Currently available high-definition ultrasound systems yield good visualization of the fetal heart. Not only are major forms of congenital heart disease diagnosed prenatally with increasing frequency, but defects that are unlikely to be of hemodynamic significance can also be demonstrated with certainty.
Enhanced-flow (‘e-Flow’) imaging is a commercially available, high-definition color Doppler modality (Aloka Co. Ltd., Tokyo, Japan) that provides higher spatial and temporal resolution compared with conventional color Doppler or power Doppler mapping. With a wide transmission bandwidth, the transmission pulse length is short, which enhances axial resolution. Enhanced-flow distinguishes blood flow from the surrounding tissue, offers better edge definition and provides better sensitivity for low-velocity flow, but because it is more sensitive it is also more subject to motion artifact. Enhanced-flow has similarities to other imaging modalities, such as power Doppler, because it uses the amplitude component of the signals received to represent the number of moving blood cells, instead of analyzing the frequency shift of blood-velocity information[2, 3]. It has been routinely used in our unit to image the fetal heart in high-risk pregnancies (Aloka Alpha-10). Recently, we have introduced simple adjustments to machine settings that improved visualization of intracardiac flow and of flow across vessels. This allowed us to image the heart as ‘just-flow’. We wish to report this clinical applicability of e-Flow imaging, which is illustrated here in a fetus with a small ventricular septal defect (VSD).
A 34-year-old pregnant woman was referred for fetal echocardiography because of increased fetal nuchal translucency (4 mm) in the first-trimester scan. Fetal cardiac and anomaly scans were performed at 17 weeks' gestation to exclude major defects[4, 5]. No abnormalities were seen. At 25 weeks' gestation, a small, barely visible, apical VSD was observed on cross-sectional images. Bidirectional flow was demonstrated on color Doppler, enhanced-flow and pulsed-wave Doppler imaging. Additionally, by completely reducing B-mode gain whilst imaging with enhanced-flow, angiography-like images of the VSD were obtained (Figure 1; Videoclip S1). These ‘just-flow’ images further enhanced visualization of the defect and facilitated the timing of interventricular shunting. In the fetus, despite similar right- and left-ventricular pressures, interventricular shunting occurs throughout the cardiac cycle. Figure 1 and Videoclip S1 show that, in systole, shunting is predominantly left to right; this reverts to right to left at end-systole and in diastole.
By reducing overall B-mode gain during mapping, the B-mode information is not shown and greater priority is given to color when ultrasound images are displayed. This simple and practical technique produces good-quality B-mode subtracted images (‘just-flow’), as demonstrated in this case. The technique is easily applicable to other commercially available high-definition flow-imaging modalities. Additionally, other cardiac structures can also be imaged in a similar way (Figure 2), allowing enhanced visualization of the fetal heart over and above that obtained with conventional settings. ‘Just-flow’ images also resemble a rendered three-dimensional (3D) volume (Videoclip S1).
B-mode subtracted color Doppler angiography (‘just-flow’) offers a new way of looking at fetal heart structures and may help to further improve the diagnostic accuracy of prenatal diagnosis of other types of fetal congenital heart disease[6, 7].