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KeywordsĮvans DH, McDicken WN, Skidmore R, Woodcock JP. Of the various types of duplex transducers, the convex sequential array and the linear phased array are commonly used in obstetrics and gynecology. Duplex Doppler ultrasonography combines two dimensional imaging with the Doppler modality that allows image-guided interrogation of vascular targets. However, continuous-wave Doppler devices are seldom used at present for fetal investigation. Each approach has significant advantages and limitations. Doppler interrogation is performed by either continuous-wave Doppler transducers, which do not have range resolution, or pulse wave transducers, which are capable of sampling a specific vascular location. In addition, appropriate filtering is performed to eliminate the low-frequency/high-amplitude signals from tissue-sources and the high-frequency noise signals from the device. The steps involve reception and amplification of the returning signals, separation of the Doppler-shifted frequencies and determination of the directionality of flow in relation to the transducer, and spectral processing, which sorts the frequencies of the signal and determines their amplitude content. Spectral Doppler analysis systematically processes these signals to generate clinical utilizable Doppler waveforms. The backscattered echo signals from blood flow consist of the carrier and the Doppler-shifted frequencies along with noise generated from various sources.
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