Diagnostic ultrasound ( PDFDrive )

e24
APPENDIX
Ultrasound Artifacts: A Virtual Chapter
DOPPLER IMAGING ARTIFACTS
he appearance of color Doppler signal is afected by several
variables including color-write priority, gray-scale gain, and pulse
repetition frequency. If the color gain is too low, low might not
be visualized. If the gain is too high, artifactual low may be seen
in adjacent sot tissues, and thrombus within the vessel might
be missed.
Loss or Distortion of Doppler Information
Incorrect gain, wall-ilter, and velocity scale can all lead to loss
or distortion of Doppler signal. A rule of thumb for gain adjustment
in Doppler imaging is to turn up the color gain until noise
is encountered and then back of just until the noise clears from
the image. In estimating frequency, the rule of thumb is to use
a Doppler angle less than 60 degrees (but not 0). Wall ilters
eliminate low-frequency noise, but a high setting can lead to
loss of signal. In general, wall ilters should be kept at the lowest
practical level, typically in the range of 50 to 100 Hz.
θ = 0°
cos θ = 1.0
∆F = 1.0
θ = 60°
cos θ = 0.5
∆F = 0.5
θ = 90°
cos θ = 0.0
∆F = 0.0
FIG. A.44 Effect of Doppler Angle on Frequency Shift. At an angle
of 60 degrees, the detected frequency shift detected by the transducer
is only 50% of the shift detected at an angle of 0 degrees. At 90 degrees,
there is no relative movement of the target toward or away from the
transducer, and no frequency shift is detected. The detected Doppler
frequency shift is reduced in proportion to the cosine of the Doppler
angle. Because the cosine of the angle changes rapidly at angles above
60 degrees, the use of Doppler angles of less than 60 degrees is recommended
in making velocity estimates. 2 (See Chapter 1, Fig. 1.35.)
A (PRF = 700 Hz)
B (PRF = 4500 Hz)
FIG. A.45 Artifactual Lack of Flow. Color Doppler image of a carotid artery and jugular vein. In (A), the pulse repetition frequency (PRF) is
700 Hz and there is aliasing in the carotid artery, but slow low in the jugular vein is seen. In (B), the PRF is 4500 Hz, eliminating aliasing in the
artery but also suppressing the display of the low Doppler frequencies in the internal jugular vein. 2 (See Chapter 1, Fig. 1.43.)