Diagnostic ultrasound ( PDFDrive )

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APPENDIX
Ultrasound Artifacts: A Virtual Chapter
Aliasing and Velocity Scale Errors
Aliasing is an artifact due to undersampling of Doppler signal,
resulting in incorrect estimation of low velocity. To measure
the Doppler frequency shit appropriately by pulsed Doppler, at
least two samples from the cycle are required (Nyquist limit). If
the pulse repetition frequency is less than twice the maximum
frequency shit produced by movement of the target, aliasing
results. he Doppler signal wraps around either the spectrum
(spectral Doppler) or the color scale (color Doppler). he most
common methods for correcting for aliasing are to shit the
baseline down or up, or increase the pulse repetition frequency
(or velocity scale). A lower-frequency transducer can also be
used to help in correcting for aliasing. As angulation approaches
90 degrees, directional ambiguity can occur, suggesting bidirectional
low. Errors in Doppler angle correction can also lead to
misleading assessments of low velocity.
FIG. A.56 Tissue Vibration Artifact. Spectral waveform at site of
superior mesenteric artery stenosis shows very high systolic velocity,
and diastolic velocities of greater than 500 cm/sec. Color portion of the
image shows color bruit consisting of color outside the vessel near
the stenosis site. Color bruit is believed to be caused by tissue vibration. 23
(See Chapter 12, Fig. 12.27A.)
Click for video showing tissue vibration artifact in patient
with stenotic superior mesenteric artery with color bruit (Video
A.20). (See also Video 12.11.)