Diagnostic ultrasound ( PDFDrive )

APPENDIX Ultrasound Artifacts: A Virtual Chapter
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A
B
C
D
FIG. A.57 Aliasing. Pulse repetition frequency (PRF) determines the sampling rate of a given Doppler frequency. (A) If PRF (arrows) is suficient,
the sampled waveform (orange curve) will accurately estimate the frequency being sampled (yellow curve). (B) If PRF is less than half the frequency
being measured, undersampling will result in a lower-frequency shift being displayed (orange curve). (C) Aliasing appears in the spectral display
as a “wraparound” of the higher frequencies to display below the baseline. (D) In color Doppler display, aliasing results in a wraparound of the
frequency color map from one low direction to the opposite direction. 2 Unlike a true direction of low change, with aliasing there will be no transition
of unsaturated color. (See Chapter 1, Fig. 1.46.)
FIG. A.58 Aliasing. Note that aliasing prevents precise calculation
of the maximum systolic velocity. However, because it is greater than
500 cm/sec, the precise number is not needed. The aliasing in the image
allows for easier identiication of the region of highest velocity for
placement of the spectral Doppler gate. 6 (See Chapter 26, Fig. 26.20.)
FIG. A.59 Aliasing in Superior Mesenteric Artery (SMA) Due to
High-Velocity Flow. 23 (See Chapter 12, Fig. 12.28.)