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Chapter 11
Fetal Descending Aorta
Karel MarsÏ—l
The first reports on Doppler recordings of blood
velocity signals from the umbilical artery [1, 2] stimulated
attempts to record blood flow in fetal vessels.
In 1979 Gill presented a method combining a quasireal-time
imaging and pulsed-wave Doppler technique
for estimating the volume flow in the intraabdominal
part of the umbilical vein [3]. The ultrasound system
used (Octoson, Ultrasonic Institute, Millers Point,
Australia) did not allow the recording of high blood
velocities owing to the low repetition frequency of
the Doppler ultrasound pulses (2 kHz) necessitated
by the long distance between the ultrasound transducers
emerged in a water bath and the target vessel.
Eik-Nes et al. [4] applied the principle of intermittently
using two-dimensional and pulsed-wave Doppler
ultrasound by combining a linear array scanner
and a 2-MHz Doppler velocimeter [4]. The two transducers
were mounted firmly to a unit with an inclination
of the Doppler transducer of 458 to the linear array
transducer. By placing the linear array transducer
on the maternal abdomen so it was parallel with a
sufficiently long portion of the vessel of interest, insonation
by Doppler ultrasound under a known angle
(458) was achieved and thus the possibility of correcting
the recorded mean blood velocity for the insonation
angle (Fig. 11.1).
With the above arrangement, the Doppler transducer
could be located relatively close to the fetal body,
and so a high repetition rate (9.75 kHz) of Doppler
pulses could be used. This technique allowed velocities
up to 1.7 cm/s to be detected down to a depth of
6.5 cm [5], and the first recordings of blood velocity
signals from the fetal descending aorta were obtained
[4]. Unfortunately, during these first recordings a
high-pass filter with a cutoff frequency of 600 Hz was
employed, in agreement with the experience from
Doppler recordings in the adult aorta, where it is necessary
to eliminate disturbing low-frequency Doppler
shift signals from the vessel walls. This technique
caused an erroneous appearance of the fetal aortic
velocity waveforms noted in the two first reports
[4, 5] with only systolic velocities present. Today such
wavefroms would be recognized as highly abnormal
[6]. The estimated time-averaged mean velocity was
also influenced by erroneous high-pass filtering of
Fig. 11.1. Doppler velocimetry of fetal descending aorta:
original method combining a linear array real-time scanner
and a 2-MHz pulsed Doppler instrument according to Eik-
Nes et al. [4]. The Doppler transducer is firmly attached to
the linear array transducer at an angle of 458 (a)
Doppler signals; consequently, the calculated values of
the volume flow cannot be considered reliable.
The Malmæ research group, at that time including
Sturla Eik-Nes, evaluated and further developed the
Doppler method for estimating fetal aortic blood
flow. In an experimental study comparing the electromagnetic
and Doppler measurements of aortic flow
in pigs, the artificial distortion of the aortic signals
was recognized [7]. This discovery enabled correction
of the original reports on fetal aortic flow and the
recommendation of not using a high-pass filter with
a cutoff frequency higher than 100 Hz.
For recording Doppler shift signals from the fetal
descending aorta, a duplex system combining pulsed
Doppler ultrasound and real-time imaging is necessary
to localize the vessel and to precisely position the sample
volume. Duplex ultrasound systems employing a
sector scanner are most often used for Doppler examinations
of the fetal circulation. They are less suitable
for fetal aorta examination owing to the difficulty of
ensuring an acceptable (i.e., < 608) insonation angle.
Figures 11.2±11.4 demonstrate the problems of obtaining
a good image of a sufficiently long portion of the
fetal descending aorta and at the same time an acceptable
angle of insonation by Doppler ultrasonography.