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 ā€œVā€ 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.