Discussion
The detection of CHD remains a great challenge during routine obstetric
screenings. It is realized that some severe CHDs could be identified by
fetal echocardiography early in the 1990s [16-17]. Initially, only
the 4CV was used to determine whether a CHD was in existence in the
circumstance of asymmetrical chambers, large-in-sized VSD/AVSD, or
space-occupying lesions [18-20]. Obvious limitations had been shown
for this modality as it could only identify anomalies at the atrial and
ventricular level. The situation had been improved impressively when the
left and right outflow tract views were included in the routine
diagnostic procedures besides the 4CV, which was supported by the fact
of a significantly higher detection rate for conotruncal anomalies
[19, 21-23].
Recently, the new fetal echocardiography guideline had shown its
advantage in detecting more than 90% CHDs in the uterus, as documented
by several teams and regional studies. The addition of the 3VT view
apparently contributes to the improved performance in this new guideline
[24]. It was a great development to evaluate fetal great arteries
anatomy as the characteristic sonograms in this specific plane are apt
to recognize by general examiners. The ductus arteriosus, aortic arch,
and their convergence in a āVā configuration could be visualized on
the left side of the trachea. In fact, most arch anomalies could be
demonstrated by this plane, such as double aortic arch, right aortic
arch, aberrant left/right subclavian artery, etc [25-26]. We proved
a very high detection rate for this plane by screening the sonographers
of one-year experience.
Unfortunately, many screening sonographers concern much on the 3VT view
while anatomical structures between the level of the outflow tracts and
3VT planes are not noticed. When gradually elevating the sound beam from
the outflow tracts views, it firstly sections through the pulmonary
trunk and its branches, and then the convergence of the arch and the
ductus arteriosus. We previously reported that it is crucial to
searching around the 3VV to exclude malformations associated with the
aortic origin of the left/right pulmonary artery [27]. Also, in the
case of the pulmonary sling, the abnormal origin together with the
retroesophageal course of the left pulmonary artery from the right
pulmonary artery could only be shown at the 3VV while it could not be
shown at the 3VT view. We added these 2 cardiac planes in the current
study to explore the feasibility in routine screenings. To our point,
the 3VV is relatively easy to obtain while it is very difficult to show
the BPAB view. A learning curve is needed for scanning. The pulmonary
branching patterns could better be identified by gray-scale ultrasound
when searching along the pulmonary trunk, as far as our experience is
concerned. Though the addition of color imaging helps to determine the
flow direction of the vessels, it does not seem to help much in
improving the detection of the branching when searching along the
pulmonary trunk.
However, color techniques are necessary to investigate small or tiny
vessels, as the vascular wall is too thin to be visualized by gray-scale
ultrasound. Color Doppler has long been used in echocardiography while
it cannot show vessels with a low velocity with a good effect. Doppler
shift can be immensely affected by the angle of the sound beam which
contributes to the low sensitivity in showing small vessels with low
velocity. The introduction of HD-flow is a great innovation in fetal
color imaging. It shows a higher sensitivity to display blood flow in
better perfusion than the traditional Color Doppler technique
[11-13]. However, excessively higher sensitivity may lead to color
overflow and thus decrease the spatial resolution of the vessels. The
R-flow technique could display color flow in a three-dimensional effect
by adding an algorithm upon initially Color/HD-flow. When using
HD-R-flow, the vessels can then be displayed with both high sensitivity
and high resolution with a sharp edge.
Technical renovations can further promote the launch of a new and
improved protocol for fetal cardiac scanning. Aorta, the pulmonary
artery, and the superior vena cava are the three great vessels which
should be scanned according to the current guidelines [6]. Other
smaller vessels are existing in the upper mediastinum, such as the
tributaries of superior vena cava (SVC), branches of the aortic arch
(i.e. left/right carotid artery, left/right subclavian artery), and the
internal thoracic artery. We tried to explore these vessels in routine
cardiac scans and thus to screen more malformations in potential. The
application of the new color technique makes it possible to approach the
skill for the detection of these additional vessels with small size and
low-velocity.
Three planes above the level of 3VT view were added in the current
study. Anatomically, the right innominate vein goes down straight and
continues to be the SVC, when the LIV joins it. AzV is a small-sized
vessel that serves as a little tributary of SVC. It is hard to be
identified using Color Doppler in the past while it is relatively easy
to show in many fetuses when scanned by HD-flow. In the case of
supracardiac anomalous pulmonary venous connect, the flow of pulmonary
veins may drain into the LIV via the vertical vein. The examiner may
find the indirect sign of a dilated LIV and thus to trace its origin to
make a diagnosis [28]. In other cases, when the LIV is not shown on
this plane, the examiner should consider the possibility of a left SVC.
If a dilated AzV is in visualization in this plane, the possibility of
an interrupted inferior vena cava might be in consideration [29].
For the BSA plane, it is useful to make the diagnosis of an aberrant
left/right subclavian artery, in which the bilateral subclavian arteries
arise from almost the same position at the proximal descending aorta.
This is very different from normal fetuses, in which the origins of the
left and right subclavian artery were located at the two ends of the
arch. At 3VT view, AzV sometimes may be misrecognized as an aberrant
right subclavian artery. The scanning of the BSA plane helps to make a
differential diagnosis. In addition, the size of the thymus could be
assessed by the BITA plane since it is surrounded by BITA. A small-sized
or disappeared thymus at this plane may suggest the possibility of
Digeorge Syndrome [30].
At last, we want to comment on the efficacy of the detailed scanning
procedure performed by the screening sonographers. HD-R-flow apparently
showed a better performance than color-R-flow in showing small vessels
in the three planes above the 3VT level. The difference between the two
color-imaging modalities may be related to different degrees of
influence by Doppler shift as the difference presents only for fetuses
with apical/lateral insonation. BRAVO et al. reported a higher detection
rate for BSA and BITA plane than our results [31]. An
echocardiographer apparently has much more experience than a screening
sonographer with only one-year experience. However, we can expect a long
term training session may improve the detection of these added planes.
For all the 6 planes involved in the current study, the detection rate
is above 70%, suggesting a potential wide clinical usage in the future.