5 Extracranial Cerebral Arteries

Color duplex sonography (CDS) is the standard diagnostic tool for evaluating the extracranial cerebral arteries. Its most important application is the detection of carotid artery stenosis in the primary and secondary prevention of stroke. The interdisciplinary German S3 guidelines (see Chapter 23) on the diagnosis, treatment, and follow-up of extracranial carotid artery stenosis recommend sonography by an experienced operator as the diagnostic modality of first choice.¹⁶ Ultrasound is particularly useful for assessing degree of stenosis, which is of key importance for therapeutic decision-making³⁰ and for detecting the progression of stenosis.¹⁰ Ultrasound can also supply information on plaque morphology.²⁶

The detection of pathology in the extracranial carotid artery or vertebral artery is particularly important owing to the availability of specific treatment options. Vascular ultrasound permits the detection of vasculitis and is a useful tool for detection of dissections and arteriovenous (AV) fistulas.

5.2 Carotid Artery

5.2.1 Anatomy, Examination Technique, and Normal Findings

Introductory Note

Stenotic changes in the carotid artery are manifested mainly in vascular segments that are easily accessible to ultrasound scanning: the extracranial carotid bifurcation and the proximal segment of the internal carotid artery (ICA). Less commonly, carotid stenosis may develop in more distal extra- and intracranial segments or at the origin of the common carotid artery (CCA). These vascular segments are partially inaccessible to direct sonographic imaging, and indirect hemodynamic criteria must also be applied in order to detect obstructive lesions in those areas.

Normal Anatomy

Common Carotid Artery

The right CCA arises from the brachiocephalic trunk. Its counterpart on the left side generally arises directly from the aortic arch, runs cephalad, and divides at a variable level (usually the superior border of the thyroid cartilage) into the internal and external carotid arteries (Fig. 5.1).

Internal Carotid Artery

The large-caliber ICA gives off no extracranial branches and ascends to the skull base posterolateral to the external carotid artery (ECA), while showing a variable degree of tortuosity. After emerging from the carotid canal, it forms the curved carotid siphon and gives off its first substantial branch, the ophthalmic artery. It then divides into its terminal branches, the middle and anterior cerebral arteries. The proximal ICA, and occasionally the distal portion of the CCA, shows a variable dilatation called the carotid bulb.

External Carotid Artery

Just past its origin from the CCA, the ECA divides into multiple branches. Its first branch is the superior thyroid artery, which usually arises at the level of the carotid bifurcation; sometimes it springs directly from the distal CCA, but always on the side of the external carotid origin. Other important branches are the facial artery, occipital artery, and superior temporal artery.

Anatomic Variants

The carotid artery may show any of several variations in its normal anatomy:

•Anomalies of position and course

•Caliber variants

•Anomalous origin

Anomalies of Position

Anomalies of position are the most common normal variants of the carotid artery and predominantly affect the carotid bifurcation. The ICA lies posterolateral to the ECA in approximately 70% of cases, posteromedial in approximately 20%, medial in approximately 10%, and anterior in fewer than 1% of cases.

Caliber Variants

It is not unusual to find minor side-to-side differences in the carotid artery diameters, but an abnormally small caliber (hypoplasia) is very rare. Caliber variants most commonly affect the carotid bifurcation: the location and prominence of the carotid bulb can vary greatly in different individuals. Accordingly, normal hemodynamic findings (e.g., the presence or absence of flow separation) may also vary over a wide range.

Anomalous Origin

An important but rare variant is the ascending pharyngeal artery arising from the ICA (Fig. 5.2). This anomaly, present in 1% to 2% of the population, can make it difficult to differentiate between the internal and external carotid arteries. When this normal variant is present, the origin of the ICA may show segmental occlusions that are surgically treatable. The ascending pharyngeal artery may also arise directly from the carotid bifurcation at a site between the internal and external carotid arteries. The most common proximal normal variant of the carotid artery is the left CCA arising from the brachiocephalic trunk (truncus bicaroticus).

Examination Technique and Normal Findings

The carotid artery is imaged in anterolateral and posterolateral longitudinal sections and in transverse sections; the patient’s head is turned slightly to the opposite side (Fig. 5.3). If the ICA occupies a far medial position and is poorly visualized in standard planes, it may be possible to scan it from the anterior side with the head tilted back. The examiner sits behind the head of the supine patient, occupying the same position as in extracranial and transcranial Doppler ultrasound, or sits on the patient’s right side as in abdominal ultrasound.

B-Mode

A complete examination should include B-mode, color Doppler, and spectral Doppler views (Table 5.1). It is best to start with B-mode imaging, since a good B-mode visualization is an effective foundation for the examination as a whole. The CCA is imaged in contiguous planes, starting from the middle or distal third of the vessel and proceeding first in the caudal direction and then cephalad. The bifurcation and its branches are visualized. The vessel walls and lumen are evaluated in various B-mode planes. Occasionally the internal and external carotid arteries can be displayed simultaneously in one bifurcation view (Fig. 5.4). While still in B-mode, the operator should try to differentiate the external and internal carotid arteries by noting their relative positions and identifying their origins. The vessel walls are evaluated according to the criteria listed in Table 5.2.

Next, the same vascular regions are analyzed in color Doppler mode (Fig. 5.5). It is determined whether flow is detectable in all vascular segments. Flow is evaluated for direction and local changes (e.g., local acceleration and flow disturbances). If a flow void is noted in the pattern, that area should be closely scrutinized in various planes and at various insonation angles. If the finding is reproducible, the site is reexamined in B-mode to check for possible hypoechoic structures. The lower part of the CCA cannot always be visualized, but this should still be attempted if there is indirect evidence of a proximal flow obstruction.

The following three instrument settings are particularly important in color Doppler mode:

•Color gain

•Pulse repetition frequency (PRF) for setting the upper cutoff frequency of the color scale

•Color box angle

These settings must be changed frequently during the examination. The color gain should be adjusted just below the level at which extravascular color pixels start to appear. Whenever possible, the color scale should be set just below the level at which aliasing occurs in the vascular segment of interest. This is the best way to detect a local increase of flow velocity across a stenosis. The color box angle for carotid artery imaging should be adjusted to obtain an insonation angle that is well below 90 degrees. An insonation angle close to 90 degrees may cause an apparent flow reversal in the vessel or may even fail to detect existing flow.

Spectral Doppler

All vascular segments that are suspicious on color Doppler scans are investigated further by Doppler spectral analysis (Fig. 5.6). Doppler spectra are routinely obtained from the CCA, ECA, and ICA (at least 1 cm above the carotid bulb). If normal waveforms are found, the internal and external carotid arteries can be positively identified based on that finding alone. Equivocal cases can be resolved by the compression of ECA branches (Table 5.3). Spectra sampled from the vessels are evaluated in a side-to-side comparison so that a possible obstructive lesion outside the scannable region can be detected on the basis of indirect criteria. Attention should also be given to possible signs of abnormally increased blood flow (angioma, collaterals).

Table 5.3 Criteria for differentiating the internal and external carotid arteries

Positive differentiation of the internal and external carotid arteries is important so that stenotic lesions can be assigned to the correct vessel. Differentiation based on differences in Doppler waveforms is not possible in pathologic cases. The ECA can be positively identified by detecting multiple branch vessels or the origin of the superior thyroid artery.

Continuous-Wave (CW) Doppler

Evaluation of the periorbital arteries (supratrochlear artery, STA) is important for detecting indirect signs of stenosis. CW Doppler is best for this purpose (Fig. 5.7). A CW Doppler probe is available as an option for some duplex ultrasound systems. Another alternative would be to add a Doppler scanning unit to the system. Blood flow is assessed with a 5- to 8-MHz pencil probe positioned at the medial canthus of the eye. The probe is applied without pressure and is angled slightly toward the midline and parietal region. The probe position is varied until a maximum audible Doppler signal is obtained.

Because the supratrochlear artery often has a tortuous course, the flow direction in the artery cannot always be accurately determined based on flow direction relative to the probe. A compression test of external carotid branches will increase confidence that antegrade flow is present. While applying the Doppler probe on the supratrochlear artery, the operator manually compresses branches of the ECA—the superficial temporal artery and facial artery—on the ipsilateral side and, if necessary, on the contralateral side. Care is taken not to alter the probe position during the test (this is a common pitfall in the compression test, especially if multiple external carotid branches are compressed simultaneously).

5.2.2 Stenosis

Cerebral ischemia secondary to extracranial carotid artery stenosis is a frequent cause of stroke. Therefore, early detection of these lesions is of major importance, especially since various invasive and conservative treatment options are available. According to the Oxford Vascular Study, the conservative treatment of asymptomatic carotid stenosis with statins is of greater benefit than reported in previous studies.²⁵ This means that the early detection of carotid stenosis is important for stroke prevention, even if invasive therapy is withheld.

Extracranial carotid stenosis most commonly occurs in the proximal segment of the ICA. Stenosis is less common in the CCA, where it mainly occurs just below the bifurcation. These vascular segments are easily accessible to ultrasound.

Internal Carotid Artery Stenosis

Grading of Stenosis

The decision between invasive or noninvasive treatment for extracranial carotid artery stenosis depends primarily on the degree of stenosis. In asymptomatic stenosis, a rapid increase in degree of stenosis is believed to indicate an increased stroke risk. In the past, various methods for grading stenosis have been used concurrently. The European carotid surgery trial (ECST) method (based on the original lumen) takes into account the physiologic dilatation of the vessel at its origin, while the NASCET (North American Symptomatic Carotid Endarterectomy Trial) method (based on the ICA distal lumen) does not take proximal dilatation into account (Fig. 5.8).

Various ultrasound methods and criteria for grading carotid stenosis have been used in the past. The DEGUM (Deutsche Gesellschaft für Ultraschall in der Medizin) criteria for grading internal carotid stenosis¹⁰ employ a multiparameter approach: high-grade stenosis is described in 10% increments, which also aids in detecting the progression of stenosis. The DEGUM criteria differ significantly from the grading system used in the Anglo-American literature, a less exacting system that basically employs one parameter (plus one minor criterion) and thus defines just two categories of stenosis, moderate and high grade.¹⁸

Today, there is a consensus among many professional societies to grade internal carotid stenosis by the NASCET method while also using the DEGUM ultrasound criteria.⁴ Additionally, the DEGUM criteria have been revised and transferred to the NASCET system.¹⁰

All ultrasound criteria for grading stenosis have their limitations and may lead to misinterpretation when applied alone. The key advantage of a multiparameter grading system is that the different criteria supplement one another, so that a synoptic review of all the findings allows them to be stratified into multiple, well-defined grades of stenosis. Because the individual criteria have varying degrees of reliability, they are grouped into major and minor criteria (Table 5.4).

Nonstenotic plaques can be visualized in the B-mode image (Fig. 5.9) but cannot be accurately graded by percentage of stenosis (NASCET). For follow-up purposes, the maximum plaque length and thickness should be documented in the scan plane displaying the plaque in its greatest extent. Documentation in a transverse view is also advised whenever possible.

Color Doppler

Low-grade stenosis can be detected and differentiated from nonstenotic plaque by the presence of local flow acceleration (aliasing) in the color Doppler image (Fig. 5.10). This requires careful adjustment of the beam angle and PRF in the color Doppler mode.¹⁰ Color Doppler (with a low PRF setting) is also crucial for distinguishing stenosis from complete occlusion.

Peak Systolic Velocity in Stenosis

The site of maximum stenosis (Fig. 5.11) is indicated by aliasing in color Doppler with the proper PRF setting,¹⁰ and the peak systolic velocity (PSV) will be measured at that site in spectral Doppler. Angle correction should be adjusted for the precise direction of jet flow in or just beyond the stenosis. This criterion is subject to uncertainties in cases with a very short or long stenosis or multivessel disease. Measurements are also affected by the degree of collateralization. As a result, PSV alone is not a reliable criterion for grading the severity of stenosis.

Poststenotic Flow Velocity

PSV (Fig. 5.12) should be measured well beyond the stenosis (outside the zone of the jet and flow disturbances), if necessary by using a curved-array transducer. This criterion may not be available for stenosis occurring at a far distal level.

Detection of Collaterals

The presence of collateral circulation proves the existence of a very high-grade, hemodynamically significant flow obstruction. However, the absence of this criterion should be interpreted with caution as intracranial collateral channels are frequently absent, or ophthalmic collateral flow may be absent because an effective intracranial collateral circulation has made it unnecessary. Scanning at both the extracranial and intracranial levels is a more reliable approach for detecting collateral flow. Precursors of collateral flow such as alternating flow or unilateral slow flow in the periorbital arteries are significant diagnostic findings.

Reduced Flow Velocity in the Common Carotid Artery

This finding requires bilateral examination with a side-to-side comparison (Fig. 5.13). A slowing of flow velocity in the CCA changes the systolic/diastolic ratio in the Doppler waveform, as pulsatility is increased. The degree of this indirect stenosis criterion also depends on the pattern of collateral recruitment: in collateral flow through the contralateral ICA, a side-to-side difference is detectable at an earlier stage than in collateral flow through the ipsilateral ECA.

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