01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Vascular ultrasound answers two linked questions. B-mode and colour imaging ask what the vessel looks like: wall, plaque, lumen, thrombus, calcification, diameter and surrounding tissue. Spectral Doppler asks what that anatomy does to blood flow: direction, velocity, pulsatility, acceleration and downstream resistance. A trustworthy conclusion reconciles all three. When anatomy and haemodynamics disagree, first suspect sampling, angle, aliasing, low-flow settings or a lesion outside the field before inventing a diagnosis.
The Doppler frequency shift increases with blood velocity and with the cosine of the insonation angle. At angles approaching 90 degrees the cosine becomes small and minor cursor errors cause large velocity errors. Carotid laboratories therefore use a consistent angle, usually within a 45-to-60-degree window, and place the correction line along the actual flow stream. Colour aliasing is not automatically stenosis: it can reflect a velocity scale that is too low. Conversely, a high scale or wall filter can erase slow flow and simulate occlusion.
Arterial waveforms reflect the pump, the intervening conduit and the downstream vascular bed. A normal common carotid waveform is intermediate resistance; the internal carotid supplies a low-resistance cerebral bed with sustained diastolic flow, whereas the external carotid is relatively high resistance. At rest a healthy peripheral limb artery is often multiphasic. Distal to important inflow disease, the waveform becomes damped and monophasic with delayed upstroke. Distal vasodilatation can also lower resistance, so shape must be interpreted with the clinical setting and comparison segments.
Ultrasound is dynamic, bedside-capable and free of ionising radiation, but it is operator dependent. Obesity, wounds, bowel gas, oedema, deep vessels and heavy calcification can hide segments or prevent reliable Doppler sampling. A complete report describes the clinical indication, sides and segments examined, limitations, stenosis convention, representative velocities and waveforms, and the finding that changes management.
Key points
- Duplex combines anatomy from B-mode, flow direction and distribution from colour, and velocity plus waveform shape from spectral Doppler.
- Doppler velocity depends on insonation angle; keep the angle correction cursor parallel to the vessel or flow jet and use the laboratory's validated angle convention.
- For stable suspected lower-limb peripheral arterial disease, examine pulses and measure ankle-brachial pressure index first, then use duplex as first-line anatomical imaging when revascularisation is being considered.
- A near-occlusion can have a deceptively low velocity because distal collapse reduces flow; never equate low velocity with mild disease without looking at morphology.
- NICE requires carotid stenosis reports to state whether NASCET or ECST criteria were used because the percentage values are not interchangeable.
- For suspected lower-limb peripheral arterial disease, history, pulse examination and ankle-brachial pressure index usually precede anatomical duplex mapping.
- A normal or high ankle-brachial pressure index does not exclude peripheral arterial disease in diabetes, where incompressible calcified arteries can elevate ankle pressure.
- When revascularisation is being considered, NICE uses duplex as first-line anatomical imaging, followed by contrast-enhanced MRA or CTA when more detail is needed.
- Waveform change is physiological evidence: progressive damping, delayed systolic upstroke and loss of normal pulsatility can reveal important proximal obstruction beyond the imaged segment.
- An emergency clinical syndrome outranks an outpatient scan result; do not let ultrasound scheduling delay stroke or acute limb-ischaemia treatment.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Plaque is characterised by site, surface, echogenicity and the residual colour-flow channel. A focal rise in internal-carotid peak systolic velocity, spectral broadening and post-stenotic disturbance supports haemodynamically significant narrowing. Confirm the grade with end-diastolic velocity and the internal-to-common carotid velocity ratio rather than a single threshold. Tandem disease, cardiac output and contralateral occlusion can shift velocities.
Near-occlusion is severe internal-carotid narrowing with reduced distal calibre or flow. Velocity may be very high at a tight jet, low when flow collapses, or difficult to detect. True occlusion requires careful low-flow colour and spectral settings and demonstration that no patent lumen remains; calcific shadow or an inadequately visualised distal segment should prompt confirmatory imaging when the distinction changes treatment.
A proximal subclavian obstruction lowers pressure beyond the lesion. The ipsilateral vertebral waveform may progress from systolic deceleration to bidirectional and then fully retrograde flow, particularly with arm demand. Compare arm pressures and subclavian waveforms; vertebral reversal is a haemodynamic sign, while symptoms and coronary bypass anatomy determine its clinical importance.
At a focal stenosis, colour may show narrowing and aliasing, while spectral Doppler shows a local velocity increase and turbulence. Compare the lesion with a nearby normal proximal segment and follow the distal waveform. A velocity ratio is more robust than an isolated absolute value, but laboratories should apply validated criteria for the vessel and intervention question.
An occluded segment lacks detectable lumen flow despite optimised slow-flow settings; collateral vessels may reconstitute a distal artery with a damped monophasic waveform. Map both ends of the occlusion, patent inflow, runoff and major collateral routes because lesion length and target vessels influence revascularisation planning.
Acute arterial occlusion may show intraluminal material and an abrupt flow cutoff, but age cannot always be assigned reliably from echogenicity alone. A pseudoaneurysm typically has swirling bidirectional sac flow and a to-and-fro neck waveform. Symptoms, distal perfusion, recent access and expansion determine urgency; do not compress a suspected pseudoaneurysm before vascular assessment.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Carotid duplex ultrasoundFirst step - Why
- Identify extracranial carotid plaque, distinguish patency from occlusion and estimate stenosis severity when the result will guide secondary prevention or carotid intervention.
- Interpretation and limitations
- Scan common, bifurcation, internal and external carotid arteries in longitudinal and transverse planes. Integrate plaque and lumen appearance with internal-carotid peak systolic and end-diastolic velocities, an internal-to-common carotid ratio, distal calibre and waveform. State NASCET or ECST grading explicitly and document limited segments.
- 02
Vertebral and subclavian Doppler - Why
- Assess proximal upper-limb inflow and vertebral flow direction when arm-pressure asymmetry, posterior-circulation symptoms or bypass anatomy suggests steal physiology.
- Interpretation and limitations
- Record bilateral vertebral direction and waveform, subclavian turbulence and distal arm waveforms. Correlate with bilateral pressures; a reversed vertebral waveform alone does not prove that a patient's symptoms arise from steal.
- 03
Ankle-brachial pressure index - Why
- Support the initial diagnosis and physiological severity assessment of lower-limb peripheral arterial disease.
- Interpretation and limitations
- Measure both brachial pressures and dorsalis pedis and posterior tibial pressures using manual Doppler. NICE defines each leg's index using the highest ankle pressure divided by the highest arm pressure. Low values support arterial disease; a normal or high result can be misleading in diabetes or medial arterial calcification.
- 04
Lower-limb arterial duplex mappingFirst line - Why
- Provide NICE-recommended first-line anatomical imaging when lower-limb revascularisation is being considered, localising and characterising inflow, femoropopliteal and tibial disease.
- Interpretation and limitations
- Follow vessels segment by segment, recording focal velocity ratios and waveform transitions, then describe stenosis, occlusion length, reconstitution, inflow and runoff. State which clinically relevant segments were not visualised.
- 05
Toe pressure or toe-brachial index and pulse-volume recordings - Why
- Provide additional physiological evidence when ankle arteries are incompressible or symptoms, tissue loss and ankle pressure disagree.
- Interpretation and limitations
- Digital arteries are often more compressible than tibial arteries, but cold, vasospasm and severe microvascular disease affect results. Use these as part of a limb-perfusion assessment rather than as an isolated guarantee of wound healing.
- 06
Contrast-enhanced MRA or CT angiography - Why
- Define anatomy beyond duplex when intervention planning requires a complete arterial map or ultrasound is limited.
- Interpretation and limitations
- NICE recommends contrast-enhanced MRA after duplex when further lower-limb imaging is needed, with CTA when MRA is contraindicated or not tolerated. Select according to renal function, implants, calcification, radiation, local expertise and the likely procedure.
04Clinical next stepsHow the result changes management or prompts escalation.
01Symptomatic carotid pathwayMove from focal symptoms to time-critical carotid decision-makingFirst stepA resolved focal neurological episode is clinically compatible with TIA and carotid endarterectomy could be appropriate.+
- 1Arrange immediate specialist assessment within 24 hours and begin the indicated secondary-prevention pathway; ultrasound is not a substitute for confirming that the event was focal and vascular.
- 2Obtain urgent carotid imaging through the specialist pathway, usually duplex first where a validated service can provide reliable grading quickly.
- 3Report the symptomatic side, plaque, patency, stenosis category, technical limits and whether NASCET or ECST criteria were used.
- 4ConfirmatoryIf the result is uncertain, suggests near-occlusion or occlusion, or an intervention is planned, obtain confirmatory arterial imaging according to the stroke and vascular team's protocol.
- 5NICE directs symptomatic patients with 50–99% NASCET stenosis, or 70–99% ECST stenosis, to urgent carotid endarterectomy assessment; lower grades do not receive surgery for that lesion.
02Stable intermittent claudication pathwayStable PAD: confirm physiology before mapping anatomyExertional leg discomfort, impaired walking or pulse findings suggest lower-limb peripheral arterial disease without an acutely threatened limb.+
- 1AlternativeTake a vascular history, inspect feet and skin, palpate pulses and assess cardiovascular risk and alternative musculoskeletal, neurological or venous explanations.
- 2Measure ankle-brachial pressure indices with manual Doppler using the highest ankle and highest arm pressures, noting diabetes or calcification that may produce falsely high values.
- 3First lineUse duplex as first-line anatomical imaging if revascularisation is being considered, mapping lesion location and haemodynamic consequence rather than scanning an isolated painful point.
- 4EscalationEscalate to contrast-enhanced MRA when more anatomical detail is needed and to CTA if MRA is contraindicated or not tolerated, then align imaging with the intended endovascular or surgical strategy.
03Chronic limb-threatening ischaemia pathwayLink tissue threat, perfusion and a reconstructable routeIschaemic rest pain, ulceration or gangrene suggests chronic limb-threatening ischaemia.+
- 1Seek urgent vascular assessment, document tissue loss or infection, pulses and neurological status, and distinguish the chronic syndrome from sudden acute limb ischaemia.
- 2Obtain physiological measures such as ankle and, where needed, toe pressures while recognising falsely reassuring incompressible ankle vessels.
- 3Map inflow, femoropopliteal and below-knee vessels with duplex, including potential target arteries and any segment that cannot be assessed.
- 4Use cross-sectional angiography when required for a complete treatment plan; imaging should serve prompt limb-salvage decisions rather than create serial-test delay.
04Discordant ultrasound pathwayResolve disagreement before assigning a high-impact gradeB-mode, colour and spectral findings disagree, or the scan conflicts with symptoms and examination.+
- 1Repeat the critical segment with optimised gain, pulse-repetition frequency, wall filter, sample volume and angle correction, and compare orthogonal views.
- 2Look for systemic and remote explanations such as arrhythmia, low cardiac output, contralateral carotid disease, proximal inflow obstruction or distal high resistance.
- 3State the uncertainty and technical limitation explicitly rather than averaging discordant observations into a precise percentage.
- 4ConfirmatoryRecommend confirmatory CTA, MRA or catheter angiography only when the result is likely to change urgent or procedural management.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- After carotid intervention, use the local vascular surveillance protocol and compare with the earliest technically adequate post-procedure study; velocity criteria in a stent are not automatically identical to those in a native artery.
- For conservatively managed carotid disease, repeat imaging only at a clinically justified interval and use the same laboratory and stenosis convention where possible so apparent progression is not caused by method change.
- Following lower-limb revascularisation, record symptoms, wounds, pulses and physiological indices alongside duplex; a patent graft can coexist with poor tissue perfusion from distal or microvascular disease.
- Trend focal velocities and ratios together with waveform change. A velocity increase without reproducible technique or anatomical narrowing should prompt review of angle and sampling before calling progression.
- Escalate surveillance when new focal neurological symptoms, rest pain, tissue loss, graft-related symptoms or a sudden fall in limb perfusion occurs; scheduled follow-up must not replace acute assessment.
- Preserve technical comparability by documenting Doppler angle, stenosis convention, measured segment and inaccessible regions in every study that will become a baseline.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Velocity is a measurement, not the diagnosis
A stenosis accelerates flow through a narrowed lumen, but measured velocity also varies with cardiac output, vessel tortuosity, collateral demand, tandem lesions and insonation angle. The safest grade emerges when morphology, several Doppler parameters and distal effects point in the same direction.
Near-occlusion breaks the simple velocity rule
When severe narrowing collapses the distal internal carotid, volume flow falls. The jet may no longer generate the expected very high sampled velocity, so distal calibre and anatomy are essential. This is a classic reason not to interpret one number outside the full study.
Waveform transitions localise hidden disease
A damped delayed waveform at the common femoral artery suggests proximal aortoiliac inflow disease even when the stenosis itself is obscured by bowel gas. A normal proximal waveform followed by a focal velocity rise and distal damping localises disease within the scanned limb.
A percentage needs a denominator
NASCET compares the narrowest residual lumen with the normal distal internal carotid, whereas ECST estimates the original bulb diameter. The same artery therefore receives different numerical percentages. Naming the method prevents thresholds from being applied to the wrong scale.
Optimising slow flow prevents false occlusion
Reduce the velocity scale and wall filter, increase colour gain until just below noise, use power Doppler if appropriate and inspect multiple planes. If patency remains consequential and uncertain, recommend another modality rather than converting a technical limit into certainty.
The scan should answer a management question
Carotid duplex after TIA asks whether ipsilateral disease could benefit from urgent intervention. Limb duplex before revascularisation asks whether there is a treatable inflow-to-runoff route. Stating the question improves acquisition and makes limitations clinically meaningful.
07Common pitfallsFrequent interpretation and management errors.
- 01
Requesting routine carotid ultrasound for non-focal dizziness or syncope without a vascular neurological syndrome, creating incidental plaque findings that do not explain the presentation.
- 02
Applying a single peak systolic velocity threshold despite poor angle correction, tandem disease, arrhythmia or severe low-flow near-occlusion.
- 03
Reporting a carotid stenosis percentage without stating NASCET or ECST convention.
- 04
Calling occlusion behind calcific acoustic shadow without optimising slow-flow settings or acknowledging that the lumen was not adequately seen.
- 05
Treating colour aliasing as proof of stenosis without checking pulse-repetition frequency, gain, vessel tortuosity and the spectral waveform.
- 06
Excluding peripheral arterial disease in a person with diabetes solely because the ankle-brachial pressure index is normal or high.
- 07
Mapping limb anatomy without examining inflow and runoff, leaving the intervention team unable to judge whether a reconstructable route exists.
- 08
Allowing an outpatient duplex appointment to delay acute stroke evaluation or emergency assessment of a threatened limb.
- 09
Comparing serial velocities acquired with different angles or at different segments and labelling the difference as biological progression.
- 10
Assuming a reversed vertebral waveform explains symptoms without correlating arm pressure, proximal subclavian disease and the neurological history.