Page 33 - Libro vascular I

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PERIPHERAL VASCULAR ULTRASOUND
proportional to the relative velocities of the source and the observer.
History behind the discovery of the
Doppler effect
This effect was first described by an Austrian physi- cist named Christian Doppler in 1842. He used the Doppler effect to explain the ‘color of double stars’. A rival Dutch scientist working at the same time tried to prove Doppler’s theory wrong by hiring a train and two trumpeters. One trumpeter stood on the train while the other stood by the track, and an observer compared the pitch of the trumpeter who passed by on the train with that of the stationary trumpeter. This experiment actually verified Doppler’s theory, although Doppler’s use of this effect to explain the ‘color of double stars’ was actually incorrect. The Doppler effect is very important in modern cosmology, as it is used to estimate the velocity of stars, which shows that the universe is expanding.
DOPPLER EFFECT APPLIED TO VASCULAR
ULTRASOUND
In the case of vascular ultrasound, the Doppler effect is used to study blood flow. The simplest Doppler ultrasound instruments use transducers consisting of two piezoelectric elements, one to transmit ultrasound beams and the other to receive the returning echoes back-scattered from the mov- ing blood cells (Fig. 3.2). In this situation, the Doppler effect occurs twice. First, the transducer is a stationary source while the blood cells are mov- ing receivers of the ultrasound waves (Fig. 3.1B). The ultrasound is then back-scattered from the blood cells, which now act as a moving source, with the transducer acting as a stationary observer (Fig. 3.1D). The Doppler shift observed depends on the frequency of the ultrasound originally trans- mitted by the transducer and the velocity of the blood cells from which the ultrasound is back- scattered. The observed frequency also depends on the angle from which the movement of the blood is observed (i.e., the angle between the ultrasound beam and the direction of the blood flow). The Doppler shift frequency, fd (i.e., the difference
Source
ft
Observer
Detected frequency
ft
ft
ft
ft
ft
A
B
C
D
E
Figure 3.1
v
v
The Doppler effect is the change in the observed frequency due to motion between the source and the observer. A: The source of the sound and the observer are both stationary, so the observed sound has the same frequency as that transmitted. B: The source is stationary and the observer is moving toward the source (with velocity v), so that the observer witnesses a higher frequency than that emitted. C: The observer is moving away from the source, so the frequency detected is lower than that emitted. D: The source is moving toward a stationary observer, so the detected frequency is increased. E: The source is moving away from the observer, thus decreasing the frequency observed.