Evolution of Doppler ultrasound in obstetric imaging: a slow(flow) step forward.

Early applications of fetal Doppler evaluation drew on parameters derived from adult vascular examination, primarily applied to identifying fetuses at risk for growth restriction and hypoxia. The integration of color Doppler into real-time imaging facilitated a shift toward structural-functional correlation, enabling clinicians to visualize fetal cardiac outflow tracts, valve regurgitant jets, and the complex umbilical-venous connections alongside standard grayscale anatomy. Power Doppler extended these capabilities to slow flow and small caliber vascular structures. Doppler modalities have been crucial in delineating the fetal cardiovascular system, increasingly detecting complex congenital heart disease (CHD) prenatally, and illuminating cardiac and non-cardiac vascular structures that previously posed a challenge to visualization, including cerebral, hepatic, and placental capillary beds. Highly sensitive Doppler technologies such as SlowflowHD as well as MV-Flow and LumiFlow, exemplify the convergence of anatomical and physiological imaging, such as fetal venous sinuses and deep cerebral venous return, to allow brain perfusion assessment at earlier gestational stages. Although SlowflowHD has been shown to have added value for diagnosis, it seems to be an underused modality. Here we show several examples drawn from our experience with SlowflowHD, in both routine scanning and anomalous cases spanning various organ systems and stages of gestation. While there is a learning curve in optimizing image acquisition and interpretation of microvascular anatomy, it should not deter practitioners proficient in Doppler techniques. We recommend incorporation of SlowflowHD and comparable microvascular imaging technologies into the sonographic armamentarium.