer - it took longer for the information to get back
- a posterior displacement*
ex: a fatty tumor (propagation speed of fat is slower than ST)
What is acoustic shadowing?
the absence or reduced intensity of echo information, distal to a reflector
what causes acoustic shadowing?
what the sound beam intersects a highly attenuating (highly reflective) structure
ex: calcium stone / bone
- most or the entire sound beam is redirected to the transducer = black space behind the strong echo structure
What is a useful artifact?
***acoustic shadowing
***enhancement
What is enhancement?
an artifact that occurs when the sound beam intersects a very low attenuation (weak reflector) structure i.e., fluid
what is focal banding?
an artifact that occurs when focusing characteristics of the transducer, creating a region of increased brightness.
- caused by greater intensity in the focal zone
- improper TGC settings
What does reverberation cause? how can you correct it?
- *near field artifact
- seen in the anterior walls of organs that are close to the skin surface
- correct by using harmonics
What causes an image to appear too bright or too dark?
improper gain or TGCs - operator dependent
what happens with excessive reject?
all the low-level echoes are eliminated - operator dependent
examples of electrical interference
patient noise - snoring, talking, coughing etc
patient rooms - machines and monitors
what is aliasing and how does it occur?
- it is a color artifact - in PW and color Doppler
- occurs when the Doppler shift frequency exceeds half the PRF - pulse repetition frequency
What is the Nyquist limit?
it is the limit of aliasing where the Doppler shift frequency exceeds 1/2PRF
What does aliasing look like? how do you correct it?
- when using color Doppler, aliasing is a wrap-around of the display = apparent reversal of flow direction
- increase the PRF
with mirrow imaging or ghosting: what is the relationship between the true anatomical structure vs artifact?
the true, anatomincal vessel, is closer to the trasnducer. The duplicated, artificat, vessel is below it.
What artifact happens when the Doppler gain is set too high or the Doppler angle is close to 90 degrees? What does it look like?
- mirror imaging or crosstalk
- the spectral Doppler shows waveform above and below the baseline
What is color ghosting or flash?
What causes it?
How can it be corrected?
- is is when color is being improperly assigned. The sudden burst of color that encompasses the frame.
- any movenment causes it: pt breathing, talking etc
- can correct it by increasing the color filter, decreasing the persistence, and reducing the width of the color field of view
what is color bleeding or blossoming?
how do you eliminate it?
- it is the extension of color beyond the region of flow to the adjacent tissue
- decrease the overall color gain or the transmit power (ALARA, which we will not do)
What is color noise/clutter?
how can it be eliminated?
- a random variation in signal detection which causes area with no flow to be color encoded
- by using filter, threshold, wall filter to eliminate low level echoes
What ultrasound Doppler is angle dependent?
- color Doppler
- pulse wave Doppler
a feature that appears on the image but does not correspond to an anatomical structure is called ____________?
an artifact
which artifact interferes with the ability to detect a low-contrast object?
acoutstic speckle
what artifact occurs from the thickness of the scanned tissue volume, perpendicular to the scan plane?
elevational resolution
definition: multiple, equally-spaced reflections on the image, decreasing in intensitiy is called ____________
reverberations
what is it called when the acoustic energy is emitted in a direction different than from the main axis of the soud beam?
side lobes
the extension of color beyond the regoin of flow, to the adjacent tissue is called:
blossoming
when dealing with acoustic speckle - what do the dots/pixel represent?
each dpt or pixel represents a collection of scatterers
name some types of artifacts
- not real
- missing
- improperly located
- improper brightness
- improper shape and size
- define interference
- what factors are at play?
-a group of very closely spaced reflectors are positioned in the path of the beam - the average signal obtained from these reflectors depends on many factors
- **1.acoustic impedance differences between scatteres and surrounding medium
2. frequency of insonation
3. scatter size
4. number of scatters per unit volume
What is referred to as the “dead zone”
the area close to the trasnducer face - where is there poorest resolution
a shallower depth has what relationship to PRF
an increased PRF (not far into the body)
