Avoid using angles greater than 60° and remember no Doppler shifted signals are generated at 90°. In practice, when taking measurements of blood flow, a Doppler beam angle of between 30 and 60° is important to ensure reliable Doppler shifted signals. Theoretically, when θ = 90° this means the blood flow is perpendicular to the Doppler beam, cosθ = 0 and no Doppler shifted signals will register. This corresponds to a Doppler beam which is parallel with the vessel, which can rarely be achieved in practice. Maximum value of cosθ corresponds to a Doppler beam angle of 0°.įor a constant flow velocity (V), the maximum value of cosθ and therefore the highest value of the Doppler shifted signal (F d) is at an angle of 0°. Table 11.1 Variation of the value of cosθ over a range of angles of insonation.
The value of cosθ varies with the angle from 0 to 1.
Table 11.1 shows the relationship between the angle of the Doppler beam (θ) and the value of cosθ. Very small signals are produced as the Doppler beam angle approaches a 90° angle. The smaller the angle between the Doppler beam and blood vessel, the larger the Doppler shifted signal. When the Doppler beam is pointing towards the direction of blood flow a positive Doppler shifted signal is observed, but once the Doppler beam is pointed away from the direction of blood flow a negative Doppler shifted signal is seen. 11.4 Graphically demonstrating the relationship between the Doppler shifted frequency with respect to the angle of the insonating Doppler beam
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Therefore highly sensitive and sophisticated hardware and processing software is required to ensure that these signals can be detected and processed.įig. The amplitude of the backscattered signal from blood can be smaller by a factor of between 1. It is important to appreciate that the amplitude of the backscattered echoes from blood is much weaker than those from soft tissue and organ interfaces which are used to build up our B-mode anatomical images. Therefore F r = F t and F d = F r − F t = 0, resulting in no Doppler shifted signals. When there is no flow or movement detected then the transmitted frequency (F t) is equal to the received frequency (F r). This time the Doppler shifted frequencies (F r − F t) produces a negative Doppler shifted signal. In this arrangement blood flow moving away from the transducer produces received signals (F r) which have a lower frequency than the transmitted beam (F t). The Doppler shifted signal (F d) can be calculated by subtracting F t from F r and produces a positive Doppler shifted signal.Ĭonversely, Figure 11.3b illustrates blood flow which is moving away from the Doppler beam and the transducer. In this arrangement blood flow moving towards the transducer produces received signals (F r) which have a higher frequency than the transmitted beam (F t). In Figure 11.3a the relative direction of the blood flow with respect to the Doppler beam is towards the transducer. Figure 11.3 illustrates the change in the received backscattered signals and the resulting Doppler shifts for blood moving towards and away from the transducer. 11.3 Demonstrating the resulting Doppler shifted signals for a) blood flow moving towards the transducer b) blood flow moving away from the transducerīlood flow moving towards the transducer produces positive Doppler shifted signals and conversely blood flow moving away from the transducer produces negative Doppler shifted signals.
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