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Definitions
Aliasing: aliasing of a Doppler signal occurs when the Nyquist limit has been exceeded. This resulting "wrapped around" appearance of either the pulsed Doppler or color Doppler signal leads to ambiguity in the direction and velocity information being displayed.
Angle of insonation: the angle between the long axis of a vessel and direction of the ultrasound beam. This value is dependent upon fetal position in relation to transducer placement.
B-flow: an angle-independent technique by which blood flow is detected by analyzing the signal amplitudes of moving red blood cells within vessels. This technique uses a subtraction algorithm that displays only moving blood flow but does not display stationary structures such as the vessel wall.
CDUS/PDUS: color Doppler ultrasonography/power Doppler ultrasonography.
Glassbody rendering: a three-dimensional volume rendering technique that displays vessels using color or power Doppler ultrasonography. These vascular structures are simultaneously displayed with surrounding anatomic structures that are reconstructed from variably transparent gray-scale voxels.
Nyquist limit: the theoretical limit to the rate that is required to sample an ultrasound signal that contains data with a specified maximum frequency. It represents the highest frequency that can be coded for a given sampling rate (PRF) in order to fully reconstruct the ultrasound signal. If the frequency is greater than half the sampling frequency, the Nyquist limit is exceeded.
Pulse repetition frequency (PRF): the number of transmitted pulsed Doppler pulses per second. The greatest velocity that can be measured by a specific transducer depends on the distance between the probe and vessel, as well as the PRF.
Umbilical coiling index (UCI): the UCI is obtained by dividing the total number of complete vascular coils by the umbilical cord length. Hypercoiling has been defined as a UCI >0.3 coils/cm, whereas hypocoiling as a UCI <0.1 coils/cm.
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Screening ultrasonography is generally performed with gray- scale two-dimensional (2D) ultrasonography, whereas more advanced studies can now utilize a vast array of technologies including three-dimensional (3D) ultrasonography, color Doppler sonography, power Doppler sonography, pulsed-wave Doppler sonography, continuous-wave Doppler sonography, tissue Doppler sonography, and B-flow imaging. Most of these techniques are mastered by experienced operators only, but color Doppler (CDUS) and power Doppler (PDUS) ultrasonography can also be used during the screening examination to reduce examination time and increase diagnostic confidence, especially in the case of an inadequate acoustic window (mainly due to maternal obesity).
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Recently, CDUS/PDUS have also been used with 3D and four-dimensional (4D) techniques to display the vascular architecture of several organs and of the heart. In this chapter, the use of CDUS/PDUS is demonstrated to provide sonologists with some useful tools to use in their everyday practice. The use of 3D and 4D CDUS/PDUS for the prenatal diagnosis of congenital heart disease and vascular abnormalities is also discussed.
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In the advanced prenatal diagnostic setting, CDUS/ PDUS can be used in several ways and with different objectives, as listed below.
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To demonstrate normal/abnormal fetal cardiovascular anatomy. ...