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a Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 41
Alternative Methods of Doppler Flowmetry
Measured Instantaneous Velocity Profile
Method ±The Multigated Doppler
In this technique, the individual velocity components
across the vascular lumen are measured using multigated
pulse-wave Doppler. As in the previous technique,
it is also necessary to measure the beam-vessel
angle in order to transform the Doppler frequency
shift information into velocity data. The summation
of these velocity components constitutes the volumetric
flow. An advanced implementation of this
approach (Cardiosonix, Neoprobe Ltd, Dublin, Ohio,
USA) has been reported in which two ultrasound
beams set at a fixed known angle are used and numerous
small (< 200 mm length) sample volumes at
successive depths are collected and analyzed by fast
Fourier transform in real time [10] (Fig. 4.7). The angle
of insonation and the flow diameter are automatically
computed allowing measurement of volumetric
flow in real time. This technique is at present limited
to assessing superficial vessels such as the carotid arteries
and is not available for maternal or fetal applications.
Vector Doppler ±Multiple Beam Insonation
Doppler methods as described above measure the
component of flow velocity parallel to the vascular
axis but not the velocity components from the secondary
flows resulting from vessel curvatures, bifurcations,
and pulsatility. One of the techniques that attempts
to resolve this limitation utilizes multiple
transducers to measure multiple velocity vectors in a
circulation and is also known as vector Doppler. Systems
utilizing a single transmitter and two or more
receivers to measure the various velocity vector components
have been described [11, 12]. Experiments
using a dual-beam vector Doppler system based on a
modified linear array show that this system has low
dependence on angle and similar reproducibility to
that of single-beam systems [13]. These devices are
not available yet for clinical use in obstetrics.
Non-Doppler Flowmetry ± Time Domain
Processing Velocity Measurement
Time domain processing is an emerging field with a
tremendous potential for offering a viable alternative
to Doppler sonography. The basic implementations of
time domain processing for measuring velocity and
volume flow are briefly discussed below. Most of
these still remain experimental in nature.
Ultrasound B-Flow
As the name implies, this innovative technology is
based on B-mode ultrasound and allows imaging of
blood flow. It utilizes digital ultrasound which allows
encoding of the signals on transmission and decoding
of the returning echoes into separate tissue and blood
signals. As the echogenicity of blood is substantially
lower than that of tissue, the returning signals are
processed and equalized to enhance blood flow signals
over those from the tissue. Whereas color Doppler
duplex ultrasound imaging requires separate activation
of the transducer elements for the Doppler
Fig. 4.7. Multigated angle-independent Doppler flow measurement
as implemented in a commercial system. Schematic
representation of the FlowGuard technology. The
ADBF technology uses 2 ultrasound beams (L1and L2)
crossing the vessel lumen with a known angle (h) between
them and allows the simultaneous acquisition of hundreds
of small sample volumes (small dots) of less than 200 lm
in depth (D) along L1 and L2. This in turn authorizes accurate
determination of the flow velocity profile, vessel diameter,
and isonation angle (h). (With permission from [10])
Fig. 4.8. Ultrasound B flow imaging of blood flow in the
umbilical vessels. The flow image is simultaneously derived
along with B imaging and not superimposed. (With permission
from General Electric Health Care System, Milwaukee,
WI)