Doppler Ultrasound
The doppler shift principle has been used for a long time in fetal heart rate detectors. Further developments in doppler ultrasound technology in recent years have enabled a great expansion in its application in Obstetrics, particularly in the area of assessing and monitoring the well-being of the fetus, its progression in the face of intrauterine growth restriction, and the diagnosis of cardiac malformations.
Doppler ultrasound is presently most widely employed in the detection of fetal cardiac pulsations and pulsations in the various fetal blood vessels. The "Doptone" fetal pulse detector is a commonly used handheld device to detect fetal heartbeat using the same doppler principle.
Blood flow characteristics in the fetal blood vessels can be assessed with Doppler 'flow velocity waveforms'. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the fetus. Various ratios of the systolic to diastolic flow are used as a measure of this compromise. The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries, the uterine arcuate arteries, and the inferior vena cava.
The use of color flow mapping can clearly depict the flow of blood in fetal blood vessels in a realtime scan, the direction of the flow being represented by different colors. Color doppler is particularly indispensible in the diagnosis of fetal cardiac and blood vessel defects, and in the assessment of the hemodynamic responses to fetal hypoxia and anemia.
A more recent development is the Power Doppler (Doppler angiography). It uses amplitude information from doppler signals rather than flow velocity information to visualize slow flow in smaller blood vessels. A color perfusion-like display of a particular organ such as the placenta overlapping on the 2-D image can be very nicely depicted. Doppler examinations can be performed abdominally and via the transvaginal route. The power emitted by a doppler device is greater than that used in a conventional 2-D scan. Its use in early pregnancy is therefore cautioned.
Doppler facilities are generally an integral part of modern ultrasound scanners. They merely would need to be switched on to function. One does not need to 'go' to another machine for the doppler investigations