1-The resistive index (RI) can be calculated from spectral measurements by using the equation RI = (PSV − EDV)/PSV, where PSV is the peak systolic velocity and EDV is the end-diastolic velocity (1). The pulsatility index (PI) can be calculated by using the equation PI = (PSV − EDV)/MV, where MV is the mean flow velocity during the cardiac cycle.

2-The RI and the PI provide information about blood flow and resistance that cannot be obtained from measurements of absolute velocity alone. The effects of variation in vessel angulation and size are nullified in the calculation of these indexes .

http://radiographics.rsna.org/content/28/3/691.full