Resistive intrarenal index: myth or reality?

Abstract

In renal diagnosis, the B-mode ultrasound is used to provide an accurate study of the renal morphology, whereas the colour and power Doppler are of strategic importance in providing qualitative and quantitative information about the renal vasculature, which can also be obtained through the assessment of the resistive index (RI). To date, this is one of the most sensitive parameters in the study of kidney diseases and allows us to quantify the changes in renal plasma flow. If a proper Doppler ultrasound examination is carried out and a critical analysis of the values obtained is performed, the RI measurement at the interlobar artery level has been suggested in the differential diagnosis between nephropathies. The aim of this review is to highlight the pathological conditions in which the study of intrarenal RI provides useful information about the pathophysiology of renal diseases in both the native and the transplanted kidneys.

Figure 1

The tardus et parvus waveform in a patient with renal artery stenosis. Note that the systolic acceleration is slow, and the systolic peak is dampened (arrow). The waveform has a rounded appearance.

Figure 2.

Two cases of acute kidney injury (AKI). Left side of the image “Case A”: a patient treated for transient AKI showed a clinical worsening to persistent AKI despite medical therapy with concomitant progressive elevation of resistive index (RI; 0.53–0.87). Right side of the image “Case B”: an oliguric patient with a remarkable urea increase (RI, 0.78); the great response to therapy with clinical resolution of renal failure was anticipated by RI reduction (0.59).

Figure 3.

A patient with left flank pain radiating in the groin and microscopic haematuria. No hydronephrosis was found on standard ultrasound examination. With the Doppler ultrasound study, a difference between the (b) left kidney resistive index (RI) (0.80) and (a) right kidney RI (0.59) emerged. (c) On urography, the left kidney was functionally excluded (upper arrow) because of a calculus (lower arrow).