3. Refraction Artifacts
•Snell’s law predicts refraction
•Bending of ultrasound beams at interface of two dissimilar
materials with different velocities.
•Leads to;
1. Misregistration
2. Edge shadowing (defocusing)
3. Ghosting.
•Refraction can cause a reflector to be positioned improperly
(laterally) on the display
4. 1. Misregistration
•It happens all time so we shouldn’t trust the positions
of organs on screen.
•Beam traveling from higher velocity medium to low
leads to misregistration artifacts.
•e.g. sound waves transition from fat rich to soft tissue
rich interface.
6. Mis-registration
1 2
3 4
• The blue line
indicates the
actual position
but it has been
shifted to where
you see it now
• The image of the
kidney is actually
shifted from
where it is
7. 2. Ghosting
•This is likely to occur, for example, when the transducer
is placed on the abdominal midline/ abdominal fascia
plane
•This leads to doubling of single objects. Beneath are the
rectus abdominis muscles, which are surrounded by fat.
These tissues present refracting boundaries because of
their different propagation speeds.
•Refractive artifacts lead to phenomenon such as double
aorta and double gestation sacs.
8. 2. Ghosting
Real Artif
act
Artif
act
One real structure is
imaged as two artifactual
objects because of the
refracting structure close
to the transducer. If
unrefracted pulses can
propagate to the real
structure, a triple
presentation (one correct,
two artifactual) will result.
Refractin
g
Structure
9. 2. Ghosting
A, Refraction (probably
through the rectus
abdominis muscle) has
widened the aorta (open
arrow) and produced a
double image of the celiac
trunk (arrows).
B, Refraction has produced
a double image of a fetal
skull (arrows). Refraction
also may cause a single
10. 3. Defocusing (Edge Shadowing)
•Beam may bend at curved surface and lose intensity, producing
a shadow
•Due to refraction, beam traveling from higher velocity medium
to low velocity gives narrow shadow
•On the other hand, a wider shadow results from beam travel
from lower to higher velocity regions
•Occur after striking a large curved reflector. Extends downwards
from curved reflectors edge
•A portion of incident sound beam lost to refraction leaving less
echo signal returning to transducer along beam path (vessel or
gallbladder).
11. 3. Defocusing
B
C
R
S
B
A, Edge shadows (arrows) from
a fetal skull. B, While a sound
beam (B) enters a circular
region (C) of higher
propagation speed, it is
refracted, and refraction
occurs again while
it leaves. This causes
spreading of the beam with
decreased intensity. The
echoes from region R are
13. Potential solutions to refraction
artifacts
•Vary angle of insonation or interrogate across a
wider area to ascertain artifacts existence
•Misregistration and defocusing artifacts need to be
considered but difficult to eliminate completely.
14. Lobe Artifacts
•Reflections generated at improper, off-axis locations
along side the main beam. Added and assigned to the
main beam
•Occur in both single (side lobes) and array transducers
(grating lobes). Grating lobes are stronger than the side
lobes of individual elements
•Lobes are weaker than the primary beam. Normally do
not produce echoes that a displayed
•However, if they encounter a strong reflector, they may
appears as hyperechoic/ hypoechoic density with in
hypoechoic/ anechoic background
15. Lobes
A, The
primary
beam (B)
and grating
lobes (L)
from a
linear array
transducer
B
B, side lobes or grating lobes can produce and receive
a reflection from a “side view.” C, This will be placed on
the display at the proper distance from the transducer
but in the wrong location (direction) because the
instrument assumes that echoes originate from points
along the main beam axis.
C
16. Lobe Artifacts
A, A real amniotic
sheet (arrow).
B–C, Grating lobe
duplication (open
arrows) of fetal bones
(curved arrows)
resembles amniotic
bands or sheets.
D, Grating lobe
duplication of a fetal
17. Lobe Artifacts
E, Artifactual grating lobe echoes (arrow)
cross the
aorta. We observe that the grating lobe
artifact is always weaker than the correct
presentation of the structure.
F, Apical two-chamber view of grating-lobe
artifact (arrow) in left ventricle.
G, Short-axis view of aortic and tricuspid
valves with grating lobe artifact (arrow).
H, At first glance, this seems to be a mirror
image artifact, similar to what is seen in
abdominal imaging. However, it is not for
two reasons: (1) There is no apparent
echogenic mirror, and (2) The repeat on the
left side is not horizontally reversed as
would be the case with mirroring. Rather, it
18. Potential Solutions to Lobe
Artifacts
•Apodization
◦Making the gain at the center of transducer be
higher than at the edges.
◦Deferential amplification of excitation pulse. Inner
elements are driven at higher amplitude voltages
than outer ones
Note: Grating lobes duplicate structures laterally
19. Multipath Artifacts
•Reflectors may appear at incorrect depths due to oblique
reflection of ultrasound waves following a longer or shorter
path than the incident beam
20. Potential Solutions to Multipath
artifacts
•Change angle of insonation to avoid oblique angle
•Interrogate object of interest (e.g. bladder) from
multiple views to ascertain what is real from artifact
21. Speed Error Artifacts
•US machine assumes a speed of 1540m/s in all structures
which is not true. If the beam passes through a structure that
is mainly fat, velocity is 1450m/s
•Therefor, the structure will appear further away from
transducer surface (vice vaser). Conversely, if the speed in the
tissue is higher than 1540 m/s, calculated distance to the
reflector is too small
•The reflector will be placed too close to the transducer
•Increased speed, causes echoes to arrive sooner. Reduced
speed causes echo arrival delay
•The phenomenon can cause abdominal step-off of anatomic
22. Speed Error Artifacts
The propagation speed over the
traveled path (A) determines the
reflector position on the display
(B).
The reflector is actually in
position 1. If the actual
propagation speed is less than
that assumed, the reflector will
appear in position 2. If the actual
speed is more than that
assumed, the reflector will
23. Speed Error Artifacts
Diaphragm step-off
is because the beam
passed through
hyperechoic
gallbladder and
portal vein where the
speed is high.
With normal
diaphragm, the
beam passed
24. Potential Solutions of Propagation
Speed error
•Be mindful of this possibility
•Newer US machines with multi-beam features and
improved signal processing capabilities usually
minimize this artifact
Note: Propagation speed error displaces structures
axially
25. Range Ambiguity
•In sonographic imaging, it is assumed that for each pulse
all echoes are received before the next pulse is emitted.
•If this were not the case, error could result i.e. range
ambiguity.
•The maximum depth imaged correctly by an instrument is
determined by its pulse repetition frequency (PRF).
•To avoid range ambiguity, PRF automatically is reduced in
deeper imaging situations. This also causes a reduction in
frame rate.
Note: Range ambiguity artifact places structures much
closer to the surface than they should be.
26. Range Ambiguity
Abdominal ascites produces a large echo free region
in this scan. A structure is located at a depth of
approximately 13 cm (straight arrows). Located in the
anechoic region at a depth of approximately 6 cm is a
structure (curved arrows) shaped like the structure at
13 cm.
How could this artifact appear closer than the actual
structure, implying that these echoes arrived earlier
than those from the correct location?
It turns out that the artifact is actually a combination
of two: reverberation and range ambiguity.
The artifact seen is a reverberation from the deep
structure and the transducer. But a reverberation
should appear at twice the depth of the actual
structure, that is, at about 26 cm. However, the arrival
27. Potential Solutions to Range
Ambiguity
•Avoid use of high PRF when measuring deep
structures
•When measuring shallow structures use high
frequency linear probes
28. Question
A reflector is at a depth of 12 cm from the
transducer surface, but the US machine displays it
as though it is at 10 cm. Most likely this is due to:
a. Range ambiguity
b. Speed of sound > 1540 m/s
c. Posterior acoustic enhancement
d. Speed of sound < 1540 m/s
29. Points to Note
•If V > 1540 m/s (FAST), object will appear closer
TOWARDS transducer surface than it actually is
•If V < 1540 m/s (SLOW), object will appear further
AWAY from transducer surface than it actually is
•Acronym SAFT
•Slow Away Fast Towards
