Downstream from a significant renal artery stenosis, the acceleration time is prolonged due to delayed systolic upstroke (“tardus-parvus waveform”). This is a sensitive Doppler parameter for detecting hemodynamically significant stenosis. Spectral broadening usually occurs at the stenotic site, not distal to it.

According to Zwiebel’s Introduction to Vascular Ultrasound:

“Prolonged acceleration time and reduced acceleration index characterize tardus-parvus waveforms distal to renal artery stenosis.”