PRINCIPLES OF DOPPLER ECHOCARDIOGRAPHY
Doppler color flow imaging has evolved as the method of noninvasively imaging blood flow through the heart by displaying flow data superimposed on the two-dimensional (2D) echocardiography image. Although 2D echocardiography provides structural or anatomical details, Doppler imaging (spectral, color flow, and tissue Doppler) is extensively utilized for physiological and hemodynamic information, thus allowing identification of valvular, congenital, and other forms of heart disease.
Doppler echocardiography provides an accurate assessment of speed and direction of motion of blood and myocardial tissue utilizing the Doppler effect. Therefore, it allows assessment of valves; quantification of stenosis or regurgitation; and calculation of filling pressures, cardiac output, and intracardiac shunts. It has largely replaced conventional cardiac catheterization for many such hemodynamic assessments.
The Doppler effect is characterized by a change in the apparent frequency of a wave as a result of relative motion between the observer and the source. It is attributed to Johann Christian Doppler, an Austrian mathematician and scientist, who in his 1842 paper titled, “On the Colored Light of the Binary Stars and Some Other Stars of the Heavens,” was the first to notice the colored appearance of stars, which emitted blue light while moving toward the earth and red light while moving away from earth. Thus, he postulated that the observed frequency of a wave depends on the relative speed of the source and the observer. Although Doppler himself never extended the principle to other natural phenomena, the Doppler effect can be observed for any type of wave, including water waves, sound waves, light waves, and electromagnetic waves.
The Doppler effect can be illustrated by a common phenomenon: when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer, the pitch of the siren sound (a measure of the siren's frequency) varies as high pitch when approaching, actual pitch at the point of passing, and low pitch after the car passes the observer. The relative changes in frequency can be explained as follows: when the source of the sound waves is moving toward the observer, each successive wave is emitted from a position closer to the observer than the previous wave, reducing the time taken to reach the observer. As a consequence, the distance between successive wave fronts is reduced leading to clustering, or “bunching together,” and resulting in an increase in the frequency. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, increasing the arrival time between successive waves, thus “spreading them out” and reducing the frequency. Similarly, when evaluating blood flow, the frequency (cycles/second) of the reflected sound waves increases when the red blood cells are moving toward the transducer and decreases when red blood cells are moving away from the transducer (Fig. 5-1).