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a Chapter 40 Doppler Ultrasonography for Gynecologic Malignancies 607
and shadings for each area of study. These maps can
be stored and used again. Some instruments offer a
ªgreen tagº function, which allows the operator to
tag, in green, a specific velocity/frequency or to set a
threshold level above which all the velocities are colored
green. The green tag is often used to help identify
areas of increased velocity more rapidly or to aid
identification of aliasing by making more obvious the
highest frequency before the alias point.
The wall filter is a high-pass filter that allows the
higher frequencies to be detected and allows the operator
to selectively remove low frequencies due to
motion of the tissue. The filter is usually displayed in
hertz units. Some wall filters are directly connected
to the PRF and increase automatically as the PRF is
increased; they may need to be manually decreased
with decreasing PRF. It is generally suggested to keep
the wall filter as low as possible during most pelvic
studies, although the wall filter may need to be increased
to reduce color ªflashº from bowel noise or
transmitted pulsation from large pelvic vessels.
The gain is a front panel control that adjusts the amplification
of the displayed Doppler signal. Color and
pulsed-wave Doppler gains should be constantly adjusted
throughout the examination to accommodate
the changing signal strengths. If the gain is set too
low, some Doppler information may not be detected,
or if it is set too high too much ªnoiseº is observed. Increased
color gain is suggested in situations of suspected
low-velocity blood flow. An ªovergainº adjustment
may be useful in areas of questionably low flow
or no flow, as this change can enhance any flow that
may be present. Power settings also affect the gain; that
is, increasing the power allows lower gain settings.
As mentioned earlier, aliasing, the mirrorimage
artifact, consists of a similar time-velocity spectrum
appearing above and below the zero line (Fig. 40.9).
It occurs at large angles of interrogation, particularly
at low signal-to-noise, when a large receiver gain is
used to detect weak Doppler signals. It results in saturation
of one quadrate phase detector due to strong
clutter signals from stationary scatters.
Flow direction artifacts occur in tortuous vessels,
such as the uterine or umbilical artery. Hence the
same vessel can produce diverse color signals, depending
on the temporary position of the vaginal
probe. As the angle of insonation decreases at each
end of the scan the velocity appears to increase. Experience
allows us to recognize such artifacts. In a
color Doppler study, any movement is modulated into
color, which may produce an artifactual impression
of blood flow. Bowel loop movements, such as peristalsis,
can produce such an artifact, called color
modulation of biologic movement. Several studies
have presented the most common Doppler pitfalls
and artifacts [33±37]. Potential solutions to avoid artifacts
are also suggested. There is an urgent need for
more standardization of the Doppler technique.
Artifacts in color and spectral Doppler imaging
can be confusing and lead to misinterpretation of
blood flow information. Inappropriate equipment settings,
anatomic factors, and physical and technical
limitations of the modality are the major causes. Incorrect
gain, wall filter, or velocity scale settings can
cause loss of clinically important information or distortion
of the tracing. Reflection of the Doppler signal
from highly reflective surfaces can create a color
Doppler mirror image. Vascular motion can introduce
Fig. 40.9. Aliasing is a common artifact
in Doppler studies. The mirror-image
artifact shown here is at least as common.
Arrows indicate the peak systolic
and diastolic velocity on the Doppler
waveforms