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a Chapter 6 Sonographic Color Flow Mapping: Basic Principles 79
Fig. 6.10. Color Doppler Nyquist effect. The image shows
Doppler color flow mapping of the left ventricular outflow.
Note the change of color from red to yellow and then from
bright blue to dark blue. With the declining velocity, blue
changes first to yellow and then to red. LVOT left ventricular
outflow tract
Fig. 6.11. Doppler color flow depiction of change in the
direction of flow. The image shows Doppler color mapping
of flow through the ascending aorta (AA) and the arch of
the aorta. In the ascending aorta the flow is toward the
transducer and is therefore depicted as red according to
the color bar setting. As the direction of flow changes, the
color becomes blue with a dark line separating the two
hues. LV left ventricle
Fig. 6.9. Color Doppler Nyquist effect. The image shows
Doppler color flow mapping of the left ventricular outflow.
Note the change of color from red to yellow and then from
bright blue to dark blue. With the declining velocity, blue
changes first to yellow and then to red. LVOT left ventricular
outflow tract
is depicted in reversed color surrounded by the nonaliased
flow (Fig. 6.9). This pattern mimics the color
flow appearance of separate streams in differing directions.
The two patterns, howerver, are clearly distinguishable.
In an aliased flow, the higher velocity
generates a higher Doppler shifted frequency, which
is depicted with greater brightness: the higher the
frequency shift, the brighter the color. The brightest
level in the color calibration bar (the uppermost for
the flow toward the transducer and lowermost for the
flow away from the transducer) represents the Nyquist
limit. As the velocity, and therefore the frequency
shift, exceeds this limit, the color wraps
around the calibration bar and appears at the other
end as the most luminous color of the opposite direction
(Fig. 6.10). For example, flow toward the transducer
with an increasing velocity is depicted with an
increasingly bright red color changing to yellow
(Fig. 6.9). As the Nyquist limit is reached, the color
flow shows brightest yellow in the color bar; and as
the limit is exceeded, flow is shown in the brightest
blue. Thus in an aliased flow, the bright or pale color
of one direction is juxtaposed against the bright color
of the opposite direction. In contrast, with genuine
flow separation the distinct flow streams are depicted
in the directionally appropriate colors separated by a
dark margin (Fig. 6.11). Note that the hue that demarcates
an aliased flow depends on the choice of the
color mapping scheme.
When demonstrating aliasing, an apparent contradiction
may be seen between the spectral Doppler
and the Doppler color flow interrogations. Doppler
color flow mapping may show a nonaliased flow pattern
when aliasing is observed with the spectral Doppler
waveform. This phenomenon is explained by the
fact that the mean frequency is less than the peak frequency,
and Doppler color flow depiction is based on
the use of mean frequency, so the Nyquist limit is not
reached as readily as with the peak frequency depiction
by the spectral Doppler method.
Color Doppler aliasing can be eliminated by elevating
the Nyquist limit, which can be achieved by
either increasing the PRF or decreasing the transducer
frequency. However, an increasing PRF may eventually
result in such shortening of the interpulse in-