Extracranial Cerebrovascular Evaluation

Aortic Arch

The ascending aorta originates from the left ventricle of the heart. The transverse aortic arch lies in the superior mediastinum and is formed as the aorta ascends and curves posteroinferiorly from right to left, above the left mainstem bronchus. Three main arteries arise from the superior convexity of the arch in its normal configuration. The first branch is the innominate artery (brachiocephalic), the second is the left common carotid artery (CCA), followed by the left subclavian artery.

The innominate artery divides into the right CCA and the right subclavian artery, which gives rise to the right vertebral artery. The left CCA originates slightly to the left of the innominate artery, followed by the left subclavian artery, which likewise gives rise to the left vertebral artery.

Anatomic variants of the major arch vessels exist. The most common variant is the left CCA forming a common origin with or originating directly from the innominate artery, with an incidence of approximately 13%. Less frequently, the left vertebral artery arises directly from the arch, the right subclavian artery originates from the arch distal to the left subclavian artery, the right CCA originates directly from the arch, and a left innominate artery may exist, from which the left common carotid and the left subclavian originate.

Common Carotid Artery

The right and left common carotid artery (CCA) ascend through the superior mediastinum anterolaterally in the neck, located medial to the jugular vein ( Fig. 37.1 ). The CCA usually measures between 6 and 8 mm in diameter. The left CCA is typically longer than the right, as it originates from the aortic arch. Bilaterally in the neck, the CCA, jugular vein, and vagus nerve are enclosed in a connective tissue called the carotid sheath . The vagus nerve lies between and dorsal to the artery and vein. The CCA typically does not have branches, but occasionally it is the origin of the superior thyroid artery. The CCA terminates at its bifurcation into the internal and external carotid arteries (ECAs). This occurs in the vicinity of the superior border of the thyroid cartilage at approximately C4; however, this level may be asymmetric and ranges from T2 to C1. At its bifurcation, the CCA has a slight dilation, known as the carotid bulb. The carotid bulb may include the distal CCA, the proximal internal, and the proximal ECAs.

Internal Carotid Artery

The internal carotid artery (ICA) originates at the CCA bifurcation and is usually the larger of the CCA terminal branches. It serves as the main conduit of perfusion to the brain. The ICA is divided into four main segments: cervical, petrous, cavernous, and cerebral. The cervical portion of the ICA is evaluated during carotid duplex imaging examinations. It begins at the CCA bifurcation (carotid bulb) and extends to the base of the skull. The ICA is located within the carotid sheath and travels deep to the sternocleidomastoid muscle. In the majority of individuals, the ICA is posterolateral to the ECA and courses medially as it ascends the neck. The cervical ICA usually does not have branches and measures between 5 and 6 mm in diameter. The first branch of the ICA is located in the cavernous portion, which cannot be visualized during an extracranial examination; however, it can be an important collateral in cases of cervical ICA obstruction. With advancing age and progressive disease, the cervical ICA may become tortuous, coiled, and/or kinked ( Fig. 37.2 ). This may make visualization, vessel differentiation, and obtaining accurate Doppler signals difficult. Agenesis, or absence of the ICA, can occur, although it is rare. Agenesis of the ICA can be bilateral or unilateral, with a higher proportion of cases being unilateral.

External Carotid Artery

The external carotid artery (ECA) originates at the CCA bifurcation. It is typically the smaller of the CCA terminal branches and perfuses the majority of the neck and face. It is located anteromedial to the ICA at its origin but courses posterolaterally as it ascends. In approximately 15% of the population, the ECA originates lateral to the ICA. This anatomic variation occurs three times more frequently on the right side. The normal ECA measures 3 to 4 mm in diameter.

There are eight named branches of the ECA (in ascending order): the superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, and terminal branches; the superficial temporal branch; and the internal maxillary branch. The first branch of the ECA, the superior thyroid artery is the most commonly visualized branch during carotid duplex imaging. The abundant number of anastomoses between the branches of the ECA and the intracranial circulation underscores the clinical significance of the ECA as a collateral pathway for cerebral perfusion when significant, flow-limiting disease is present in the ICA.

Vertebral Artery

The vertebral arteries are large branches of the subclavian arteries, with atherosclerotic changes usually occurring at their origin. Occasionally, the vertebral artery arises directly from the aortic arch (3% of cases on the left side and rarely on the right side). The two vertebral arteries are asymmetric in size in about 75% of cases, with the left vertebral artery being the dominant artery. The vertebral artery can be divided into four segments: extravertebral, intervertebral, horizontal, and intracranial.

The extravertebral segment is evaluated during carotid duplex imaging. This segment courses superior and medial from its subclavicular origin and enters the transverse foramen of the sixth cervical vertebra. The proximal segment of the vertebral artery is approximately 4 to 5 cm in length and usually has no branches. The vertebral artery ascends within the transverse foramina of the upper cervical vertebrae (intervertebral segment), emerges from the transverse foramen of the atlas (horizontal segment), and becomes the intracranial portion as it pierces the spinal dura and arachnoid, just below the base of the skull at the foramen magnum. At this level, the right and left vertebral arteries combine to form the basilar artery.

Procedure

The transducer is placed above the clavicle on the neck. First, GS imaging is used to initially determine the locations of the arteries and CCA bifurcation, evaluate vessel tortuosity, and extent of atherosclerotic plaque. This may be performed in a transverse or longitudinal view, depending on the preference of the operator. Doing so provides global information regarding the anatomy and pathology, which will aid while imaging in a longitudinal plane.

Imaging longitudinally, the CCA is located and followed proximally as far as the clavicle will permit ( Fig. 37.3 ). The CCA can be distinguished from the internal jugular vein, as the jugular will change shape with respiration and compresses with transducer pressure. Although the origin of the right CCA is often located as it arises from the innominate artery, the left CCA originates from the aortic arch and is not typically visualized using ultrasound imaging. The origin of the left CCA may be located in some cases by using a lower-frequency transducer with a smaller footprint angled inferiorly. The CCA is imaged superiorly to the level of the carotid bifurcation (carotid bulb). The CCA bifurcation is a common site for the development of atherosclerotic disease. At the bifurcation, the ultrasound transducer is moved slightly anterior and posterior to image the origin of the ICA and ECA. The ICA and ECA are individually followed distally to the angle of the mandible or as far as quality imaging is attainable.

The vertebral arteries are located by angling the transducer slightly lateral from a longitudinal view of the mid or proximal CCA. The vertebral artery lies deep to the CCA. Once correctly identified, it should be followed as far proximally as possible. Using CD will greatly assist in locating the vertebral artery and its origin, as well as determining the direction of vertebral artery blood flow. Decreasing the CD velocity scale (PRF) may be helpful while locating the vertebral artery. The more sensitive power Doppler may also prove helpful.

Multiple scanning approaches (anterior, lateral, and posterior to the sternocleidomastoid muscle) can be used to acquire carotid images. It is not uncommon for a carotid duplex to require the use of all four approaches to obtain high-quality longitudinal and transverse images to perform a complete assessment of the arteries of the neck due to vessel tortuosity and the eccentric shape and shadowing of atherosclerotic plaque. The lateral approach provides best visualization of the carotid system, while the distal ICA is typically best visualized from a posterior approach. The transverse plane provides a cross-sectional view of the artery; therefore, any measurement of vessel diameter or plaque should be performed in this plane.

The CD and SD interrogation of the carotid system is performed in the longitudinal plane using a 60-degree SD angle between the ultrasound beam and the vessel walls. Using constant and standard SD angle permits study reproducibility and proper comparison. SD angles greater than 60 degrees are not recommended, as they cause an increase in measurement error. The sample volume, or SD gate, should be placed in the center of the artery, and parallel to the vessel walls. Although SD images may only be obtained at certain areas within the carotid arteries, the SD gate should be moved slowly throughout the entire length of the artery while searching for the highest velocity. Spot Doppler checks at specified locations will result in errors, as areas of increased velocity and stenosis may be missed. The CD display will help guide proper placement of the SD gate and is useful in visually locating sites of increased velocity (aliasing). Although CD is helpful to locate areas of increased velocity and possible stenosis, care must be taken to evaluate that area to obtain its maximum velocity. This is done by slowly moving the SD gate in proximity of and throughout the color jet.

Commonly, SD waveforms are obtained in the proximal, mid, and distal CCA; the carotid bulb; the origin of the ECA; the proximal, mid, and distal ICA; the vertebral artery; and, in some institutions, the subclavian artery. This is most often done bilaterally, so a comparison between sides can be made. Additionally, if an area of stenosis is discovered, additional SD signals may be necessary in that area to further interrogate the extent of disease. This can be done by obtaining SD signals just proximal to, within, and distal to the stenotic area. In these situations, additional GS images should also be obtained, both in the longitudinal plane (to evaluate plaque characteristics and extent) and in the transverse plane (to provide information regarding the severity of stenosis). This should be done in any vessel in which pathology is visualized. Manipulation of GSs via B-mode gain and time gain compensation is critical to obtain an accurate depiction of plaque that is present.

Common sources of technical errors that occur when performing carotid duplex imaging include the following: too high of insonating frequency for vessel depth, focal zone(s) inappropriately set, CD angle set too steep, CD gain set too low, and CD velocity scale (PRF) set too high. Cases of severe stenosis may not be detectable using CD; in these situations, power Doppler should be used to ensure the absence of flow before a determination of occlusion is made.