Resistive index (vascular ultrasound)
The resistive index (Pourcelot index) is a calculated flow parameter in ultrasound, derived from the maximum, minimum, and mean Doppler frequency shifts during a defined cardiac cycle. Along with the pulsatility index (PI), it is typically used to assess the resistance in a pulsatile vascular system.
- RI = (PSV - EDV) / PSV
Resistive index is one of the most common vascular ultrasound indices used owing to its simplicity. As a vessel narrows and resistance to flow increases, the RI will increase. The RI is proportional to not only vascular resistance but also vascular compliance. Different vessels and vascular beds have different flow requirements, so there are different normal RI values depending on the target organ. So it is used in a multitude of clinical scenarios.
Specific RI uses include: