General principles of carotid Doppler ultrasonography

Carotid Artery Anatomy and Tips forDifferentiating the Internal Carotid Arteriesfrom the External Carotid Arteries

The right carotid artery arises from the right brachiocephalic artery.Ultrasonography can show the most proximal segment of thecommon carotid artery. The left common carotid artery arises fromthe aortic arch. Ultrasonography cannot show the proximal segmentof the left common carotid artery. The examiner should be ableto differentiate the internal carotid artery (ICA) from the externalcarotid artery (ECA). The ICA is located posterior and lateral to theECA. The ICA is slightly larger than the ECA. The ECA has branchessuch as the lingual artery, but the ICA does not. The Dopplerspectrums from the ICA show a lower resistive pattern (Fig. 1A). Thevelocity difference between the systolic phase and diastolic phase ofthe ICA is smaller than that of the ECA. Another way to differentiatethe ECA from the ICA is that during the acquisition of the ECADoppler spectrum, placing the fingertips on the ipsilateral temporalartery generates a serration-like artifact on the Doppler spectrumfrom the ECA. This temporal artery tapping-induced artifact is notseen from the ICA. This so-called “temporal tapping” is a useful toolin differentiating the ICA from the ECA (Fig. 1B). Being certain ofwhich is the ECA and the ICA is important in case one of them is occluded.

Intima-Medial Thickness

The intima-medial thickness (IMT) has been widely used as one ofthe parameters of atherosclerosis [1,2]. The IMT is measured ona two-dimensional (2D) gray-scale image. The optimal gray-scaleimage of the longitudinal scan of the carotid artery, which passes bythe center of the carotid artery, shows two bright interfaces alongthe artery wall. In the far wall, the upper bright line is the interfacebetween the blood and intima, and the lower bright line is theinterface between the media layer and adventitia layer. The interfacebetween the intima and media does not produce any interface.The distance between the upper and lower bright line representsthe thickness of the intima and media layer. It would be better thatthe carotid artery is parallel to the probe surface to minimize theoverestimation of the IMT from the diagonal measurement. The IMTis generally measured on the distal common carotid artery at the farwall because the common carotid artery is easier to image and lessvariable than the ICA due to the angle of the beam or depth of thevessel. In one study, the success rate for far wall measurement was89% (109/122) in the common carotid artery and 38% (140/366)in the ICA [1]. The IMT should be measured on a segment withouta focal lesion. Focal atherosclerotic lesions are much more commonin the ICA than in the common carotid artery. Nowadays, manyvendors provide an automated tool for measuring the IMT (Fig. 2).Carotid artery atherosclerosis as measured by IMT is an independentrisk factor for stroke and myocardial infarction [1-3].

Plaque Morphology and Plaque Volume

The plaque morphology, such as the echogenicity of the plaque, thesurface, presence of ulceration, as well as the presence of plaqueand stenosis, is important for predicting future cardiovascular events[4]. A description of the plaque morphology from a gray-scale image

is highly recommended during carotid Doppler ultrasonography (Fig.3). The description should include the echogenicity of the plaque,the surface, and the presence of ulceration. The echogenicity ofthe plaque could be described as one of echogenic plaque, isoechoic plaque, echolucent plaque, or heterogeneous plaque. Isoechoic plaque means that the echogenicity of the plaque is the same as that of the intima-media complex. The plaque surface canbe described as smooth, irregular, or ulcerated. Plaque ulceration isassociated with an increased risk of stroke [5,6].

It is, however, very difficult to detect plaque ulceration byultrasonography examination, and it is operator dependent. It isknown that the sensitivity of detecting carotid plaque ulcerationranges from under 30% to over 80% when it is compared withpathological specimens [7,8]. The effort to detect ulceration shouldbe applied to increase accuracy in the assessment of risk of thepatient with carotid plaque. Another problem is that such an ulceris not clearly correlated with symptoms and is difficult to detect

without careful gray-scale examination. The depression of the plaquesurface by more than 2 mm is thought to indicate ulceration. Thepattern of plaque ulceration can be cystic, bridge-shaped, spongeshaped, or a simple depression [8-11].

To go into more detail on plaque echogenicity, it has been notedthat symptomatic lesions are typically associated with purely orpredominantly hypoechoic plaques. There has been an effort tomeasure plaque echogenicity quantitatively. Biasi et al. [12] usedlongitudinal images of the plaque and vessel wall and measuredthe gray-scale median (GSM). The GSM of the blood pool was 0to 5, and the GSM of the adventitia of the wall was 185 to 195.What they found was that the stroke risk during carotid stentingprocedures is dependent on the GSM of the plaque. Plaque withGSM values of 25 or less showed a 7.1% stroke risk while plaquewith GSM values more than 25 showed only a 1.5% stroke risk. Thismeans that echolucent plaque is more vulnerable [11] (Fig. 3B).

Recently, three-dimensional (3D) ultrasonography has beenused for measuring plaque volume [13-15] (Fig. 4). On a 2D grayscale image, plaque size can be measured based on length andheight, but the total volume of the plaque cannot be measured.3D ultrasonography showed good intra- and interobserverreproducibility for measuring total plaque volume [15].

The plaque volume can be used as a monitoring tool foratherosclerosis treatment. The plaque volume is known to increasewithout treatment and decrease with statin therapy [16]. 3Dultrasonography is thought to be useful for the monitoring of plaqueand could also be useful for the evaluation of new treatments [17]. 3D ultrasonography volume measurements are more sensitive thanIMT for the evaluation of carotid plaque progression posttreatment.More specifically, while there was a significant change in the 3Dplaque volume during the follow-up period, there was no change inthe IMT.

3D ultrasonography also can be used for plaque characterization.The limitation of 2D gray-scale evaluation of plaque is that singleor even multiple images cannot represent the entire plaquevolume. Heliopoulos et al. [18] tried to measure the echogenicityof the entire volume of plaque with 3D ultrasonography. In 110symptomatic and 104 asymptomatic patients with carotid plaquedisease, they assessed the mean gray value of the whole plaqueand found a higher incidence of low echoic plaque in symptomaticpatients than in asymptomatic patients, suggesting a higher risk ofcerebral ischemia from the low echoic plaque.

Color Doppler Ultrasonography and PulsedWave Doppler Ultrasonography

Color Doppler is color-encoded velocity information on a gray-scaleimage. Color Doppler is a good tool for visualizing the blood flow inthe vessel and finding stenotic segments.

To obtain a proper color Doppler image, an adequate acousticangle is essential. With a linear probe, to generate a gray-scaleimage, the sonic beam needs to be perpendicular to the skin.However, to obtain proper velocity information from color Dopplerultrasonography, the Doppler angle should be between 30° and 60°.The carotid artery is not a deeply located structure, and ensuring theproper angle of the Doppler probe surface relative to the commoncarotid artery is not easy. In contrast to measuring the IMT position,in which it is better for the vessel wall to be parallel to the probesurface, there should be an angle between the probe surface andvessel in color Doppler ultrasonography. One helpful technique forachieving this angle is the heel and toe technique. The heel and

to etechnique is a way of steering the probes. In the usual positionfor carotid artery scanning without any pressure, the probe surfacewill be parallel to the common carotid artery in most patients. Justpushing the head side edge or foot side edge will create a bit ofan angle between the probe surface and vessel, and the optimalDoppler angle can be achieved (Fig. 5).

Adjusting the velocity range is one of the important ways ofcontrolling Doppler ultrasonography parameters. One of thepurposes of color Doppler ultrasonography is finding any stenoticsegment in the vessel. Because the flow volume through thevessel is constant, the velocity of the flow is fastest at the stenoticsegment. If the upper limit of the color velocity scale is just belowthat of the flow velocity in the normal vessel, the increased flowvelocity in the stenotic segment will be above the upper limit ofthe velocity scale and there will be an aliasing artifact. If there is asegment showing an aliasing artifact at the proper velocity scalesetting, it means that the segment is stenotic. The usual normalvelocity of the common carotid artery is 30-40 cm/sec [19], but thevelocity scale setting should be adjusted for each patient.

However, to measure the exact flow velocity, we cannot rely oncolor Doppler imaging; we need pulsed wave Doppler. In pulsedwave Doppler, a small sample volume in the center of the vesselor in the stenotic segment will be used to check the velocity of thesegment. The peak velocity is used for detecting significant stenosis.Angle correction is essential to measure the true flow velocity, andthe angle correction should be along the flow direction, not alongthe vessel wall. The flow direction is usually not different from thevessel direction, but in cases of eccentric atherosclerotic plaque, theflow direction and vessel direction can be different.

Many criteria have been used for detecting significant stenosis

of the ICA. In 2003, a consensus conference was held for Dopplerultrasonography diagnosis of carotid artery stenosis [20], and theyproposed consensus velocity criteria for carotid artery stenosis,which has been used since then (Table 1). In these criteria, the peaksystolic velocity is important. A peak systolic velocity of 125 cm/sec or higher in the ICA or twice as fast as that of the commoncarotid artery is thought to indicate possible significant stenosisof the ICA. A good example of a report form for carotid Dopplerultrasonography, as shown in Table 2, should include all theinformation on these criteria.The vertebral artery Doppler spectrum has a low resistive patternlike that of the ICA (Fig. 6). In examining the vertebral artery, theflow direction is important because it can be reversed in case ofsubclavian steal syndrome [21].

Pitfalls in Carotid Doppler Ultrasonography

A calcified atheroma of the carotid artery can make it difficult tofind a proper sonic window for color Doppler ultrasonographyor pulsed wave Doppler. Trying different sonic windows such asposterolateral projection is helpful in some cases. If the obscuredsegment is less than 1 cm in size and there are normal waveformsat the proximal and distal parts of the plaque, it can be concludedthat no significant stenosis exists. However, when the calcification isobscuring a large vascular segment, another imaging modality suchas magnetic resonance angiography could be required.In case of contralateral carotid artery stenosis, the PSV can befalsely elevated without significant stenosis. If there is critical 

stenosis of the ICA such as a residual luminal diameter of less than0.7 mm, the peak systolic velocity of the affected segment can showpseudo-normalization. Color Doppler ultrasonography could depictthis critical stenosis of the ICA.

Carotid Doppler ultrasonography is a powerful modality forevaluating the carotid arteries. Carotid Doppler ultrasonographyis useful not only for detecting carotid artery stenosis, but also fordetecting atherosclerotic plaque including visualizing the intimamedia thickness as a biomarker for atherosclerosis. Radiologistsshould be familiar with the physics and clinical findings of Dopplerultrasonography in performing Doppler ultrasonography studies ofthe carotid arteries.

ORCID: Whal Lee:http://orcid.org/0000-0003-1285-5033

Conflict of Interest

No potential conflict of interest relevant to this article was reported.

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