1. Aliasing occurs when the Doppler shift is higher than half the pulse repetition frequency, causing signals to become indistinguishable and display with the wrong color or velocity. It can be corrected by increasing the pulse repetition frequency or adjusting the baseline. 2. Mirror artifacts occur when the ultrasound beam reflects off a structure like bone or diaphragm before returning to the transducer, creating a duplicate image. Changing the angle of insonation can help avoid it. 3. Motion artifacts appear as random color flashes caused by movement of the patient, transducer, or tissue during imaging. They can be minimized by comfortable patient positioning and a stable scanning arm.

1. Doppler Artefacts (1)
Dr. Kamal Sayed MSc US UAA
RN/aliasing/mirror/reverberation/comet tail/

2. •
Doppler ARTEFACTS
•
1) Random noise (RN)
•
Random noise is produced in all electrical circuits.
•
When the gain is too high, this noise becomes detectable in
Doppler circuitry.
•
In the image it is seen as colour foci appearing randomly in
the image.
•
It is easily identified as an artefact because the colour foci do
not reappear in the same location as true flow does.
•
The random noise is used to set the Doppler gain correctly
(just at or below the level that generates little RN).

3. 2) Aliasing
•
Aliasing is a phenomenon inherent to Doppler modalities
which utilize intermittent sampling in which an insufficient
sampling rate results in an inability to record direction and
velocity accurately
•
.
Aliasing is an effect that causes different signals to become
indistinguishable when sampled
•
. Aliasing is an error or distortion created in a digital image
that usually appears as a jagged (rough, uneven shape or
edge with lots of sharp points) outline) .
•

4. We commonly observe aliasing on television.
•
Aliasing is one of the most well known artefacts in both
colour and spectral Doppler examinations and arises when
the Doppler shift is higher than half of the PRF (Nyquist
limit).
•
Aliased signals are displayed with @ the wrong directions (red
instead of blue and vice versa) @ and with incorrect relative
velocity (the hue of the colour).
•
Slides (7/8)

5. •
ALIASING
•
Pulsed wave (PWD) systems suffer from a fundamental
limitation.
•
When pulses are transmitted at a given sampling frequency
(known as the pulse repetition frequency {PRF} ), the
maximum Doppler frequency fd that can be measured
UNambiguously is half the pulse repetition frequency :
•
{fd = ½ PRF}.
•
If the blood velocity and beam/flow angle being measured
combine to give a fd value greater than half of PRF,
ambiguity in the Doppler signal occurs.
•
.
•

6. •
This ambiguity is known as aliasing.
•
A similar effect is seen in films where wagon wheels can
appear to be going backwards due to the low frame rate of
the film causing misinterpretation of the movement of the
wheel spokes.
•
•
slides (13/14)

7. •
Waveforms with aliasing :
•
with abrupt termination of the peak systolic and display this
peak below the baseleine Sonogram.
•
Correction : @ increase the pulse repetition frequency
•
@ and adjust baseline (move down)
•
Figure 6 (a) & (b) : Example of aliasing and correction of the
aliasing.
•
Slides (10/11)

8. Figure 6 (a,b): Example of aliasing and correction of the aliasing.
(a) Waveforms with aliasing, with abrupt termination of the peak systolic and
display this peaks bellow the baseleineSonogram clear without aliasing.

9. Figure 6 (b) Correction: increased the pulse repetition frequency and adjust
baseline (move down)

10. •
The PRF is itself constrained by the range of the sample
volume.
•
The time interval between sampling pulses must be sufficient
for a pulse to make the return journey from the transducer to
the reflector and back.
•
If a second pulse is sent before the first is received, the
receiver cannot discriminate between the reflected signal
from both pulses and ambiguity in the range of the sample
volume ensues.
•

11. •
As the depth of investigation increases, the journey time of
the pulse to and from the reflector is increased, reducing the
pulse repetition frequency for unambiguous ranging.
•
The result is that the maximum fd (maximum doppler
frequency) measurable decreases with depth.
•

12. •
Low PRFs are employed to examine low velocities (e.g. venous
flow).
•
(The longer interval between pulses allows the scanner a
better chance of identifying slow flow).
•
Aliasing will occur if : @ low PRFs @or velocity scales are used
and high velocities are encountered .
•
. (Figure 4,5 and 6) slides (13/14/15/16/17/18).
•
Conversely, if a high pulse repetition frequency is used to
examine high velocities, low velocities may not be identified
•
•

13. figure 4 : Aliasing of color doppler imaging and artefacts of color. Color
image shows regions of aliased flow (yellow arrows).

14. figure 5 : figure 4 is corrected by Reducing color gain and increase pulse
repetition frequency.

15. Figure 6 (a,b): Example of aliasing and correction of the aliasing.
(a) Waveforms with aliasing, with abrupt termination of the peak systolic and
display this peaks bellow the baseleineSonogram clear without aliasing.

16. Figure 6 (b) Correction: increased the pulse repetition frequency and adjust
baseline (move down)

17. Figure 7 (a): Color flow imaging: effects of pulse repetition frequency or scale. (above)
The pulse repetition frequency or scale is set low {11.0} (yellow arrow). The color
image shows ambiguity within the umbilical artery and vein and there is extraneous
noise.

18. Figure 7 (b) The pulse repetition frequency or scale is set appropriately for the
flow velocities {27.5} (bottom). The color image shows the arteries and vein
clearly and unambiguously.

22. •
Aliasing cannot occur in PD.
•
Aliasing is not important in rheumatology and should not be
avoided by increasing the PRF. This would lead to under-
detection of flow because of decreased sensitivity to slow
flow.
•
Other potential aliasing causative factors include:
•
@ use of higher frequency transducers
•
@ inappropriate angle of insonation
•
@ large sampling volume

24. •
Aliasing : High velocities appear negative.
•
Nyquist frequency : The Doppler frequency at which aliasing
occurs.
•
Equal to ½ the PRF. Also called Nyquist limit.
•
Equation: Nyquist limit (kHz) = PRF/2
•
To eliminate effects of aliasing :
•
1. Change the Nyquist (change the scale).
•
2. Select a transducer with a lower frequency.
•
This shrinks the spectrum

25. •
3. Select new view with a shallower sample volume to raise
Niquist. .
•
4. Use continuous wave Doppler (CWD).
•
5. Select a new view so that the angle is further away
•
from 0° and closer to 90°. This shrinks the spectrum.
•
6. Baseline shift (this is for appearance only.)
•
7- Decreasing the pulse repetition period (PRP) to increase
the PRF and the Nyquist limit.

26. •
8- Applying a low-frequency transducer to create a small
Doppler shift for blood flow velocity.
•
Numbers 1 through 5 actually eliminate aliasing.
•
Baseline shift simply makes it appear to have vanished.
•
1- Decreasing the pulse repetition period (PRP) to increase
the PRF and the Nyquist limit.
•
2- Applying a low-frequency TXR to create a small Doppler
shift for blood flow velocity.
•
3- It is also possible to adjust (by lowering or elevating) the
baseline of the US image to reduce aliasing; doing this will
adjust the PRF.
•

27. Summary
•
1- Decreasing the pulse repetition period (PRP) to increase
the PRF and the Nyquist limit.
•
2- Applying a low-frequency TXR to create a small Doppler
shift for blood flow velocity.
•
3- It is also possible to adjust (by lowering or elevating) the
baseline of the US image to reduce aliasing; doing this will
adjust the PRF.
•
THUS Aliasing can be remedied by reducing the frequency of
the US TXR or increasing the PRF.
•

28. •
3- Motion artefact
•
Movement of the patient, TXR or movement of the tissue or
vessel wall caused by arterial pulsation during Doppler
imaging give motion relative to the transducer and produce a
Doppler shift.
•
The movements are slow and produce low frequency Doppler
shifts that appear as random short flashes of large confluent
areas of colour.
•

29. •
One way to avoid these low frequency flash artefacts are
•
by means of : 1- filters, eg, wall filters. Such filters, however,
also remove information from slow moving blood.
•
When the Doppler is at its highest sensitivity just at or below
the noise level, intermittent motion artefacts must be
accepted.
•
Motion artefacts are minimised when 2- the patient is
comfortably positioned with 3- the area under investigation
resting and it is also mandatory that 4- the examiner’s
scanning arm is resting comfortably as well.

30. •
4) Mirror image artifact is seen when there is a highly
•
Reflective surface (e.g. diaphragm) in the path of the primary
beam.
•
The primary beam: @ reflects from such a surface
(e.g. diaphragm) but instead of directly being received by
the transducer, @ it encounters another structure (e.g. a
nodular lesion) in its path @ and is reflected back to the
highly reflective surface (e.g. diaphragm).
•
@ It then again reflects back towards the transducer
•
•
•

31. •
. To avoid this mirror image artifact, change the position and
angle of scanning to change the angel of insonation of the
primary ultrasound beam
•
Slide (32).

33. In rhematology, mirrors nearly always are bone surfaces.
•
The mirror artefact is easily seen as such when the true image
as well as the mirror and mirror image are all in the image.
•
The mirror image is slightly trickier when only the mirror and
mirror image are present
•
. The ultrasound machine makes a false assumption that the
returning echo has been reflected once and hence the
delayed echoes are judged as if being returned from a deeper
structure, thus giving a mirror artifact on the other side of the
reflective surface.
•

34. •
It is a friendly artifact that allows the sonographer to exclude
pleural effusion by the reflection of the liver image through
the diaphragm. Examples :
•
@ reflection of a liver lesion into the thorax (the commonest
example)
•
@ reflection of abdominal ascites mimicking pleural effusion
•
@ duplication of gestational sac (either ghost twin or
heterotopic pregnancy)
•
@ duplication of the uterus
•
Images slide (35)
•

35. Mirror artefact Scrotum (upper image)
Liver lesion (lower image)

36. •
5) Blooming or colour bleed artifact occurs when the colour
signal indicating blood flow extends beyond its true
boundaries, spreading into adjacent regions with no actual
flow
•
. This artifact mainly affects the portion of the image distal to
the vessel and the transducers. It is somewhat similar to
its MRI namesake as both phenomena denote something that
seemingly appears larger than its true physical limits.

37. •
Blooming occurs because the spatial resolution of colour
Doppler is lower than grey scale ultrasound .
•
It can be exaggerated by inappropriately high colour gain.
•
It is important to be aware of as it may obscure partial
occlusions such as thrombi resulting in misdiagnosis 2.
•
It can be avoided by lowering the colour gain until the bleed
of signal outside of the vessel disappears 3.
•
Slide (38)

38. Blooming artefact

39. •
6) Reverberation artifact occurs when an US beam
encounters two strong parallel reflectors.
•
When the US beam reflects back and forth between the
reflectors ("reverberates"), the US TXR interprets the sound
waves returning as deeper structures since it took longer for
the wave to return to the transducer.
•
Reverberation artifacts can be improved by changing the
angle of insonation so that reverberation between strong
parallel reflectors cannot occur

40. •
It is advisable always to let the colour box go to the top of the
image to be aware of possible reverberation sources
•
Slides ( 41/42/43).
•
7) Comet tail artefact is a specific type of reverberation artifact .
•
This results a short train of reverberations from an echogenic
focus which has strong parallel reflectors within it (e.g.
cholesterol crystals in adenomyomatosis).
•
Slide (44/45)

41. Reverbertion aretfact pelvic scan

42. Reverbertion aretfact

43. Reverbertion aretfact

44. Comet tail artefact

45. Comet tail artefact

46. •
8) Twinkling artifact
•
Twinkling artifact is seen with (CFD) colour flow Doppler
ultrasound . It occurs as a focus of alternating colours on
Doppler signal behind a reflective object (such as a calculus),
which gives the appearance of turbulent blood flow .
•
It appears with or without an associated colour comet-tail
artifact
•
slides (32)

47. TWINKLING

48. •
The underlying mechanism is thought to be a result of
inherent noise within the US scanner, specifically phase
(a.k.a. clock) jitter within the Doppler electronics .
•
Twinkling artifact is more sensitive for detection of small
stones (e.g. urolithiasis, cholelithiasis) than is acoustic
shadowing. It is most pronounced when the reflecting surface
is rough and highly dependant on machine setting
•
slide (34)
•

49. TWINKLING

50. •
@ when the focal zone is located below a rough reflecting
surface, the twinkling artifact becomes more obvious than
when it is above it.
•
@ decreased pulse repetition frequency facilitates better
visualisation of the artifact.
•
@ The presence of renal twinkling artifact on sonography has
a high positive predictive value (78%) for the presence of
nephrolithiasis as unenhanced CT.
•
Slide (36)
•

51. TWINKLING

52. •
Doppler systems convert frequency shifts into colors and
•
spectra.
•
Frequency shifts generally arise from moving red
•
blood cells.
•
However, low velocity motion from pulsating
•
vessel walls can also produce small Doppler shifts that
•
‘bleed’ into surrounding anatomy.
Slide (38)

54. •
Wall filter (high pass filter) : @ Eliminates low magnitude
Doppler shifts that are
•
Created from moving anatomy rather than red blood cells.
•
Wall filters serve as :
•
@ a “reject” for Doppler.
•
@ Wall filters exclude low level dopp shifts around the
baseline, while having NO effect on large dopp shifts.
•
@ wall filters are used to reject “clutter”.
•
Color flash is called ghosting on the exam.

55. •
# Reducing color Doppler gain will not cure ghosting artifact
•
because red blood cell reflections are weaker than tissue
•
reflections.
•
# Reducing color Doppler gain will eliminate
•
reflections from tissues before those from blood cells.

56. •
9) beam width artifact
•
occurs when a reflective object located beyond the widened
ultrasound beam, after the focal zone, creates false
detectable echoes that are displayed as overlapping the
structure of interest.
•
To understand this artifact, it is important to remember that
the ultrasound beam is not uniform with depth, the main
beam leaves the transducer with the same width as it, then
narrows as it approaches the focal zone and widens again
distal to this zone .
•
Slide (42)
•

57. BEAM WIDTH ARTEFACT

58. •
Usually, it occurs when scanning an anechoic structure and
•
some peripheral echoes are identified, i.e. gas bubbles in the
duodenum simulating small gallstones and peripheric echoes
in the bladder.
•
It is possible to avoid this artifact BY : @ adjusting the focal
zone to the depth level of interest @ and by placing the TXR
at the centre of the object being studied.