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**IMAGE CHARACTERISTICS AND ARTIFACTS**
Terms in this set (24)
portions of the image that are brighter than surrounding tissues, or tissues that appear brighter than normal
portions of an image that are not as bright as surrounding tissues, or tissues that appear less brighter than normal.
without echoes; echo-free
Looks like a ladder or a venetian blind.
2. equally spaced
3. parallel to the sound beam
4. deeper and along a straight line
COMET TAIL OR RING DOWN
Appears as a solid line directed downward, merged reverberation.
1. single, solid hyperechoic line
2. long echo
3. parallel to sound beam
4. a reverb with the 'space' squeezed out
US beam becomes very weak because the structure it passses through has a HIGHER THAN USUAL ATTENUATION. A hypoechoic region appears distal to a hyperechoic structure.
EDGE SHADOW/SHADOWING BY REFRACTION
Refraction at the edge of a circular structure can create an artifact.
Occurs when the medium through which the sound travels has a lower attenuation rate than soft tissue.
The correct number of anatomic reflectos appear on the image.
Sound may bounce off a reflector, called a mirro, in its path and be redirected.
Artifact is located deeper than the true reflector.
PROPAGATION SPEED ERRORS/RANGE ERRO ARTIFACT
Speed errors appear as a step-off, split or cut.
Faster than soft tissue-when the propagation sheed is greater than 1,540 m/s the reflector will be placed too SHALLOW on the display.
Slower than soft tissue-when the speed is less than soft tissue's, the reflector will be placed too DEEP on the display.
Artifact appears side-by-side with the true anatomic structure.
Sound changes direction striking a boundary when the media have different propagation speeds.
DEGRADES LATERAL RESOLUTION
Extra acoustic energy may be transmitted in directions other than the beam's main axis. Side lobes occur with MECHANICAL TRANSDUCERS.
With lobe artifact, the assumption that reflections arise from the beam's main axis is violated.
Extra acoustic energy may be transmitted in direction other than the beam's main axis.
Occurs with ARRAY TRANSDUCERS.
Grating lobe artifact can be reduced or cured by dividing each element into even smaller, miniature pieces.
Grating lobes are further reduced by exciting the subdiced elements with different voltages. Subelements closer to the center of the sound beam are excited with higher voltages, while the outermost subelements, further away from the center of the beam, are excited with lower voltages.
SLICE THICKNESS/elevational resolution
Occurs when beam has a greater width than the reflector.
Linear array transducers have poor elevational resolution.
This artifact is cured with the 1 1/2 dimensional array transducer.
GRAINY APPEARANCE in the SHALLOW part of the image
Created by INTERFERENCE EFFECTS of scattered sound
caused by DOPPLER GAIN SET TOO HIGH found in spectral DOPPLER only
special form of mirror image where the Doppler spectrum appears above and below the baseline. Flow that should appear only on one side of the baseline, incorrectly appears on both sides.
2 causes: 1. doppler gain set too high
2.incident angle near 90 degrees
corrected by lowering the gain
The reflections from moving red blood cells are weak. Reflections from stationary anatomic structures such as issue and vessel walls are much stronger. These strong reflections are called clutter.
Artifacts are created when the sound beam is larger than the reflector. "SMALL IS GOOD"
LONG pulses create artifacts and degrade AXIAL RESOLUTION
WIDE pulses create artifacts and degrade LATERAL RESOLUTION
THICK pulses create artifacts and degrade ELEVATIONAL RESOLUTION
low velocity motion from pulsating vessel walls can also produce small Doppler shifts that 'bleed' into surrounding anatomy
WALL FILTER/HIGH PASS FILTER
determines whether low velocity flows are displayed. Serve as a reject for Doppler. Wall filters exclude low level Doppler shifts around the baseline, while having no effect on large Doppler frequency shifts.
Used to reject clutter
REVERB AND RING DOWN
BOTH RESULT FROM SOUND WAVES REFLECTING BETWEEN 2 STRONG REFLECTORS
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