Carotid Duplex

Carotid duplex ultrasound combines B-mode imaging and pulsed wave Doppler (with colour Doppler) to examine the common carotid, the carotid bifurcation, the internal and external carotid arteries, the vertebral arteries and the jugular veins. A 5 to 12 MHz linear probe is used, with the patient supine and the head turned slightly away.

Vessel wall thickness, plaque morphology, lumen diameter, flow direction, peak systolic velocity (PSV), end diastolic velocity (EDV) and the internal to common carotid PSV ratio are recorded. The examination takes 20 to 30 minutes, with no radiation or contrast.

Atherosclerotic carotid stenosis causes 10 to 20% of ischaemic strokes. The risk of stroke in symptomatic stenosis of 70 to 99% is about 20 to 25% over 2 years on medical therapy without intervention, and endarterectomy within 2 weeks of symptoms gives the greatest benefit. In asymptomatic stenosis, annual stroke risk is about 1 to 2% on current best medical therapy.

Carotid duplex has a sensitivity and specificity of 85 to 95% for detecting 70 to 99% stenosis and is the first line test, supported by CT angiography or MR angiography before surgery or stenting.

Indications. Transient ischaemic attack, amaurosis fugax or ischaemic stroke in the carotid territory (urgent, within 24 to 48 hours). Carotid bruit with symptoms or high vascular risk. Preoperative assessment before cardiac surgery in selected patients. Follow up after endarterectomy or stenting (restenosis).

Suspected carotid dissection (limited sensitivity), carotid body tumour, neck mass pulsation, subclavian steal syndrome (vertebral flow reversal), Takayasu arteritis and giant cell arteritis (wall thickening). Pre-assessment of fibromuscular dysplasia. Not for routine screening of asymptomatic people without risk factors.

Contraindications and precautions. None. The neck is exposed and the patient lies flat; severe breathlessness or neck deformity may limit positioning. Probe pressure on the carotid bifurcation should be light, as hypotension and bradycardia can result from carotid sinus stimulation (rare, in elderly patients). Pressure over a severely stenotic plaque is avoided.

Preparation and technique. No preparation is needed. Both sides are examined in transverse and longitudinal planes from the clavicle to the angle of the jaw. Intima media thickness is measured in the far wall of the distal common carotid.

Plaque is described by site, length, surface (smooth, irregular, ulcerated), echogenicity (echolucent, mixed, echogenic, calcified), and degree of narrowing. Doppler samples with an angle of 60 degrees or less are taken in the common carotid, throughout the internal carotid (proximal, mid and distal, and at the point of maximum velocity), external carotid and vertebral arteries.

Spectral broadening is noted.

Normal appearances. The common carotid artery has a diameter of 5 to 7 mm, internal carotid 4 to 6 mm, with a bulb at the origin of the internal carotid, and a thin smooth wall (intima media thickness under 0.8 mm; over 0.9 to 1.0 mm is abnormal).

The internal carotid shows low resistance continuous forward flow in diastole and has no branches in the neck, whereas the external carotid shows high resistance flow with a dicrotic notch and gives off branches (superior thyroid and others), and shows the temporal tap sign. Normal PSV in the internal carotid is under 125 cm each second, with an internal to common carotid PSV ratio under 2.

Vertebral arteries show antegrade, low resistance flow (PSV 30 to 60 cm each second). Flow is laminar with a clear spectral window.

Interpretation.

• Degree of internal carotid stenosis by velocity criteria: under 50% (PSV under 125 cm each second, ratio under 2, EDV under 40, plaque visible), 50 to 69% (PSV 125 to 230, ratio 2 to 4, EDV 40 to 100), 70 to 99% (PSV over 230, ratio over 4, EDV over 100, with visible narrowing and spectral broadening), near occlusion (high, low, or string like flow, with an extremely narrowed lumen, and flow velocities that may be low or high), and occlusion (no lumen flow, no Doppler signal, with reduced or absent diastolic flow in the common carotid and a to-and-fro or thump pattern at the stump). Local laboratories should validate velocity criteria.
• Plaque characterisation: echolucent, ulcerated or heterogeneous plaque with intraplaque haemorrhage and irregular surface carries a higher stroke risk than echogenic or calcified plaque; calcification causes shadowing and limits Doppler.
• Carotid dissection: tapering, flame shaped stenosis, intramural haematoma (crescent shaped, hypoechoic wall thickening), double lumen, and pre-occlusive flow.
• Vertebral artery: reversed flow indicates subclavian steal (proximal subclavian stenosis or occlusion) with arm pressure differences over 20 mmHg and the reversal increased by arm exercise or cuff release; high resistance flow suggests distal obstruction; absent flow suggests occlusion or hypoplasia; diameter under 2 mm suggests hypoplasia.
• Takayasu and giant cell arteritis: long segment concentric, smooth, homogeneous wall thickening (the "macaroni" sign) of the common carotid.
• Fibromuscular dysplasia: "string of beads" with high velocities in the mid or distal internal carotid, tortuosity and kinking.
• Carotid body tumour: hypervascular mass at the bifurcation splaying the internal and external carotid ("lyre" sign).
• Kinks, coils and tortuosity: increase velocities without true stenosis.
• Post-endarterectomy or stenting: restenosis with velocity rise (stent velocities are higher, so thresholds are raised, such as PSV over 300 for 70% stenosis), patch aneurysm or pseudoaneurysm, and haematoma.
• Contralateral compensation: velocities rise in the contralateral artery when the other side is occluded, overestimating stenosis.
• Intima media thickness over the 75th centile or plaque indicates subclinical atherosclerosis and raises cardiovascular risk.

Critical values and findings needing immediate action.

• Symptomatic stenosis of 50 to 99% (especially 70 to 99%): urgent vascular surgery or stroke team review for endarterectomy, ideally within 2 weeks of the event, antiplatelet therapy and high intensity statin.
• Near occlusion or occlusion with recent symptoms: urgent CT or MR angiography and brain imaging; stroke team.
• Carotid dissection: antithrombotic therapy and stroke team assessment.
• Free floating thrombus or mobile plaque: urgent vascular referral.
• Subclavian steal with symptoms of vertebrobasilar ischaemia: vascular referral.
• Pseudoaneurysm or patch rupture after surgery: emergency surgical review.
• Pulsatile expanding neck mass after trauma: emergency imaging and surgical care.
• Acute neurological deficit with carotid occlusion: stroke code (thrombolysis or thrombectomy when eligible).

Limitations and artefacts.

• Velocity based grading varies with equipment, angle of insonation (angle over 60 degrees overestimates velocities), cardiac output (low output with heart failure underestimates; aortic regurgitation, hyperdynamic states and anaemia raise velocities), arrhythmias, and contralateral occlusion.
• Heavy calcification with acoustic shadowing, a high bifurcation, short neck, obesity, and previous radiotherapy limit views; CT or MR angiography is used.
• Distinction between severe stenosis and near occlusion is difficult and can wrongly label a near occlusion as occlusion.
• Tandem stenoses (intracranial or in the common carotid) can alter velocities and hide the true severity.
• Ultrasound cannot assess the aortic arch, the origins of the carotid in the chest, or intracranial vessels; transcranial Doppler, CT angiography or MR angiography are needed.
• Intima media thickness measurement needs training, and is not a routine clinical decision tool.
• Dissection is poorly detected, especially in the high cervical segment and skull base.
• Operator dependence is important, and accredited laboratories with internal validation give the most reliable results.

Special groups.

• Young stroke patients: dissection, fibromuscular dysplasia, arteritis and cardiac sources are considered; duplex is combined with MR angiography.
• Women of childbearing age: pregnancy related vascular events, safe with ultrasound.
• Patients with diabetes: higher plaque burden and multi-vessel disease.
• Patients awaiting coronary bypass: screening in those with prior stroke or bruit.
• Patients with HIV: accelerated carotid atherosclerosis.
• Older adults: multi-segment disease; the choice between surgery and stenting depends on age and anatomy.
• Regions: Takayasu arteritis, sickle cell disease, rheumatic heart disease and Chagas disease cause stroke in younger people; duplex is often the only vascular imaging, so thorough technique is important.

Follow up tests. CT angiography or MR angiography of the head and neck, brain CT or MR (diffusion weighted) for infarcts, digital subtraction angiography in selected cases, transcranial Doppler for intracranial flow, echocardiography and cardiac rhythm monitoring for embolic sources, lipid profile, glucose or HbA1c, blood pressure assessment, inflammatory markers for arteritis,

surveillance duplex at 6 to 12 months for moderate stenosis, and referral to vascular surgery, neurology or stroke service.