Renal Artery Doppler

Renal artery duplex ultrasound is a non-invasive screening test for renal artery stenosis and renovascular hypertension. It combines direct spectral interrogation of the main renal arteries with indirect intrarenal (downstream) waveform analysis. Elevated peak systolic velocity and renal-aortic ratio indicate a haemodynamically significant stenosis, while a tardus-parvus intrarenal waveform provides supportive downstream evidence.

Indications

Suspected renovascular hypertension, especially resistant or early/late-onset hypertension
Unexplained or progressive renal impairment
Suspected fibromuscular dysplasia in younger hypertensive patients
Flash pulmonary oedema or worsening renal function after ACE inhibitor/ARB
Surveillance after renal artery stenting or angioplasty
Assessment of the transplant renal artery

Contraindications & Cautions

No absolute contraindications; ultrasound is safe and non-ionising
Bowel gas, obesity and patient motion frequently limit visualisation of the main renal artery origins
Technically demanding and operator-dependent; a negative or incomplete study may need CT/MR angiography
Accessory renal arteries are commonly missed and can cause false-negative direct criteria

Patient Preparation

Fast for 6-8 hours to reduce bowel gas obscuring the renal arteries and aorta
Schedule in the morning where possible
Supine and decubitus (flank) positions for anterior and posterolateral windows
Allow adequate time; the study is technically demanding
Record blood pressure medications and renal function

Technique & Parameters

Curvilinear 2-5 MHz probe for deep abdominal penetration
Sample the aorta at the level of the renal arteries for peak systolic velocity to calculate the renal-aortic ratio
Interrogate each main renal artery from origin to hilum with angle-corrected spectral Doppler at an angle <=60 degrees
Record segmental/interlobar intrarenal waveforms in upper, mid and lower poles
Measure intrarenal acceleration time, acceleration index and resistive index for tardus-parvus assessment
Measure bipolar kidney length and note asymmetry

Systematic Review

Aortic peak systolic velocity at the renal artery level
Main renal artery PSV at origin, mid and distal segments bilaterally
Renal-aortic ratio (renal artery PSV divided by aortic PSV)
Intrarenal waveforms for tardus-parvus (prolonged acceleration time, low acceleration index)
Resistive index in each pole and side-to-side comparison
Kidney length and cortical thickness for chronic atrophy

Key Findings & Significance

Main renal artery peak systolic velocity above ~180-200 cm/s suggests significant (>=60%) stenosis
Renal-aortic ratio greater than 3.5 supports a haemodynamically significant proximal stenosis
Post-stenotic turbulence and elevated velocity with distal spectral broadening
Tardus-parvus intrarenal waveform: acceleration time >0.07 s (prolonged upstroke) and reduced acceleration index
Resistive index side-to-side difference and low downstream RI may accompany proximal stenosis
Small kidney (<9 cm) with reduced flow suggests chronic ischaemic nephropathy

Differential Considerations

Elevated renal artery PSV: true stenosis vs tortuosity/angle error vs high-flow state
Tardus-parvus: proximal renal artery stenosis vs aortic stenosis (systemic upstroke change)
Renovascular disease: atherosclerotic (ostial/proximal, older) vs fibromuscular dysplasia (mid/distal, younger, string-of-beads)
Reduced intrarenal flow: stenosis vs medical renal disease vs poor cardiac output
Non-visualised artery: technical failure vs occlusion vs accessory artery

Pearls & Pitfalls

Keep the Doppler angle at or below 60 degrees; angle error is the commonest cause of spurious velocity elevation
Use both direct (PSV, RAR) and indirect (tardus-parvus, acceleration time) criteria for reliability
Beware accessory renal arteries as a cause of false-negative direct criteria
Fibromuscular dysplasia typically affects the mid-to-distal artery, unlike ostial atherosclerotic disease
Correlate with kidney size; a small kidney with slow flow suggests established ischaemic nephropathy

Structured Report

State fasting status, probe, positions and technical adequacy
Report aortic and renal artery PSV, renal-aortic ratio and side of any abnormality
Describe intrarenal waveforms with acceleration time, acceleration index and resistive index
Give bipolar kidney lengths and note asymmetry
Give an impression on the presence and likely aetiology of stenosis and recommend CTA/MRA or angiography when indicated

References

AIUM Practice Parameter for the Performance of Native Renal Artery Duplex Sonography
Radermacher J et al. Use of Doppler Ultrasonography to Predict Outcome of Renal Artery Stenosis. NEJM
Grant EG et al. Duplex Sonography of the Renal Arteries. Radiographics
ACC/AHA Guidelines on the Management of Peripheral and Renal Artery Disease

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

Renal Artery Doppler

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References