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Chapter 2
Physical Principles of Doppler Ultrasonography
Dev Maulik
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].
Propagation of Sound
Sound is a form of mechanical energy that travels
through solid or liquid media as pressure waves
(Fig. 2.1). Sound waves are generated when an object
vibrates in a medium. For example, percussion causes
a drum membrane to vibrate and to generate sound
waves in the air. During vibration the forward movement
of the sound source causes a pressure rise in
the adjacent medium, so the molecules of the medium
become crowded. As the source moves backward
there is a pressure drop in the medium, so the molecules
now move apart. This phenomenon of alternating
molecular compression and rarefaction accompanies
the waves of sound energy as they propagate
along the medium (Fig. 2.2). Although the molecules
vibrate, they remain in their original location and are
not displaced. Sound travels faster in solids than in
liquids and faster in liquids than in gases.
Although usually considered in a unidimensional
plane, in reality sound is transmitted in a three-dimensional
space. Sound waves from a vibratory
source or from a reflector are moving surfaces of
high and low pressures. These surfaces are called
waveforms. The shape of a waveform depends on the
shape of the source or the interface. Thus a plane
waveform emanates from a flat source and a spherical
waveform from a spherical source. With Doppler
ultrasonics, the scattered waveform is spherical, as
red blood cells (RBCs) behave as spherical sources
during the scattering of an incident beam.
Fig. 2.1 A, B. Propagation of sound. A Passage
of a sound pulse through point ªAº in
the medium. B Consequent changes in the
pressure at that point