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DOPPLER ARTIFACTS

DOPPLER ARTIFACTS. Parameters for doppler. THE TRANSDUCER FREQENCY. The choice of transducer frequency is paramount because the intensity of the scattered sound varies in proportion to the fourth power of the Doppler frequency.

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DOPPLER ARTIFACTS

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  1. DOPPLER ARTIFACTS

  2. Parameters for doppler THE TRANSDUCER FREQENCY • The choice of transducer frequency is paramount because the intensity of the scattered sound varies in proportion to the fourth power of the Doppler frequency. • For superficial structures 7 to 10 MHz may be ideal, whereas for deep abdominal structures, 3 MHz or lower may be needed.

  3. The Doppler Angle • The angle between axis of flow and the USG beam. • In Doppler US, the strongest signals and best spectra result when the motion is parallel to the beam. • A Doppler angle of 90° does not display flow because no component of the frequency shift is directed back toward the transducer. • The Doppler beam angle must always be kept as low as possible. Ideally, it should be less than 60°and always less than 70°.

  4. Sample volume • 3-Dimensional space from which the Doppler frequency shifts are measured. In color or power Doppler it is the color box, and in pulsed wave Doppler it is the cursor one places within the vessel.

  5. With spectral Doppler, too wide a sample which encompasses the entire vessel lumen includes the normal turbulence and slower velocities along the vessel margins , result in spectral broadening, loss of spectral window. • If the spectral sample is too small and if it is not placed in the area of greatest flow, the resulting measured velocity is too low. • The ideal sample volume size for survey of a vessel is abouttwo thirds of the vessel width. positioned in the center of the vessel excluding as much of the unwanted clutter from near the vessel walls as possible.

  6. Wall Filters • The Doppler frequency shift can be detected from moving blood vessel walls and from the blood itself. • These wall echoes are large amplitude causing a loud “wall thump” on the audio Doppler output. • These signals are also low frequency. • By using a threshold that cuts off these low frequency noises, a cleaner high-velocity blood-flow signal is displayed. • However, if the wall filter threshold is set too high, true blood flow also is discarded from the display.

  7. Spectral Analysis Parameters Power • Increase power to improve • sensitivity. • Excess power will cause spectral • broadening and mirroring.

  8. Spectral Analysis Parameters Colour Gain • Increase gain to improve sensitivity. • Excessive gain causes noise at all frequencies and overestimation of velocity and vice-versa is also true. • Excess gain will cause spectral broadening and mirroring.

  9. Spectral Analysis Parameters Dynamic Range • Compress the dynamic range to improve • S/N ratio. • Over compression will mask true spectral • broadening.

  10. Artefact • Is defined as a feature appearing in an ultrasound image that does not correspond to or represent an actual anatomical or pathological structure. • Classification based on: • Appearance: dot, streak like, ring like, • Cause of artefact

  11. Doppler Artifacts

  12. Aliasing • Sometimes doppler shift freq. of moving blood is greater than detectable freq. of pulsed doppler. This produces an artifactual ,lower freq. signal ,phenomenon known as “Aliasing”. • Aliasing can be eliminated by increasing PRF which increases sampling rate, but relation of PRF and depth plays an important role .

  13. Aliasing in doppler spectrum • Hence the PRF must be at least twice the freq. of the doppler signal • Aliasing is manifested on a doppler spectral display as “wrap around” display with high velocities being converted to reversed flow at the pt. of aliasing and still higher velocities appearing as progressively low velocities .

  14. Aliasing on Color Doppler image • Aliasing within a vessel is displayed as adjacent colours from red to yellow to light blue to dark blue. • Aliasing isdisadvantageous in that high velocities may not be accurately measured. • Colour Doppler aliasing can be advantageous because it quickly localizes the highest velocity region within a vessel for spectral sampling.

  15. Ways to eliminate aliasing: • Increase the PRF by increasing the velocity scale. • Adjusting the spectral baseline. • Lowering the freq. of transmitted beam to reduce doppler Freq. • Increasing the doppler angle to reduce doppler freq.( as cos Q decreases.)

  16. Blooming Artifact • In common terms this is known as “color bleed” because the color spreads out from within the vessel and bleeds beyond the wall into adjacent areas. • It is caused by abnormally high colour gain settings. • The unwanted result, is that the information within the vessel ie, partial thrombus can be “written over” and obscured.

  17. Directional ambiguity • Directional ambiguity or indeterminate flow direction refers to a spectral Doppler tracing in which the waveform is displayed with nearly equal amplitude above and below the baseline in a mirror image pattern. • Directional ambiguity should not be confused with true bidirectional flow. In the latter case, blood actually flows in two directions, such as in the neck of a pseudo-aneurysm or high resistance flows.

  18. The diff. betn true bidirectional flow and an indeterminate direction spectral tracing is that bidirectional flow is never simultaneously symmetric above and below the baseline. Direction varies within the cardiac cycle.

  19. Partial volume Artifact • Partial volume artifact results from a slice thickness that is not infinitely thin. • Echoes and Doppler signals can be acquired from objects that may be partly within the slice and partly outside of it. • Echoes can appear within anechoic structures and Doppler signals are acquired in an area in which no vessels are perceived on gray-scale.

  20. On color flow imaging, an example of partial volume artifact is visualization of a portion of the iliac artery within the ovary giving the impression of abnormal cyst wall flow. • Spectral analysis of this vessel shows waveform typical of an iliac artery and imaging from the 90° plane clearly shows the vessel separate from the ovary.

  21. MIRROR-IMAGE ARTIFACT Patient related artefacts : • The mirror image artifact displays objects on both sides of a strong reflector, though they are located only on one side of it. • With Doppler, mirror image artifact commonly occurs adjacent to the highly reflective lung in the supraclavicular region. Reflection off the pleura causes an apparent duplication of the subclavian artery or vein. • The phantom vessel is always projected deeper in the image.

  22. Pseudoflow • Pseudo-flow is defined as presence of flow of a fluid other than blood. • Pseudo-flow can mimic real blood flow with color or power Doppler US, but no true vessel containing the fluid exists. • Spectral Doppler tracing does not exhibit a normal arterial or venous waveform. • Spontaneous examples of pseudoflow include ascites, amniotic fluid and urine -bladder jets.

  23. FLASH ARTIFACT • Flash artifact is a sudden burst of random color that fills the frame, obscuring the gray-scale image. • This artifact may be caused by object motion or transducer motion . • Flash artifact is most commonly seen in the left lobe of the liver and in hypoechoic areas such as cysts or fluid collections.

  24. TWINKLING ARTIFACT Machine related artefacts : • Color Doppler signals that imitate motion or flow behind a stationary strongly reflecting interface. • The twinkling Doppler is a mosaic of rapidly changing colors located deep to an echogenic reflector.

  25. Guidelines for an optimal color flow Doppler examination • Adjust the gain and filter settings to obtain an optimal color signal and minimal color noise. •    Adjust the velocity scale (PRF) and baseline according to the flow conditions. • A low scale is used for low flows and velocities; however, it may produce aliasing. A high scale reduces aliasing but is less sensitive for slow flows.

  26. Obtain an optimal Doppler angle by adjusting the beam steering and probe position. The angle should be less than 60° or less if velocity measurements are to be made. • The color flow box should be kept as small as possible to allow better frame rate for better resolution and sensitivity.•    Adjust the pulsed Doppler sample volume size appropriately two thirds of the vessel diameter to obtain accurate velocities.• Avoid transducer motion.

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