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a Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 37
First Moment Envelope
Of the additional definitions of the Doppler envelope,
the first moment of the Doppler power spectrum is
of particular interest. The Doppler-shifted frequency
is proportional to the speed of the backscattering erythrocytes,
and the power of a Doppler spectrum is
proportional to the density of erythrocytes moving at
a corresponding speed. On this theoretic basis, Saini
and associates [1] proposed that being a summation
of the power-frequency product the first moment
represents the integrated cell count-velocity product
within the sample volume and is therefore indicative
of instantaneous volumetric flow. Maulik and colleagues
[2] investigated the potential of the first moment-derived
Doppler waveform from the umbilical
artery and observed that the first moment-based pulsatility
index was twice that derived by the maximum
frequency shift envelope. This phenomenon may be
explained by the relative flattening of the velocity
profile during early systole and by changes in the arterial
diameter.
The utility of first moment remains uncertain.
Although this approach contains more hemodynamic
information, its practical application is limited by the
impracticality of ensuring uniform insonation. Moreover,
as pointed out by Evans and coinvestigators [3],
the total power of the Doppler spectrum may be affected
by many factors other than the vascular crosssectional
dimension.
Hemodynamic Information
from Doppler Sonography
The Doppler frequency shift estimates, but does not
directly measure, blood velocity. Doppler ultrasound
can generate a wide range of hemodynamic information,
including the presence of flow, directionality of
flow, flow velocity profile, quantity of flow, and the
state of down-stream flow impedance.
Flow Detection
Fig. 4.3. Color Doppler identification of umbilical vessels.
Left: Gray-scale imaging shows apparently cord-free amniotic
fluid space. Right: Color Doppler interrogation shows
the presence of the umbilical cord in this space
Identification of the presence of blood flow is one of
the common uses of Doppler ultrasound. Pulsed duplex
Doppler insonation, especially with the two-dimensional
color flow mapping mode, is capable of
identifying flow that has multiple utilities in clinical
practice. For example, color Doppler insonation is
used to identify loops of umbilical cord in the amniotic
cavity, especially when gray-scale imaging is inconclusive
(Fig. 4.3). This information is useful for
conducting invasive procedures, such as amniocentesis
or cordocentesis, and for evaluating the amniotic
fluid volume. In the human fetus, detection of anomalous
flow using pulsed Doppler echocardiography
may significantly supplement the two-dimensional
echocardiographic technique for diagnosing cardiac
abnormalities (see Chap. 33). For example, Doppler
ultrasound detection of flow in an echo-deficient area
adjacent to the main pulmonary artery of the fetus
was noted to assist the diagnosis of congenital aneurysm
[4].
A major application of duplex Doppler flow identification
is the diagnosis of proximal vein thrombosis. An
absent or anomalous flow in a proximal vein suggests
complete or partial occlusion. The technique has been
used to assess peripheral vascular competence with
varying degrees of success. However, caution should
be exercised when interpreting failure to detect Doppler
shift because in this circumstance the flow may
be present but not within the sensitivity of the Doppler
device. The maternal and fetal placental circulation offer
prime examples of this problem. Although these circulations
carry an immense amount of blood flow,
Doppler interrogation fails to produce any recognizable
Doppler shifts from most of the placental mass. Recent
introduction of the power mode of color Doppler ultrasound
has significantly improved the capability of recognizing
slow- and low-flow states. This subject is
further discussed in Chap. 6.
Flow Direction
The demodulation technique allows determination of
the directionality of flow; forward and reverse flows
are displayed on the opposite sides of the baseline.
Flow directionality provides basic hemodynamic information
that can be of significant clinical utility.
For perinatal applications, flow directionality allows
assessment of the status of continuing forward flow
during end-diastole in umbilical and uteroplacental
circulations. Absence or reversal of this end-diastolic
forward flow is of ominous prognostic significance
and is further discussed in Chap. 25. Depiction of