Article

Renal artery Doppler: how to assess it

Renal artery Doppler: technique, indices (RI, acceleration, renal-aortic ratio), stenosis and renovascular hypertension, normal report and limitations.

What it is and when to indicate it

Renal artery Doppler assesses flow in the main renal arteries and the parenchyma to investigate renal artery stenosis and renovascular hypertension, especially in difficult-to-control hypertension, early or late onset, worsening renal function with ACE inhibitors/ARBs, and renal asymmetry.

This page organizes the protocol in clear language and does not replace the original guidelines (AIUM, ACR, CBR), supervised training or clinical correlation.

Preparation and technique

• Fasting of 6 to 8 hours reduces gas and improves the window to the main renal arteries
• Curved transducer of 2 to 5 MHz; Doppler preset with PRF and filter adjusted for slow flow
• Correct the insonation angle to 60 degrees or less when measuring velocities
• Anterior, flank and decubitus approaches; use the liver and spleen as a window
• Sample the aorta at the level of the renal arteries, the ostium, the course and the parenchyma (interlobar/segmental arteries)

Indices and reference values

Use the indices together, not in isolation, and always with clinical correlation. Cutoffs vary between services and equipment.

• Peak systolic velocity (PSV) in the renal artery usually up to about 180 to 200 cm/s
• An elevated renal-aortic ratio (RAR above about 3.5) suggests significant stenosis
• Parenchymal resistive index (RI) typically up to about 0.70, with symmetry between kidneys
• A prolonged acceleration time (above about 70 ms) and a tardus-parvus pattern suggest upstream stenosis
• Asymmetry in RI and renal size reinforces the suspicion

Signs of stenosis and limits of the method

• Focal velocity increase at the ostium/course with post-stenotic turbulence
• Tardus-parvus pattern and prolonged acceleration time in the parenchyma distal to the stenosis
• Accessory renal arteries may be missed and are a cause of false-negative results
• Gas, obesity and respiratory motion limit the window — report honestly
• Operator-dependent method; doubtful cases go to CT/MR angiography

Normal report template

• Patent main renal arteries, with velocities within the reference limits
• Normal renal-aortic ratio, without direct signs of significant stenosis
• Symmetric parenchymal resistive indices within normal range
• Normal acceleration time, without a tardus-parvus pattern
• Kidneys of symmetric dimensions (assessment limited by gas/habitus when applicable)

Common pitfalls

• Inadequate insonation angle overestimating or underestimating velocities
• Not searching for accessory arteries and concluding normality prematurely
• Mistaking physiological turbulence for stenosis without the parenchymal indices
• Elevated RI from diffuse parenchymal disease interpreted as stenosis
• Concluding without documenting the technical limitations of the exam

Do not overdiagnose

Combine direct signs (velocity, RAR) and indirect signs (tardus-parvus, acceleration time, RI) and correlate with the clinical picture. In clinically relevant doubt, recommend CT or MR angiography. The conclusion should be proportional to the findings.

Sources

Educational content; it does not replace original guidelines, medical evaluation or supervised training. Main references:

• AIUM. Practice Parameter for the Performance of Native Renal Artery Duplex Sonography.
• ACR-AIUM-SPR-SRU. Practice Parameter for the Performance of an Ultrasound Examination of the Abdomen and/or Retroperitoneum.
• Brazilian College of Radiology (CBR). Standardization and technical guidelines for ultrasound.
• Rumack CM, Levine D. Diagnostic Ultrasound (renal Doppler).
• Radiopaedia. Renal artery stenosis and Doppler (didactic support, not copied).

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