Carotid duplex ultrasound
Common, internal and external carotid arteries bilaterally; vertebral flow direction
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
- Stroke or TIA where carotid endarterectomy would be considered.
Technique
- Linear transducer of roughly 5-12 MHz; a lower-frequency curvilinear or phased-array probe is used to reach the proximal common carotid and the origins in a short or deep neck.
- Patient supine with the head turned away from the side being examined and the neck slightly extended, the sonographer seated at the head or at the side; each vessel is examined in transverse sweep first for plaque and then longitudinally for spectral measurement.
- Doppler angle kept at or below 60 degrees with the sample gate small and placed in the centre of the lumen, corrected parallel to the vessel wall — angle error is the single largest source of velocity error, and velocity is what the stenosis grade is derived from.
- Peak systolic and end-diastolic velocities recorded in the distal common carotid, at and immediately beyond the point of maximum stenosis in the internal carotid, and in the external carotid, with the internal-to-common peak systolic velocity ratio calculated rather than relying on the absolute internal carotid velocity alone.
- Vertebral arteries interrogated between the transverse processes for flow direction as well as presence — retrograde or bidirectional flow is the finding in subclavian steal and is easily missed if only patency is checked.
- Grading criteria are laboratory-specific and the report should name the criteria used and the degree of stenosis in the terms the local vascular service acts on (NASCET-equivalent in most UK and US practice).
Where it goes wrong
- Heavy calcification causes acoustic shadowing and can render grading unreliable, in which case CTA or MRA is needed.
- Velocity thresholds for grading stenosis vary between laboratories.
Clinical questions that reach this study
Contrast
None