Resistive Index (RI): Doppler ratio = (PSV − EDV) / PSV; measures vascular resistance; elevated in renal obstruction, transplant rejection, and parenchymal disease.
The resistive index (RI) is a dimensionless Doppler-derived ratio calculated as (PSV − EDV) / PSV. It quantifies downstream vascular resistance and is used to assess renal parenchymal disease, transplant rejection, elevated intracranial pressure, and uteroplacental circulation. Normal renal RI is 0.50–0.70; values >0.80 suggest obstruction or parenchymal disease.