Renal artery duplex — haemodynamic assessment
Aorta at the level of the renal arteries, both main renal arteries from origin to hilum where insonation allows, and intrarenal segmental and interlobar arteries in upper, mid and lower poles of both kidneys. Renal length and cortical thickness are part of the study, not an extra.
Typical, not policy
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Resistant hypertension, or hypertension of abrupt onset in a young patient where fibromuscular dysplasia is in question.
- Deteriorating renal function after starting an ACE inhibitor or angiotensin receptor blocker.
- Flash pulmonary oedema with preserved left ventricular function.
- A patient in whom the imaging question is real but iodinated contrast and gadolinium are both unattractive — which describes much of the population that actually harbours atherosclerotic renal artery stenosis.
Technique
- The diagnosis is haemodynamic, not anatomical, and that is the point of choosing duplex: peak systolic velocity in the renal artery (a threshold around 180-200 cm/s is widely used), the renal-aortic ratio (commonly above 3.5), and post-stenotic turbulence. Confirm local thresholds — they vary between laboratories.
- Intrarenal tardus-parvus waveforms — acceleration time beyond roughly 70 ms with a slow systolic upstroke — are the indirect sign, and are what lets an ostial stenosis be inferred when the origin itself cannot be insonated.
- Scan fasted where practical to reduce bowel gas, and allow a longer slot than a routine renal ultrasound: this is an operator-dependent study that rewards time.
- A kidney under about 8 cm with cortical thinning changes the question — that is established parenchymal loss, and revascularising it will not recover function.
Where it goes wrong
- Operator dependence is the real limitation and it is not a small one: sensitivity falls away sharply outside high-volume laboratories, and a negative study from an inexperienced operator does not exclude the diagnosis.
- Accessory renal arteries are present in a substantial minority and are frequently missed on duplex — a normal study in a patient with a strong clinical picture is not the end of the pathway.
- Fibromuscular dysplasia is a mid-vessel and branch disease, which is precisely where duplex is weakest; a young hypertensive with a normal duplex may still need CT or MR angiography.
- Body habitus and bowel gas defeat direct insonation of the origins in a meaningful proportion of patients, which is why the intrarenal waveform matters and why the report must say which segments were seen.
Clinical questions that reach this study
Contrast
None
Acquisition
- Preparation
- Supply the ankle-brachial pressure index and the claudication distance or rest-pain history: duplex is graded against the haemodynamic question being asked. Declare previous bypass grafts, stents and amputations — graft surveillance is a different examination from native-vessel mapping.