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The origins of modern medical technology may be traced to nineteenth-century europe, when the industrial revolution ushered in sweeping changes in every aspect of life. Of all the momentous discoveries and inventions of this period, there was one relatively obscure scientific event that laid the foundation for the subsequent development of Doppler technologies in the twentieth century — the discovery of a natural phenomenon that came to be known as Doppler effect. Another critical event was the discovery of the piezoelectric phenomenon by Pierre Curie and Jacques Curie, which enabled the development of ultrasonic transducers many decades later. This chapter briefly describes the origin of the Doppler theory during the nineteenth century and traces the development of diagnostic Doppler ultrasound technology during the second half of the twentieth century to the present.

This chapter presents the basic concepts of sound and ultrasound propagation and discusses the physical principles of the Doppler effect and Doppler sonography, which are essential for understanding their diagnostic uses. Although this book focuses primarily on clinical utilization of the diagnostic Doppler technology, developing an understanding of the basic principles is imperative for its proficient use. The following is a brief introduction and is not intended to be a comprehensive treatise on the subject. For a more in-depth discussion, there are several excellent textbooks that comprehensively examine the physics of Doppler ultrasonography [1–4]

Spectral Doppler ultrasound velocimetry involves systematic analysis of the spectrum of frequencies that constitute the Doppler signal. This chapter presents a general perspective on Doppler signal anlyses and describes the spectral Doppler ultrasound devices commercially available for clinical use. They include continuous-wave (CW) Doppler, pulsed-wave (PW) Doppler and duplex Doppler devices. Within the realm of obstetric usage, the application needs are diverse and require various choices of equipment. For example, fetal Doppler echocardiography requires advanced duplex ultrasound instrumentation, which combines the capabilities of high-resolution two-dimensional imaging with the PW Doppler mode and an acoustic power output appropriate for fetal application. For umbilical arterial hemodynamic assessment, simpler, substantially less expensive CW Doppler equipment with a spectral analyzer may be sufficient. It is essential therefore that one develop a basic understanding of the implementation of Doppler ultrasound technology.

The spectral Doppler power waveform contains an immense amount of hemodynamic information from the sampled circulation. As we saw in Chap. 3, the spectral information consists of three fundamental variables: frequency, amplitude, and time. Spectral frequency reflects the speed of blood flow. The amplitude approximately represents the number of scatterers traveling at a given speed and is also known as the power of the spectrum. Amplitude depends on the quantity of moving red blood cells (RBCs) in the sample and therefore reflects the volume of blood flow. The time over which the frequency and amplitude of the Doppler spectrum is therefore three-dimensional (see Chap. 3). Such a display is complex, however, and not particularly useful for clinical application. Fortunately, there are alternative approaches for effectively characterizing the spectral waveform, including two-dimensional sonograms and various frequency envelope waveforms. These approaches can be utilized to evaluate various aspects of hemodynamics of flow to downstream impedance. This chapter discusses these subjects and other related issues.

In recent years evaluation of the venous system has become a compulsory part of the haemodynamic assessment of the fetus, but the underlying mechanisms of our Doppler recordings are still incompletely understood. The present chapter is not intended to solve all that, but rather to address important hemodynamic issues from a clinical point of view to help clinicians use and interpret venous Doppler recordings. For the interested reader more extensive discussions of blood flow dynamics are available in the literature [1–5]

The introduction of Doppler color flow mapping has ushered in an exciting era for diagnostic sonographic imaging. As discussed in the relevant chapters of this book, application of this technique to obstetric and gynecologic imaging has proved highly useful. A prerequisite for optimal utilization of this technique is an in-depth knowledge of its principles and limitations. Furthermore, it is important to appreciate that the appearance of color flow images is influenced by the operational setting of the equipment, which must be taken into account for reliable interpretation. Finally, there are even newer modalities of flow imaging that present exciting possibilities for the future of noninvasive hemodynamic investigation in clinical practice.

The umbilical artery was the first fetal vessel to be evaluated by Doppler velocimetry and has since become the most widely investigated component of the fetal circulation. The attention paid to this vessel may be explained partly by its ready accessibility to Doppler interrogation, even without guidance by duplex imaging, and because it is a vital component of the fetal circulation, acting as the lifeline between the fetus and the placenta. Numerous studies have reported the methodogy of Doppler interrogation of the umbilical artery and have elucidated the factors that modulate the umbilical arterial Doppler indices under normal conditions. This chapter describes the procedure of Doppler sonography of the umbilical circulation and reviews the normative data on the umbilical arterial Doppler indices.