Ultrasonography technique
An ultrasound system (Voluson E10, GE Healthcare, Kretztechnik, Zipf,
Austria), equipped with a 2 to 9 MHz transabdominal transducer was used
in this study. Each patient was scanned by two screening sonographers
with one year of experience, independently, using gray-scale ultrasound
and different color techniques. They were both adept at the traditional
procedures of fetal cardiac transverse scanning using the 4CV, the left
and right outflow tracts view, and the 3VT view. Then, they were both
trained for three months to become familiar with scanning the added
planes included in the current study. The specific scanning procedures
are achieved by continuously moving the sound beam cephalad along the
vertical axis of the fetal thorax. When scanning the root of the great
arteries, the sound beam sections through the long-axis of the ductus
arteriosus and the right pulmonary artery, together with the short-axis
of the aorta and superior vena cava. It is the 3VV. Slight rotating the
sound beam to the left, the bifurcation of the pulmonary artery should
be demonstrated to show the normal pulmonary artery branching patterns.
It is the branch of pulmonary artery bifurcation (BPAB) view. When the
beam sections through the arch, the ductus arteriosus, and their
convergence, the 3VT view is obtained. When continuing elevating the
sound beam, it demonstrates that the superior vena cava is joined by
both the left innominate vein and the posteroanterior drainage of the
azygos vein. When the beam is turned further cephalad, it sections
across the transverse aortic arch forming a sausage-like structure
[14]. The origin of bilateral subclavian arteries (BSA) could be
identified in this plane. Scanning around the BSA, the origin of
bilateral internal thoracic arteries (BITA) from the ipsilateral
subclavian artery can be demonstrated [15]. The related anatomical
structures that should be recognized in each plane were listed in Table
1.
For both sonographers involved in the study, the first recognized
anatomical structures (including the course of the great vessels) by
gray-scale ultrasound. Then, different color-imaging techniques were
used to assess the vessels in detail, including the course and flow
direction. For sonographer A, he used Color Doppler combined with the
radiant flow (Color-R-flow modality) to show the vessels if needed,
while HD-flow combined with the radiant flow (HD-R-flow modality) was
used by sonographer B when scanning these planes. For both modalities,
the scale was set to 20-30 cm/s and 40-50 cm/s when showing the veins
and arteries respectively. The number of planes visualized was recorded
for each sonographer to make a comparison.
As cardiac apex orientation might pose some effects on the detection of
the cardiac views, we also made comparisons among different fetal
positions when necessary. Three subgroups were made according to the
supine position (cardiac apical pointing to 10-2 o’clock), the prone
position (cardiac apical pointing to 4-8 o’clock), and the lateral
position (cardiac apical pointing to 2-4 and 8-10 o’clock).