Visualization of an AVF can be obscured by "aliasing" (wraparound artifact). What parameters can be adjusted on an ultrasound machine to decrease aliasing?
The pulse repetition frequency (PRF, pulses per second) can be increased. When the PRF is too low, high velocities are converted into reversed flow at the point of aliasing. At a minimum, the PRF must be at least twice the frequency of the Doppler signal to reconstruct it accurately. When the PRF = 2 x Freq, this is called the Nyquist Sampling Rate.
Color & spectral Doppler waveforms
Grayscale, color, and spectral Doppler appearance of an arteriovenous fistula (AVF) in the left upper extremity.
The spectral Doppler waveform of the brachial artery upstream from the AVF shows a normal waveform with perhaps mildly increased diastolic flow.
The spectral Doppler waveform in one of the adjacent paired brachial veins, upstream from the AVF shows "arterialization" with a pulsatile waveform.
Similarly, the spectral Doppler waveform in the basilic vein, upstream from the AVF shows "arterialization" with a pulsatile waveform.
Cine image at the site of the AVF shows near simultaneous pulsatile color flow in the brachial artery and vein, with turbulence at the fistula.