Errors in imaging
- not real
- missing reflectors
- improper brightness
- improper shape
- improper size
- improper position
causes of artifacts
violation of assumptions (acoustic artifacts)
equipment malfunction or design
physics of ultrasound
six basic assumptions of imaging systems
- sound travels in a straight line
- sound travels directly to a reflector and back
- sound travels in soft tissue at exactly 1540 m/s
- reflections arise only from structures postitioned in the beams main axis
- the imaging plane is very thin
- the strength of a reflection is related to the characteristics of the tissue creating the reflection
multiple echoes appearing on the display as a result of US ping-poning between two reflectors. Looks like a ladder or a venetian blind.
multiple, equally spaced, parallel to the sound beam, and deeper and along a straight line
Comet tail or ring down
appears as a soild line directed downward, merged reverberation
signle soild hyperechoic line, long echo, parallel to the sound beam, a reverb with the space squeezed out
US beam unable to pass through a structure because the structure has a higher than usual attenuation.
hypo- or anechoic postior shadowing, prevents visualization of true anatomy on the scan
Edge shadow or shadowing by refraction
refraction at the edge of a circular structure can also create an artifact.
Hypoechoic, due to refraction and beam spreading, absence of true anatomy
occurs when the medium through which the sound travels has a lower attenuation rate than soft tissue. the correct number of anatomic reflectors appear on the image.
parallel to the sound beam, hyperechoic
Sound may bounce off of a strong reflector and be redirected. Ultrasound systems assume that sound travels directly to a reflector and back to the transducer.
A second copy of a true reflector, the artifact appears deeper than the true reflector, true reflector and artifact are equal distances from the mirror.
Propagation speed errors (range error artifact)
If US doesnt propagate at 1550 m/s, assume relationship between the distance and time is invalid.
correct number of reflections on scan, improper depths
When the propagation speed is greater than 1540 m/s, the reflector will be placed too ______.
When the speed is less than soft tissues, the reflector will be placed too ______ on the display.
Sound changes direction striking a boundary: obliquely and when the media have different propagation speeds.
Second copy of reflector
Artifact appears side-by--side with the true anatomic structure
What tyoe of resolution does refraction artifact degrade?
Side lobes and grating lobes
Extra acoustic energy may be transmitted in directions other tha the beams main axis.
Second copy of the true reflector
artifact appears side-by-side with the true anatomic structure
Transducer associated with side lobes?
Mechanical or single crystal transducers create side lobes
Arrays create grating lobes
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 voltagesSubelements closer to the center of the sound beam are excited with higher voltages, further away from the center of the beam are excited with lower voltages
Lobe artifact degrades what type of resolution
The assumption that reflections arise from the beams main axis is violated by what artifact?
occurs when beam has a greater width than the reflector
what type of resolution is associated with slice thickness
Reflections produced by structures above or below the ideal imaging plane appear in the image?
partial volume artifact or section thickness artifact
cured with 1 1/2 dimensional array
Grainy appearance not directly from reflections from tissues.
Created by interference effects of scattered sound, both constructive and destructive.
Range ambiguity artifact
Caused when very deep reflections arrive at transducer after the next pulse was created.
Places late arriving reflection too shallow on the image
How can range ambiguity artifact be cured?
by lowering the PRF because the PRF is too high
Artifacts are created when the sound beam is _____ than the reflector?
Laminar flow are aligned and parallel and may have what tyoe of patterns?
Parabolic or plug
When small regions of turbulent flow have vastly different velocities?
Doppler Spectral Broadening
Blood flows when the total fluid energy at one location differs from the total fluid energy at another location?
Units of viscosity
- changes in flow direction
- increased velocity, highest at the point of max narrowing
- turbulence at exit
- pressure gradient across the stenosis
- arterial gradient across the stenosis
- arterial flow loses its pulsatile nature and becomes more continuous
When velocity at the point of max narrowing. Pressure is the lowest here.
Factors that determine resistance
radius of lumen, length, viscosity of fluid
circulatory pressure + hydrostatic pressure
diaphragm moves down
venous flow from legs decreases
venous return to the heart increases
Diaphragm moves up
Venous flow from legs increases
Venous return to heart decrease
-50 mmHg at fingertip with hand elevated
-30 mmHg at head
0 mmHg at heart
15 mmHg at knee
100 mmHg at ankle