They have a biconcave disc shape and a diameter of 7 µm, which is much smaller than the wavelength of ultrasound used to study blood flow. Red blood cells occupy between 36% and 54% of the total blood volume. Table 3.1 Variation of the cosθ term of the Doppler equation with the angle of insonationīlood is made up of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets suspended in plasma. The smaller the angle of insonation, the larger the frequency shift detected, but as the angle of insonation approaches a right angle, very small frequency shifts are detected. When the transducer is pointing toward the flow, a positive frequency shift is seen, but once the transducer is pointing away from the direction of flow, a negative frequency shift is seen.
Figure 3.3 shows how the detected Doppler shift frequencies change as the Doppler angle changes. When the angle of insonation is 0° (i.e., the Doppler beam is parallel to the direction of flow), the cosθ term is 1, giving the maximum detectable Doppler shift frequency for a given velocity of blood and transmitted frequency. When the angle of insonation is 90°, the cosθ term is 0, so virtually no Doppler shift is detected. Table 3.1 shows how the cosθ term varies between 0 and 1 as the angle changes from 0° to 90°. The Doppler equation shows that the detected Doppler shift depends on the angle of insonation, θ, through the term cosθ. The simplest Doppler systems can extract the Doppler shift frequency and output it to a loudspeaker, enabling the operator to listen to the Doppler shifts produced from the blood flow. In fact, it is a useful coincidence that the typical values of blood velocity found in the body and the transmitted frequencies used in medical ultrasound result in Doppler shift frequencies that are in the audible range (from 20 Hz to 20 kHz). Taking the speed of sound in tissue to be 1540 m/s, the Doppler equation can be used to estimate that the Doppler shift frequency produced will be 1.6 kHz. The factor of 2 is present in the Doppler equation as the Doppler effect has occurred twice, as explained above.Ĭonsider, for example, a 5 MHz transducer used to interrogate a blood vessel with a flow velocity of 50 cm/s using an angle of insonation of 60°. Where v is the velocity of the blood, θ is the angle between the ultrasound beam and the direction of blood flow (also known as the angle of insonation), and c is the speed of sound in tissue. The Doppler shift frequency, f d (i.e., the difference between the transmitted frequency, f t, and received frequency, f r) is given by:įigure 3.2 Simple Doppler ultrasound instruments use transducers consisting of two piezoelectric elements, one to transmit ultrasound and the other to receive the returning echoes back-scattered from the moving blood cells. 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 observed depends on the frequency of the ultrasound originally transmitted by the transducer and the velocity of the blood cells from which the ultrasound is back-scattered. 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. First, the transducer is a stationary source while the blood cells are moving receivers of the ultrasound waves ( Fig.
In this situation, the Doppler effect occurs twice. 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 moving blood cells (Fig. In the case of vascular ultrasound, the Doppler effect is used to study blood flow. DOPPLER EFFECT APPLIED TO VASCULAR ULTRASOUND