Chapter 13: THE ECHOCARDIOGRAM
DOPPLER ECHOCARDIOGRAPHY: PRINCIPLES AND APPLICATIONS The Doppler Principle
Although 2D and M-mode echocardiography provide abundant information about cardiac structure and movement, they supply no direct data concerning blood flow. This is a significant limitation, as the presence and severity of conditions such as valvular regurgitation and intracardiac shunting can be suspected or inferred only indirectly by 2D imaging. Using the principle first delineated by the physicist Johann Christian Doppler,50 one can use ultrasound to determine the velocity and direction of blood flow by measuring the change in frequency produced when sound waves are reflected from red blood cells.51-53 In this way, information regarding the presence, direction, velocity, and turbulence of blood flow can be acquired by cardiac ultrasound.54
The Doppler principle states that when a sound (or light) signal strikes a moving object, the frequency of that signal will be altered, and the increase or decrease in frequency will be proportional to the velocity and direction at which the object is moving. This is illustrated in Fig. 13-24. If a stationary transducer at the apex emits a sound wave with a transmitted frequency of fo and the wave is reflected by nonmoving red blood cells (RBCs) in an isovolumic phase of the cardiac cycle, then the received frequency fr will be identical to fo. If the signal is reflected by RBCs that are moving toward the transducer, as through the mitral valve in diastole, the returning waves will be compressed so that fr will be greater than fo. Conversely, if the target RBCs are moving away from the transducer, as in the outflow tract in systole, the returning sound waves will be elongated and the received frequency will be decreased. Of importance, the magnitude of change in the received frequency is directly related to the velocity at which blood is flowing toward or away from the transducer.53 If the velocity of sound and the angle 8 between the direction of RBC flow and the beam path are known, then the velocity of the RBCs is described by the Doppler equation:
where fd is the frequency shift recorded, fo the transmitted frequency, and c the velocity of sound. Note that the denominator is doubled because the sound wave does not originate with the RBC but must travel back and forth from the transducer. By measuring Doppler shift frequencies, the velocity and direction of blood flow can be calculated, displayed, and recorded.
The angle between the direction of blood flow and the course of the sound beam is a most important factor in Doppler ultrasound Fig. 13-25). Velocity is a vectorial entity, having magnitude and direction, and Doppler will detect only those velocities parallel or near parallel to the interrogating signal. Since the relationship between velocity and the angle is a cosine function and the cosine of angles up to 20 degrees is 0.9, little error is introduced within this range.53 Because the processor that calculates blood velocity assumes that the angle is 0 degrees, however, considerable errors occur when it is greater than 20 degrees. Moreover, the angle of incidence in 3D space usually cannot be determined with certainty from 2D echocardiographic images. Therefore, in order to obtain accurate velocity determination by Doppler, it is crucial to position and direct the transducer so that the beam is as parallel to flow as possible.
In clinical use, the frequency of transmitted ultrasound is in the range of 2 to 7 MHz, the velocity of sound in tissue is approximately 1540 m/s, and the Doppler shift frequency is relatively small (approximately 1 to 4 kHz) as compared with the transmitted frequency. As the Doppler shift frequencies are in the audible range, a speaker integrated into the Doppler echocardiography system can present them as an audible signal. Normal signals are tonal or musical. The Doppler shift also can be presented graphically to provide a hard copy printout and enable measurement.
-H ; Figure 13-26 shows the typical graphic pulsed Doppler pattern of normal systolic blood flow through the RV outflow tract into the pulmonary artery, with flow velocity on the y axis and time on the x axis. The location and size of the area from which Doppler recordings are derived is determined by the operator by positioning a sample volume on the echo image. The absence of flow is represented by the zero or no-flow line, termed the baseline. By convention, flow toward the transducer is displayed above the baseline and flow away from the transducer is displayed below the baseline. The velocities above and below baseline represent flow toward or away from the transducer and not forward or backward in the circulation. Because of the effects of viscous friction, the sample volume almost invariably includes RBCs flowing at slightly different velocities. Even normal laminar blood flow in the great vessels varies in velocity across the lumen, as RBCs in the center of the vessel move at higher velocity than those exposed to viscous friction at the wall, and this creates a parabolic rather than a flat flow profile. Therefore, any returning Doppler shifted signal contains a spectrum of velocities, each of which can be displayed by means of fast Fourier transform analysis. The graphic output of the Doppler signal displays the range of velocities within the sample volume site at any time in gray scale and the number of RBCs moving at any velocity as relative intensity. Normal laminar flow is characterized by a uniformity of velocity and direction of individual RBCs, and therefore a narrowly dispersed signal, while disturbed or turbulent flow is manifest by marked variability in velocity and direction and therefore a broad signal, which is multitoned, dissonant, and harsh.
Recently, echographs have been modified to enable recording of the low-velocity, high-amplitude Doppler signals produced by moving tissue as well as those of RBCs. The ability to asses tissue velocity provides an evaluation of transmural rate of contraction and relaxation.53^ Also, Doppler tissue recordings permit assessment of regional function, and appear to be less susceptible to the influence of LV loading conditions than are Doppler blood flow recordings.531',53'
Time-velocity spectral recordings of blood flow are generally obtained with two types of Doppler interrogation: continuous wave and pulsed wave (Fig. 13-27).5455 In the continuous-wave (CW) mode, sound waves are both transmitted and received continuously. This requires two piezoelectric crystals in each transducer, one for transmitting and one for receiving. Because all flow velocities along the beam are recorded, CW Doppler cannot define individual signals at specific distances from the transducer-a problem referred to as range ambiguity Continuous-wave Doppler, however, has no upper limit of velocity that can be accurately recorded. Thus, a CW Doppler beam can accurately measure the direction and velocity of overall flow but cannot discern the precise site of origin of individual components within the signal (Fig. 13-
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