How is the renal to aorta ratio (RAR) calculated?

The renal to aorta ratio (RAR) is calculated by dividing the highest peak systolic velocity (PSV) of the renal artery by the peak systolic velocity of the aorta. This ratio is a critical parameter used in assessing renal artery stenosis.

The reasoning behind this calculation is rooted in the hemodynamic changes that occur with renal artery stenosis. When there is significant narrowing of the renal artery, the blood flow velocity increases in the affected renal artery as a compensatory mechanism, resulting in a higher PSV. In contrast, the PSV in the aorta is generally lower and represents the baseline blood flow. By taking the ratio of these velocities, practitioners can quantitatively assess the severity of the stenosis: a higher ratio indicates a more severe stenosis.

Using the correct method to calculate the RAR provides essential diagnostic information that assists in determining treatment plans for patients, making it a key aspect of vascular ultrasound evaluations focused on renal conditions.