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velocities, especially at depth, some scanners will allow a ‘high PRF’ mode to be selected. This allows more than one pulse to be ‘in flight’ at a given time. The higher PRF allows higher velocities to be measured, but it also introduces range ambiguity (i.e., a loss of certainty as to the origin of the Doppler signal). In this mode, the scanner will typ- ically show more than one sample volume dis- played on the scan line. Using a lower transmitted frequency would produce a lower Doppler shift frequency. This lower frequency would not require as high a PRF to prevent aliasing. Therefore, reduc- ing the transmit frequency would increase the max- imum velocity that could be measured.
Limitations of CW versus pulsed Doppler
CW Doppler and pulsed Doppler have different limitations. There is no upper limit to the velocity of blood that can be detected by CW Doppler, but no information is available regarding the depth of the origin of the signal, and it is not always possi- ble to detect arterial flow without the venous flow from a nearby vein also being detected. CW is most commonly used in simple hand-held systems to lis- ten to blood flow, enabling ankle blood pressures to be measured (see Ch. 9) or for fetal heart detec- tion. It can also be used in cardiology to allow high velocities to be measured through the heart valves. Pulsed Doppler provides information regarding the origin of the signal, enabling detailed studies of a specific vessel; however, this restricts the maximum velocity that can be detected. Pulsed Doppler is used in duplex systems for both spectral Doppler and color flow imaging.
DUPLEX ULTRASOUND
Duplex ultrasound systems, combining pulse echo imaging with Doppler ultrasound, have been com- mercially available for about 25 years. Combining the pulse echo imaging with Doppler ultrasound allows interrogation of a vessel in a known location and permits close investigation of the hemody- namics around areas of atheroma visualized on the image. Ideally, to produce a good image of a vessel wall, the vessel should be at right angles to the
DOPPLER ULTRASOUND
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ultrasound beam. This is the case in the majority of peripheral vessels, as they mainly lie parallel to the skin. However, the Doppler equation shows that no Doppler signal will be obtained when the angle of insonation is at right angles to the direction of flow (as cos 0). The greatest Doppler shift is detected when the beam is parallel to the direction of flow. Therefore, there is a conflict between the ideal angle of the beam used for imaging and that used for Doppler recordings. A compromise would involve the ability to steer or angle the Doppler beam independently of the imaging beam. Some early duplex systems did this by mounting a sepa- rate Doppler element, with an adjustable angle, next to the imaging element. Modern linear array and phased array transducers overcome this by pro- ducing a steered beam, as described in Chapter 2 (see Fig. 2.15). The transducer elements are most sensitive to the returning signals that are at right angles to the front face of the element. This means that, as the beam is steered, the sensitivity of the Doppler transducer will fall to some extent, and therefore the Doppler beam can only be steered by about 20° left and right of center. There is thus a compromise between the choice of Doppler angle and sensitivity.
Velocity measurements using duplex
ultrasound
An important consequence of duplex ultrasound is that it allows the image of the vessel to be used to estimate the angle of insonation between the Doppler beam and the vessel. This enables the detected Doppler frequency to be converted into a velocity measurement using the Doppler equation (equation 3.1). Figure 3.11 demonstrates how the image of a vessel can be used to line up an angle correction cursor (which sits in the center of the sample volume display) with the vessel wall, so giv- ing the angle of insonation. Very large errors in velocity measurement can be generated by incorrect alignment of the angle correction cursor. Although there are many potential sources of errors when using Doppler ultrasound to calculate blood flow velocity (see Ch. 6), it is a powerful technique for detecting and quantifying the degree of disease present in a vessel.