o_1977r8vv9vk1ts2ms0kd8pksa.pdf

78 D. Maulik
Fig. 6.8a±d. Doppler color gain and wall filter. The examples
illustrate the effect of variations in color gain and wall
filter on color flow imaging of the fetal heart. The gain,
wall filter, and other settings of the image are displayed at
the left upper margin. a Echogram at 74% color gain and
medium level of wall filter. b Image at 86% color gain and
at the same level of wall filter and other settings. Note the
overflow of color and the appearance of color speckles.
c, d Effects of high and low filters, respectively, with the
color gain and other attributes maintained at the same
level as the image in a
or flow mapping, which may be helpful for identifying
slow-flow circulations (see below).
Limitations of Doppler
Color Flow Mapping
Despite the impressive technologic innovations of
Doppler color flow ultrasonography that have revolutionized
noninvasive cardiovascular diagnosis, there
are important limitations of the method that should
be considered. These limitations are inherent in the
physical principles and the engineering implementation
of the method. They are also responsible for
various artifacts that one may encounter during use
of this technique. An understanding of these limitations
and artifacts is essential for the appropriate use
and interpretation of Doppler color flow mapping.
The following section discusses these factors, which
include aliasing, range ambiguity, temporal ambiguity,
problems related to the angle of insonation, tissue
ghost signals, and mirror imaging.
Aliasing
As color flow mapping is based on pulsed Doppler
insonation, its ability to measure the maximum frequency
shift without ambiguity is limited by the PRF
rate of the system. The threshold level of Doppler
shift beyond which the measurement becomes ambiguous
is called the Nyquist limit, and the phenomenon
of Doppler shift ambiguity beyond the Nyquist limit
is called the Nyquist effect, or aliasing. This phenomenon
is known as aliasing because it falsely depicts
the frequency shift in terms of both magnitude and
direction. Aliasing in spectral pulsed Doppler sonography
is discussed in detail in Chap. 3.
Aliasing in a color flow system is shown in a spatial
two-dimensional plane in which the aliased flow