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82 D. Maulik
Table 6.3. Advantages and disadvantages of the Doppler
power or energy mode
Advantages
Increased sensitivity for detecting low-velocity
circulatory systems as encountered in splanchnic,
placental, or tumor vessels
Virtual independence of the angle of insonation
May allow better delineation of flow in vascular
channels
Absence of Nyquist effect
Disadvantages
Does not provide information on the directionality of
flow
Depth-dependent, so the greater the depth of the
vessel, the less available amplitude flow information
phase analysis of the returning echoes. Although it
was theoretically possible to produce two-dimensional
Doppler color flow mapping based on the intensity of
the Doppler signal, it is only recently that the systems
have seriously attempted to resolve the technical challenge
of producing an amplitude-based map that
would be independent of Doppler-shifted frequency.
The advantages and disadvantages of the amplitude
mode are summarized in Table 6.3 and are discussed
further below.
With spectral Doppler analysis the amplitude information
is generated by the spectral analysis, which
provides complete information on the Doppler power
spectrum. It is depicted in the so-called Z axis as
brightness of the waveforms. With color flow processing,
which is based on phase-difference analysis, the
amplitude of the Doppler signal is estimated by the
autocorrelator. The output is color-coded and is displayed
along with the gray-scale tissue images. The
Doppler energy or power display, however, does not
indicate directionality of flow in relation to the transducer.
In contrast to the frequency mode, which as is
apparent from the Doppler equation is angle-dependent,
the amplitude mode is virtually unaffected by
the angle of insonation.
The amplitude output is affected by the wall
(high-pass) filter as it removes high-amplitude/lowfrequency
Doppler signals generated by tissue movements.
If the filter setting were identical to the frequency-based
color flow mapping, the amplitude map
would offer no more flow information than the former.
Current filter algorithms, particularly those utilizing
the multivariate approach, have substantially
minimized this problem. An appropriate filter setting
is essential for optimal color Doppler amplitude
imaging (Fig. 6.15). A low threshold of wall filter is
needed for identifying low-flow states. Doppler power,
or energy, imaging is also affected by the gain
(Fig. 6.15). A high gain results in increased sensitivity
for detecting slow-velocity circulations but also in the
extension of color flow areas beyond the vascular
margin and the depiction of tissue or wall movements.
Other factors that interdependently or independently
affect the power or energy mode display
include the transmitted acoustic power, depth, color
sensitivity, preponderance of gray scale (write priority),
and persistence. The implementation of these
controls and the resultant changes in the amplitude
color maps vary from device to device.
Amplitude mapping may be helpful for identifying
blood flows of low volume or low velocity. Doppler
mean frequency shift based on color mapping may
not be sensitive enough to detect such a flow system.
Examples of such potential uses in obstetrics include
fetal echocardiography and demonstration of splanchnic
flow in fetal lungs and placenta. In gynecologic
practice, amplitude may optimize our ability to detect
flow in pelvic organs such as the ovaries (Fig. 6.16),
especially in postmenopausal women. The extent of
its clinical utility is still being explored, particularly
in assessing tumor vascularity. Three-dimensional reconstruction
of the vascular channels has been accomplished
utilizing the Doppler amplitude mode. This
feature may have potentially promising clinical application.
Applications of power Doppler sonography are
discussed in Chaps. 7, 39, and 40.
Fig. 6.15 a±e. The effects of wall filter and gain on color
Doppler amplitude imaging of fetal renal flow. a A reasonable
amplitude image of renal flow. b, c The effect of lowand
high-wall filter setting, respectively. d, e The effects of
low and high gain, respectively