This chapter provides a brief introduction to the physics and operating principles of instruments using the Doppler effect in cardiovascular diagnostics. Readers seeking more detail should consult Kremkau’s textbook, Hatle and Angelsen’s classic monograph, and Weyman’s comprehensive reference book.
Most Doppler signals of interest come from blood flow. Recent advances in Doppler tracking of tissue motion are covered briefly here, although the basic principles are the same. Because blood cells are much smaller than the wavelength of the ultrasound used, returned signals are generated only by scattering and accordingly are 100-fold less powerful (−20 dB) than the signals from specular reflectors. The strength of the reflected ultrasound signal increases with hematocrit up to 30%, after which interference becomes dominant. Strong signals from specular reflectors within the beam can occasionally be of diagnostic help (e.g., valve “clicks” that give important timing information) but are usually a nuisance that needs to be eliminated by filtering (pp. 23 and 28). Because the Doppler processing path in ultrasound machines must measure tiny frequency changes in extremely weak signals, Doppler measurements are particularly sensitive to environmental electrical noise. Fortunately, electrocautery and other noises are usually obvious on the Doppler display and are not likely to lead to misdiagnoses.
To obtain sufficient signal strength and penetration for a good Doppler signal, higher ultrasonic intensity levels and lower frequencies are used compared to those for two-dimensional (2-D) imaging. Because patients receive a much higher ultrasound dose during a Doppler examination, its duration should be limited to that necessary to obtain the desired information.
Doppler effect
The Doppler effect is defined as the apparent change (or modulation) in the frequency of waves, (e.g., sound or light) occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. It is named after Christian Johann Doppler, an Austrian physicist who first described the effect (for light) in 1842. To understand the Doppler effect, imagine the distance between successive sound wave fronts becoming squeezed when the source and receiver are coming closer together, resulting in a foreshortening of the wavelength or, equivalently, an increase in the frequency. Conversely, imagine an elongation of the wavelength of a sound wave when the source and receiver move apart, resulting in a decrease in frequency ( Figure 2-1 ).
Another way to understand the Doppler effect is to imagine a boat traveling through ocean waves. When the boat moves in a direction opposite to the wave motion, it encounters wave peaks more often (i.e., at a higher frequency) than if it were stationary. Conversely, when the boat moves in the same direction as the wave propagation, it encounters wave peaks less frequently. The change in frequency depends on the boat speed and the original frequency and speed of the waves in the water.
By combining electrical signals representing the transmitted and received acoustic frequencies, we obtain another signal with a frequency equal to the difference of the two frequencies, the so-called Doppler shift. The frequency of the Doppler shifted signal when a reflector or scatterer moves directly toward or away from the transmitter is given by the Doppler equation, which may be derived from the change in wavelength due to motion:
f D = f 0 ( 2 v c − v ) .
Here, f D is the Doppler frequency, f 0 is the transmitted frequency, v is the velocity directed toward the transducer from the scatterer, and c is the speed of propagation in the medium. The factor of 2 appears because the shift occurs twice, once due to the reflector’s motion when it receives the signal from the source and a second time when it becomes the source and sends the already-Doppler-shifted wave back to the transmitter. When the speed of the scatterer (in the heart, typically in the range of 1 m/sec) is much less than the speed of propagation in tissue, the v term in the denominator of Equation 2-1 can be neglected (i.e., c − v ≅ c ), giving the approximate form shown in most textbooks:
f D = 2 f 0 v c
For example, when a 2.5-MHz transmitted signal encounters scatterers moving at 1 m/sec, the Doppler frequency is 3247 Hz, that is, within the range of human hearing. Echocardiography machines use lower frequencies for Doppler measurements than for imaging, typically in the lower third of the useful bandwidth of the scanhead. The lower frequency increases the beam penetration, resulting in a larger reflected signal strength from weak scatterers, such as blood cells. Using a lower transmitted frequency also reduces the Doppler shift frequency proportionately. Because Doppler shifts encountered clinically lie within the audible range, Doppler instruments usually provide output to a loudspeaker, which can give important diagnostic clues to the operator. More advanced machines separate the Doppler shifts of scatterers moving toward the scanhead from those moving away from the scanhead and present the directionally distinct signals in separate channels of a stereo loudspeaker output.
The preceding equations are based upon the assumption that the scatterer motion is directly toward or away from the transducer. When the motion is oblique to the path between the transducer and the scatterer, only the component of velocity along the line connecting them causes a Doppler shift. A term with the cosine of the crossing angle between the velocity vector and the beam path must then be included in the Doppler equation, which is rearranged here to solve for the quantity of clinical interest, namely, velocity:
v = f D c 2 f 0 cos .
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