2.2. Acquisition of 2D B-mode video clips of the fetal cardiac four-chamber view
As a pre-requisite for 2D-STE analysis, recording of appropriate video clips using a General Electric (GE) Voluson™ E10 ultrasound device equipped with a trans-abdominal C2-9-D convex probe (2-9 MHz) as well as the software tool fetalHQ® (GE Healthcare, Chicago, IL, USA) was performed. The following predefined ultrasound settings were applied: The fetal heart occupied almost 75 % of the screen, corresponding to an appropriate image magnification. The recording of the 2D B-mode cine-loop of the four-chamber view (4-CV) was performed for at least 3 to 5 complete cardiac cycles (approx. 3 seconds (s)), with the IVS optimally oriented as horizontally (apex perpendicular, 90°) or obliquely (apex oblique up, 45°) as possible, to visualize the total thickness of the right and left lateral walls of the ventricles as well as the IVS. Care was taken to ensure high contrast between the blood-filled ventricle and the endocardial margin. The frame rate was as high as possible, usually higher than 60 frames/s (Hertz (Hz)). Cine-loops with impairing movement artifacts or insufficient macroscopic identification of endocardial border were excluded, but, to reflect clinical routine, those with suboptimal orientation of the IVS (apex up/down; apex oblique down) were not.