The arterial resistivity index (also called as Resistance index, abbreviated as RI), developed by Léandre Pourcelot [1], is a measure of pulsatile blood flow that reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement.

It is primarily used in ultrasound imaging to evaluate arteries and solid organ damage.

[4] It is used in ultrasound testing of umbilical artery for placental insufficiency.

[6] Following kidney transplantation, patients with an RI > 0.8 have an increased mortality.

[4][7] Mapping of the local arterial resistivity index from laser Doppler imaging enables unambiguous identification of retinal arteries and veins on the basis of their systole-diastole variations, and reveal ocular hemodynamics in human eyes.