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).