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Regarding aliasing in pulsed-wave Doppler imaging: a. It occurs when the Doppler shift frequency exceeds 2 PRF (pulse repetition frequency) b. It limits the range of velocities than can accurately be measured to 1.5–2m s-1 c. The risk of aliasing is higher at higher Doppler frequencies d. It can be reduced by using a higher PRF, lowering the frequency or increasing the angle of insonation e. The problem of aliasing can partially be overcome by using a high PRF mode |
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Answer» a. False. According to the Nyquist criterion, the waveform measured (shift frequency in this case) needs to be sampled at least twice in each period (time between pulses). Therefore, aliasing occurs when the Doppler shift exceeds half of the sampling frequency (PRF), not double. b. True. c. True. At higher frequencies, the Doppler shift (which is proportional to frequency) will be larger and more likely to exceed 0.5 PRF. d. True. At higher PRF, the Doppler shift that can be measured without aliasing is proportionately higher. Lowering the frequency or increasing the angle of insonation decreases the Doppler shift frequency, so it is less likely to exceed the Nyquist criterion. e. True. In this mode, the next Doppler pulse is sent before the echo from the previous pulse is received. This allows measuring of higher flow velocities, but introduces range ambiguity (loss of range resolution). |
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