How is the resistive index (RI) calculated for kidneys?

The resistive index (RI) is an essential parameter used in vascular studies to assess the resistance to blood flow in the renal arteries. It is particularly valuable in evaluating renal blood flow and can indicate perfusion abnormalities or vascular obstructions.

The correct calculation of the resistive index is achieved by taking the peak systolic velocity (PSV) and subtracting the end diastolic velocity (EDV) from it. The resulting difference is then divided by the peak systolic velocity. Mathematically, this is expressed as:

RI = (PSV - EDV) / PSV.

This formula effectively provides a dimensionless number that ranges from 0 to 1, where higher values indicate greater resistance to blood flow. A value close to 1 suggests reduced diastolic flow, typically associated with increased resistance. Conversely, a value closer to 0 indicates low resistance and better perfusion.

Using this approach helps clinicians evaluate the hemodynamics of the kidneys, enabling them to make informed decisions about potential pathologies or treatments. Understanding this calculation is crucial when interpreting ultrasound results related to renal vascular health.