Doppler sonography is used for non-invasive assessment of circulation in many clinical conditions. This technique has been used for studying most of the major fetal circulatory systems, including the umbilical artery (UA) , umbilical vein , aorta , heart , and middle cerebral artery . Doppler sonography provides a unique opportunity to investigate human fetal hemodynamics and to use these findings for fetal surveillance.
This topic will discuss Doppler sonography of the umbilical artery. In high-risk obstetrical populations, meta-analyses of randomized trials have shown that fetal umbilical artery Doppler is an effective test for improving perinatal mortality and morbidity .
PHYSICS OF DOPPLER ULTRASOUND
Doppler frequency shift refers to the change in frequency of energy wave transmission observed when relative motion occurs between the source of wave transmission and the observer. An ultrasound (U/S) beam encountering circulating blood is scattered by millions of red cells, which cause the incident beam to undergo a frequency shift proportional to the speed of red cell movement (ie, blood flow velocity). This relationship is expressed in the Doppler equation (figure 1) . If the Doppler shift and the angle of beam incidence are known, and assuming that the transducer frequency and the velocity of sound in tissue remain relatively constant, then the velocity of blood flow can be determined from the following modification of the above equation: v = fd.c / 2 ft.cos q . This equation forms the basis for clinical application of the Doppler principle.
MODALITIES OF DOPPLER ULTRASOUND
Clinical implementations of Doppler ultrasound technology for assessing circulation include continuous wave (CW), pulsed wave (PW), and color Doppler. CW and PW Doppler modes are also known as spectral Doppler. These modalities can also be used to evaluate tissue movement and strain, and are known as tissue Doppler imaging.
Continuous wave — Continuous wave Doppler ultrasound is used extensively in obstetrics for determining and monitoring the fetal heart rate, but cannot identify the exact location of the origin of the Doppler signals in the ultrasound beam path. Free-standing CW devices with a spectral analyzer had been used for insonating the umbilical arteries for velocimetric applications, but PW Doppler has replaced these devices.