RRI measurement and calculation

In each volunteer, the same designated ultrasound device (GE Vivid S70N, v202CH, US) with a curvilinear probe (1.5–6.0 MHz) was used. The volunteer was positioned on their side and first a complete view of the kidney was obtained. Colour-Doppler was applied to visualize the global organization of intrarenal blood vessels. Pulsed wave Doppler at the smallest possible width between 2 and 5 mm was used to measure flow velocities in an interlobular- or arcuate artery in the upper, middle and lower pole of each kidney. If possible, the examiners obtained a reading with at least three consecutive similar-looking waveforms in each of the three poles for each kidney. RRI was calculated for each of the three poles as [(peak systolic velocity − end diastolic velocity)/peak systolic velocity]. These values were used to compute a total mean RRI (RRI_total), a mean RRI for the right kidney (RRI_dx) and a mean RRI for the left kidney (RRI_sin). If the examiners were unable to obtain a satisfactory measurement in one pole, the mean value was calculated using the measurements obtained. Our protocol for obtaining RRI is in line with previously described protocols [23].