The pulsatility index (PI) is a calculated value derived from the spectral Doppler waveform that quantifies the variability of blood flow velocity within a vessel over the cardiac cycle. It is calculated by subtracting the minimum (end-diastolic) velocity from the peak systolic velocity and dividing by the mean velocity over the entire cycle. A higher PI indicates greater pulsatility and higher downstream resistance. The PI is widely used in obstetric ultrasound to assess uterine and umbilical artery blood flow — elevated PI in the umbilical artery may indicate placental insufficiency and fetal growth restriction. It is also used in cerebrovascular, renal, and hepatic Doppler evaluations. The PI complements the resistive index and provides valuable information about the hemodynamic state of the vascular bed being examined.