The resistive index (RI), also called the Pourcelot index, is calculated from spectral Doppler waveforms as (peak systolic velocity − end-diastolic velocity) / peak systolic velocity. It reflects the resistance to flow in the downstream vascular bed. Normal RI values vary by organ: renal RI is normally below 0.70; elevated values suggest intrarenal vascular disease or obstruction. RI is used to evaluate renal transplants, hepatic vasculature, and testicular blood flow.