Doppler artefact (1)

the differnet types of doppler physics aretefacts

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 UN ambiguously 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.

19. 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

20. 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

21. 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 (PR P ) to increase the PR F and the Nyquist limit .

22. 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 .

23. 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.

24. 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 .

25. 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.

26. 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) i n its path @ and is reflected back to the highly reflective surface (e.g. diaphragm). @ It then again reflects back towards the transduce r

27. . 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).

28. 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.

29. 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)

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

31. 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.

32. 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 s uch 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)

33. Blooming artefact

34. 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

35. 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)

36. Reverbertion aretfact pelvic scan

37. Reverbertion aretfact

38. Reverbertion aretfact

39. Comet tail artefact

40. Comet tail artefact

41. 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)

42. TWINKLING

43. 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)

44. TWINKLING

45. @ 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)

46. TWINKLING

47. 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)

48. 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.

49. # 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.

50. 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) 

51. BEAM WIDTH ARTEFACT

52. 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.