24.3 Carotid Doppler & Post-Catheterization Complications

Carotid Duplex Protocol

Carotid duplex sonography evaluates the extracranial arteries for atherosclerotic stenosis, a leading preventable cause of stroke. Scan the common carotid artery (CCA), the carotid bulb, the internal carotid artery (ICA), and the external carotid artery (ECA) in transverse and longitudinal planes with grayscale, then add spectral and color Doppler with the angle cursor parallel to flow at 60 degrees or less. The vertebral arteries are sampled in the mid-neck to document flow direction. Grayscale defines plaque burden and morphology; Doppler quantifies hemodynamic significance using peak systolic velocity (PSV), end-diastolic velocity (EDV), and the ICA/CCA PSV ratio — the highest ICA PSV divided by the highest CCA PSV.

Differentiating the ICA from the ECA

The ECA gives off branches in the neck (the superior thyroidal artery is the first), has a higher-resistance waveform with a dicrotic notch, and shows waveform oscillations with the temporal tap maneuver — fingertip taps on the superficial temporal artery in front of the ear. The ICA has no extracranial branches and a lower-resistance waveform with continuous forward diastolic flow, reflecting the brain's low-resistance vascular bed. Correct identification matters because the stenosis criteria below apply to the ICA.

Stenosis Criteria

The Society of Radiologists in Ultrasound consensus criteria grade ICA diameter narrowing as follows:

A ≥70% stenosis rests on the two SRU primary parameters: an ICA PSV above 230 cm/s together with plaque narrowing the lumen by 50% or more on grayscale. The ICA/CCA ratio (above 4.0) and the ICA EDV (above 100 cm/s) are additional parameters, brought in when clinical or technical factors suggest the PSV may not be representative — they are not all required at once, so a stenosis with a PSV of 280 cm/s and an EDV of 85 cm/s is still graded ≥70%. In near occlusion the lumen collapses and velocity may paradoxically fall to a slow "trickle" — never assume a low-velocity tight stenosis is mild. In total occlusion, no flow is detected; lower the wall filter and pulse repetition frequency and use color or power Doppler to avoid missing a string-sign lumen. Contralateral high-grade stenosis can raise velocities on the normal side through compensatory flow, so always interpret velocities together with the grayscale estimate of diameter reduction.

Plaque Morphology and Dissection

Echogenic, calcified plaque with posterior shadowing is generally fibrotic and more stable; hypoechoic, heterogeneous plaque — representing a lipid core or intraplaque hemorrhage — is more vulnerable and more often associated with embolic symptoms. Surface ulceration and mobile thrombus raise risk further. A carotid dissection appears as a tapered ("flame-shaped") narrowing or occlusion beginning distal to the bulb, sometimes with a visible intimal flap, and is a leading cause of stroke in younger patients.

Vertebral Artery and Subclavian Steal

The vertebral artery normally flows antegrade (toward the brain) with a low-resistance pattern that mirrors the ICA. A proximal subclavian artery stenosis drops pressure distal to the lesion, and the ipsilateral vertebral artery becomes a collateral route for the arm — blood flows down the vertebral instead of up. The spectrum runs from a pre-steal "bunny" waveform (mid-systolic deceleration with net antegrade flow) through alternating to-and-fro flow to complete flow reversal, which is the full subclavian steal. A blood-pressure difference of more than about 20 mmHg between the arms supports the diagnosis, and a hyperemia (blood-pressure cuff) test can unmask a latent steal.

Post-Catheterization Complications

Femoral arterial access for cardiac catheterization and endovascular procedures produces two classic complications, both diagnosed with duplex sonography.

A femoral pseudoaneurysm is a contained arterial leak: blood escapes through a defect in the arterial wall and pulsates within a hematoma sac that communicates with the artery through a neck. Color Doppler shows the pathognomonic yin-yang pattern — swirling bidirectional flow within the round sac — while spectral Doppler in the neck shows the to-and-fro waveform: blood jets into the sac during systole and returns to the artery during diastole. Treatment has shifted from ultrasound-guided compression repair (prolonged probe pressure to thrombose the sac) to ultrasound-guided thrombin injection, in which a small dose of thrombin injected into the sac under real-time visualization clots it within seconds, with success rates above 95%. Compression remains an option for small pseudoaneurysms or when thrombin is contraindicated.

An arteriovenous fistula (AVF) is a direct communication between the femoral artery and the adjacent vein. Duplex shows a high-velocity, low-resistance arterial signal feeding the fistula, marked arterialization of the venous waveform (pulsatile, high-velocity venous flow), and a perivascular color "bruit" — chaotic color speckle in the soft tissues caused by tissue vibration. Unlike a pseudoaneurysm, there is no discrete sac. Large AVFs may cause steal symptoms or high-output cardiac strain and require surgical or endovascular closure; small ones often thrombose spontaneously. Finally, a simple hematoma — avascular fluid with no internal Doppler flow — must be distinguished from a pseudoaneurysm; grayscale alone cannot separate the two, so the yin-yang sign and the neck's to-and-fro waveform are the discriminators.

Technical Factors and Reporting

A mid-frequency linear transducer (about 5-9 MHz) covers most necks; switch to a lower frequency or curvilinear probe for deep or very superior segments near the mandible. Set the color and spectral pulse repetition frequency so normal flow fills the lumen without aliasing — aliasing at a stenotic jet is expected, but the scale must be high enough to display true peak velocities, and a low scale is reserved for confirming occlusion or slow trickle flow. Sample the CCA proximally, the bulb, the ICA at and just beyond any plaque, the ECA, and the vertebral artery, keeping the Doppler angle at 60 degrees or less and consistent between serial studies. A complete report states the highest ICA PSV and EDV, the ICA/CCA ratio, the stenosis category, plaque description and extent, vertebral flow direction, and any non-atherosclerotic finding such as dissection or fibromuscular dysplasia, so that the referring clinician can compare follow-up examinations directly.