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42 D. Maulik
and for the B-mode imaging (see Chap. 6), B-flow
approach utilizes the same B-mode scan for both the
tissue and the flow imaging. This allows B-flow to
achieve high-resolution images at high frame rates
without the constraints of Doppler sonography such
as aliasing, angle dependence, wall filtering, and the
need for complex controls. The safety concerns are
also minimized because the acoustic intensity of B-
mode ultrasound is lower than that of Doppler. This
new modality holds immense potential for clinical
application in the perinatal field. An example B-flow
image of the umbilical vessels is shown in Fig. 4.8.
Time Domain A-Line Echo Tracking
One of the first commercial implementations of time
domain processing, called color velocity imaging
(CVI), measured blood flow velocity by determining
the distance traveled by a group of RBCs during a
known time period (Fig. 4.9) [14±16]. The method is
based on cross-correlating the consecutive returning
A-line echo signals from an RBC group moving in
the beam path. As the interval between the pulses is
very short, the cells in the group maintain the same
spatial inter-relationship so that their echo signatures
remain approximately identical. This allows determination
of the distance traveled by the RBC cluster in
a given time and therefore the velocity. Volumetric
flow is obtained by measuring the spatial average velocity
across the vascular lumen and multiplying this
value by the cross-sectional area of the functional
vascular lumen derived from M-mode tracking of the
vessel (Fig. 4.10). The technique was employed also to
generate real-time color flow mapping. In vitro and
in vivo (animal) validation studies of the technique
have confirmed the precision and accuracy of the
technique in measuring velocity and volumetric flow
[17]. For velocimetry, both CVI and Doppler methods
showed low variance, low intrarater variability (0.03%
and 0.09% respectively), high reliability coefficients
(97% and 96%, respectively), and a significant correlation
(r =0.96; p