Martin Necas 05 May 2003
The main focus of carotid duplex examination is to identify stenotic lesions within the carotid arteries. Significant carotid artery stenosis may lead to transient ischemic deficit(TIA), or stroke. Duplex examination is a reliable tool for assessing the degree of carotid stenosis. Carotid artery endarterectomy is often performed based on duplex findings only without confirmatory angiography. While the vast majority of hemodynamically significant lesions involve the carotid bifurcation and proximal ICA, carotid duplex examination includes the entire sonographically accessible carotid system, vertebral arteries, and often the subclavian arteries. Other pathology which can be encountered on Carotid Duplex includes: carotid artery dissection, and carotid body tumour. Ultrasound is well suited for long-term surveillance of minor carotid stenoses or post-endarterectomy patients.
Normal extracranial carotid arteries are easily accessible to ultrasound. The arterial contour is smooth with a thin intimal echo. Laminar flow profile is seen in all vessels. Minor turbulence can be encountered in the carotid bulb in the region of flow separation. The CCA is a mix-resistance vessel, ECA contains medium-to-high resistance signal, and the ICA demonstrates low resistance.
The most common carotid artery disease is the formation of carotid artery stenosis (or possibly occlusion) due to presence of atherosclerotic plaques. Other types of pathology which may be encountered on Carotid Duplex include: Subclavian Steal Syndrome, Carotid Dissection, Fibromuscular Dysplasia, and Carotid Body Tumor.
Angiography is commonly used for assessment of carotid arteries. It is particularly useful in patients who are difficult sonographic candidates such as in complex calcific carotid lesions, high bifurcation, and stenosis beyond the sonographically accessible extracranial ICA.
Initiate the examination with a greyscale survey of the CCA, bifurcation, ICA, and ECA along the entire sonographically accessible segments in transverse and longitudinal views. Close attention should be paid to vessel course, site(s) of plaque, and plaque characteristics (location, amount, echotexture, and surface).
Using colour Doppler, examine the CCA, bifurcation, ICA, and ECA. Attention should be paid to areas of narrowing, increased Doppler shifts, aliasing, and turbulence. Record colour Doppler views of CCA, bifurcation, ICA, and ECA. Flow-separation (area of non-pathologic turbulence) is often observed in the proximal ICA. Use of power-Doppler for real-time survey of the carotid arteries is discouraged due to the lack of Doppler shift data (velocity data). However, power-Doppler is useful at recording the anatomy of the vessel lumen and may be used for this purpose.
Spectral Doppler sampling is performed in the proximal CCA, distal CCA, proximal ICA, mid ICA, distal ICA, and proximal ECA. Identification of the ICA and ECA can safely be made based on visualization of vessel branches (ECA), and spectral waveform differences. In difficult cases, temporal tap of the ECA is a useful indicator. Take care to record the highest velocity for each of the above sample sites.
ICA/CCA ratio is calculated by dividing the highest ICA velocity by the distal CCA velocity. Note: In extremely high-grade critical stenoses (such as trickle flow), the below interpretation criteria may fail due to very low flow velocities. Careful review of the stenosed vessel using Colour Doppler or Power Doppler usually confirm the presence of trickle flow. Subclavian and vertebral artery sampling is also routinely performed. Vertebral artery reversal (complete or partial) are indicative of subclavian steal syndrome. Careful attention should be paid to the vertebral artery if the subclavian artery demonstrates signs of stenosis such as loss of triphasicity, high PSV, or turbulence.
The following are ASUM recommended guidelines for the interpretation of carotid Duplex. Note, that %stenosis pertains to %diameter reduction.
0%…Normal waveform and no visible plaque
< 15%…Deceleration spectral broadening, PSV < 125 cm/sec
15 - 49%…Pansystolic spectral broadening, PSV < 125 cm/sec
50 - 69%…Pansystolic spectral broadening, PSV > 125 cm/sec and EDV < 110 cm/sec or ICA/CCA > 2
70 - 79%…Pansystolic spectral broadening, PSV > 270 cm/sec or
- EDV > 110 cm/sec or ICA/CCA > 4
80 - 99%…As above plus EDV > 140 cm/sec
100%…Occluded. No flow, possible terminal thump
Note: In case of unilateral ICA occlusion, the contralateral ICA velocities may demonstrate compensatory elevation. Over-estimation of the contralateral ICA disease may therefore occur.
Duplex Scanning in Vascular Disorders