On completion of this chapter, you should be able to:
List the risk factors, signs, and symptoms for stroke and carotid artery disease
Distinguish normal from abnormal carotid anatomy
Understand the physiology of the carotid arterial system
Discuss the technical aspects of carotid duplex imaging
Recognize pathology associated with the carotid anatomy
Describe the sonographic findings associated with internal carotid artery disease, specifically stenosis and occlusion
Discuss common errors associated with the interpretation of carotid duplex imaging examinations
A stroke often dramatically affects the life of an individual. Stroke is a leading cause of serious long-term disability, and approximately 130,000 of the 795,000 strokes that occur each year in the United States result in death. The resultant financial burden is estimated to be $34 billion annually, comprised of direct costs (health care, rehabilitation, etc.) and indirect costs (missed work days, loss of productivity, etc.). Stroke is more prevalent among men, with risk increasing with age. Stroke is also almost twice as common in the black population than white, with the black, Hispanic, and native American populations all having an increased risk compared with the white population. A cerebrovascular accident, or “stroke,” is characterized by an interruption of blood flow to the brain (ischemic stroke) or by a ruptured intracranial blood vessel (intracranial hemorrhage). Approximately 85% of all strokes are ischemic; the remaining 15% are hemorrhagic. Because of the high prevalence and often fatal consequences, extracranial cerebrovascular ultrasound becomes an important imaging modality to identify disease that may be the potential cause of a stroke. Accurate and thorough imaging of the carotid artery is imperative, as prevention remains the best treatment for stroke.
Anatomy for extracranial cerebrovascular imaging
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, followed by the left subclavian artery.
The innominate artery divides into the right common carotid artery and the right subclavian artery, which gives rise to the right vertebral artery. The left common carotid artery 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 occur frequently. The most commonly occurring variant (between approximately 7% and 21%) is the left common carotid artery forming a common origin with or originating directly from the innominate artery. 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 common carotid artery 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 ( Figure 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. 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 usually does not have branches, but occasionally it is the origin to the superior thyroid artery. The CCA terminates at its bifurcation into the internal and external carotid arteries. This occurs in the vicinity of the superior border of the thyroid cartilage (approximately C4); however, this level may be asymmetric, and has been reported as low as T2 and as high as 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 external carotid arteries.
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 in the carotid sheath and runs deep to the sternocleidomastoid muscle. In the majority of individuals, the ICA is posterolateral to the external carotid artery 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 ( Figure 37-2 ). This may make visualization, vessel differentiation, and obtaining accurate Doppler signals difficult. Bilateral agenesis, or absence of the ICA, can occur, although it is rare. , However, unilateral agenesis is estimated to be 6 times more common than bilateral ICA agenesis. ,
External carotid artery
The external carotid artery (ECA) originates at the CCA bifurcation and is usually the smaller of the CCA terminal branches. It serves to perfuse 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 3 times more frequently on the right. The ECA usually measures 3 to 4 mm in diameter.
There are eight named branches of the ECA that have been identified (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 superior thyroid artery is the first branch of the ECA and 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.
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 (4% 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.
Extracranial arterial hemodynamics
The hemodynamics of arterial flow can be influenced by a variety of factors, including vessel diameter, flow volume, vessel tortuosity, blood viscosity, and resistance. One factor that aids in determining the resistance of arterial blood flow is the nature of the vascular bed it is perfusing. If an artery is supplying blood to a dilated vascular bed (present on most organs), resistance in that artery will be low. Conversely, an artery supplying blood to a more constricted vascular bed, or arterioles, will maintain a higher resistance.
The ICA and ECA are responsible for providing the majority of the head with oxygen- and nutrient-rich arterial blood. Although they both serve to supply blood to the head, the different areas of it that they are responsible for perfusing cause them to demonstrate very different hemodynamics. These hemodynamic characteristics need to be understood in order to appropriately perform, and interpret, a carotid artery examination.
After originating from the CCA, the ICA typically travels deep toward the base of the skull. Once intracranial, the ICA branches, which form portions of the circle of Willis that ultimately perfuse the brain. Because the brain requires a large amount of perfusion (approximately 13% of systemic blood flow) the ICA tends to be larger than the ECA to account for the larger amount of blood volume it transports. The ICA also maintains low-resistance arterial flow in the normal patient. This is due to its lack of extracranial branches, but more so to the low peripheral resistance placed upon it. Because the brain requires such a large amount of blood, and its adequate perfusion is critical, the vascular beds that perfuse it are of low resistance. This allows for continuous perfusion throughout systole and diastole of the cardiac cycle. Similar to the ICA, the vertebral artery also contains low-resistance arterial flow, as it perfuses a large portion of posterior circulation of the brain, which consists of vascular bed applying minimal amounts of peripheral resistance.
Conversely, the ECA supplies blood to the majority of the neck, face, and scalp. These areas of the body require much less blood than an area such as the brain. Therefore the capillary beds within these areas tend to be smaller and provide a higher amount of resistance. For this reason, coupled with its many extracranial branches, blood flow within the ECA undergoes high resistance. This is most evidently demonstrated by a reversal of flow component that is present during diastole.
Carotid disease and stroke risk factors, warning signs, and symptoms
Disease of the carotid arteries is caused by atherosclerotic plaque buildup along the lumen of the vessels. Plaque buildup can then cause their narrowing or, in more severe cases, occlusion. Furthermore, if atherosclerotic plaque dislodges from the endothelium wall, it can propagate and cause a stroke, sudden vision loss, or a transient ischemic attack (TIA). A TIA, often referred to as a “mini-stroke,” is an ischemic neurologic deficit that lasts less than 24 hours.
Risk factors for carotid disease and stroke can be categorized into those that are not modifiable and those that are modifiable or can be controlled. Nonmodifiable risk factors include age (risk of stroke dramatically increases with age), sex (incidence of stroke is higher in males), race (blacks have a higher stroke risk than other races), and family history of cerebrovascular disease. Modifiable or controllable risk factors include hypertension, atrial fibrillation, and other cardiac diseases; diabetes mellitus; elevated cholesterol; smoking; and a history of a sedentary lifestyle and poor diet.
Patients may present either symptomatic or asymptomatic, but the majority of individuals with carotid disease will have no symptoms. Asymptomatic patients are typically referred for carotid duplex imaging if they are at high risk for stroke, or due to the presence of a cervical bruit. A cervical bruit located in the carotid artery is a noise that can be heard while using a stethoscope, caused by high-velocity and/or turbulent blood flow causing the auscultation of the vessel and the vibration of surrounding tissues. Symptomatic patients commonly present with symptoms including aphasia, dizziness, dysphagia, diplopia, and hemianopsia.
The five warning signs of stroke are listed in Box 37-1 . Warning signs can also be remembered using the acronym FAST:
Sudden numbness or weakness of face, arm, or leg, especially on one side of the body
Sudden confusion; trouble speaking or understanding
Sudden trouble seeing in one or both eyes
Sudden trouble walking or experiencing dizziness, loss of balance, or coordination
Sudden headache with no known cause
This stands for facial weakness, arm weakness, speech difficulties, and time to act if these symptoms are observed. It is important to remember that symptoms of weakness or numbness of a leg or arm on one side of the body ( hemiparesis ) indicate disease in the contralateral carotid system. In other words, left body symptoms implicate the right carotid system and vice versa. Ocular symptoms, however, suggest disease in the carotid system on the ipsilateral side. For example, transient blindness ( amaurosis fugax ) of the right eye suggests disease of the carotid system on the right side. Symptoms such as blurred vision, dysarthria, ataxia, syncope, vertigo, or overall weakness are nonspecific and can be confusing as to which vascular system is involved. Bilateral symptoms such as these may be related to the vertebral system, especially if the carotid system proves to be clear. The classification of cerebrovascular symptoms includes the following: stroke, or cerebrovascular accident (CVA), is a permanent ischemic neurologic deficit; reversible ischemic neurologic deficit (RIND) is a neurologic deficit that resolves between 24 and 72 hours; and TIA is an ischemic neurologic deficit that lasts less than 24 hours.
Technical aspects of carotid duplex imaging
Before the carotid duplex imaging examination is performed, a thorough medical history must be taken from the patient, focusing on risk factors, signs, and symptoms of carotid disease and stroke. The patient should also be asked about any previous surgical interventions and any imaging he or she might have had, especially previous ultrasound studies. All information including history, surgical history, and previous imaging studies should be confirmed and augmented with a review of the patient’s chart and any previous relevant imaging that is available for comparison. Once completed, the examination is explained to the patient and the patient is positioned supine, with the head resting on a pillow and turned slightly away from the side being scanned. The head of the bed can be raised slightly if the patient has trouble lying flat, but too much of a forward angle can hinder imaging and decrease examination quality.
Before duplex imaging begins, brachial pressures may be obtained. A difference of ≥20 mm Hg between arms is suggestive of a proximal subclavian or innominate artery stenosis/occlusion, on the side with the lower pressure. Close examination of the neck should also be performed to determine the presence of a cervical bruit. Not all stenoses in the carotid arteries will cause bruits, and a bruit may be identified in a normal artery. Furthermore, the bruit may also be transmitted (cardiac).
The carotid duplex imaging examination consists of images obtained using gray-scale (GS) imaging (identifies vessels for Doppler interrogation, visualizes intimal thickening, evaluates location, extent, and characteristic of plaque, visualization of other pathology); color Doppler (CD) imaging (provides a qualitative assessment of flow patterns, evaluates the amount of vessel filling [identifies area of stenosis]); and spectral Doppler (SD) imaging (obtains qualitative and quantitative information regarding flow characteristics).
Suggested technical parameters for carotid duplex imaging are as follows:
Use a high-frequency (7- to 12-MHz) linear-array transducer.
The image is orientated such that the head is to the left of the monitor.
Although CD is based on the direction of blood flow (toward or away) in relation to the transducer, red is usually assigned to arterial, and blue to venous blood flow. Follow your institution’s guidelines.
Keep SD sample volume size (gate) small; this will provide waveforms that accurately represent flow characteristics.
Use a 60-degree SD angle (or less) to the vessel wall. Make sure your fine angle correction is parallel to the vessel walls. Any error can have a large effect on true velocity readings.
The CD and SD scale (pulse repetition frequency [PRF]) should be adjusted throughout the examination to evaluate the changing velocity patterns.
The CD and SD wall filters are set low.
The CD region of interest box size affects frame rate (number of image frames displayed per second), so the CD display should be kept as small as possible.
The CD and SD gain should be adjusted throughout the examination as the signal strength changes.
Harmonics may be used during GS imaging to improve hypoechoic plaque visualization.
If available, compound imaging may increase the quality of GS imaging.
Beware of using time gain compensation (TGC) controls to make vessels completely anechoic, as you may “erase” hypoechoic plaque or thrombus.
It is imperative that each institution develops a carotid imaging protocol that defines the standard examination. This protocol must include indications for a complete and/or limited examination, clinical applications, protocol and technique (arteries to be evaluated and the number and locations that SD waveforms are obtained within each artery), interpretation criteria, quality assurance, and equipment maintenance. A standard complete examination usually includes GS, CD, and SD evaluation of the carotid and vertebral arterial systems, bilaterally.
The transducer is placed above the clavicle on the neck. First, use GS imaging to evaluate and determine the locations of the arteries, the CCA bifurcation, vessel tortuosity, and atherosclerotic plaque. This may be performed in a transverse or longitudinal view, depending on the preference of the operator. This will provide 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 ( Figure 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. Follow the CCA 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 anteriorly and posteriorly 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 laterally 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 aid in evaluating the direction of vertebral artery blood flow. Decreasing the CD velocity scale (PRF) may be helpful in locating the vertebral artery, as well as the use of power Doppler.
Multiple scanning approaches (anterior, lateral, and posterior to the sternocleidomastoid muscle) can be used and are often required to obtain high-quality longitudinal and transverse images that completely assess the arteries of the neck. This is typically due to vessel tortuosity and the eccentric shape and shadowing of atherosclerotic plaque. Although the lateral approach provides the best visualization of the carotid system, 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. The use of a constant and standard SD angle permits study reproducibility and proper comparison. Spectral Doppler 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 to guide proper placement of the SD gate and is useful in visually locating sites of increased velocity (aliasing). Although CD is helpful in locating an area 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” (change).
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 a 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.
Common sources of technical error that occur when performing carotid duplex imaging include the following: the insonating frequency is too high for the vessel depth, the focal zone(s) is not appropriately set, the CD angle is too steep, the CD gain setting is too low, and the CD velocity scale (PRF) is 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.
In the normal patient, GS imaging should reveal smooth vessel walls with an anechoic lumen. No visual evidence of plaque formation or vessel wall deformities should be found. Furthermore, all vessels can be fully visualized in both the longitudinal and transverse plane.
Spectral Doppler waveforms obtained within the CCA will demonstrate characteristics of both the low resistive ICA and the high resistive ECA (end diastole above baseline). The CCA SD waveforms will display a positive and continuous Doppler pattern throughout the cardiac cycle ( Figure 37-4 , A and B ). At peak systole, the color will fill the artery to the vessel walls and display an increased velocity (a lighter shade) in the center of the artery. These CD and SD waveform findings will be present in all locations of the CCA evaluated (proximal, midportion, and distal [just proximal to the bifurcation]). The CCA spectral Doppler waveform is important because it may indicate proximal disease or suggest distal disease. However, investigators have shown that the peak systolic velocities (PSVs) recorded from the CCA change along its length and should not be considered a reliable marker for disease. Therefore an ICA-to-CCA peak systolic ratio is often calculated, which allows for a comparison between the CCA and ICA of the individual patient to be made. When selecting the SD waveform for the calculation of the CCA/ICA ratio, it is important that the SD obtained from the straight portion (mid to distal) of the CCA is used. Errors will occur if the SD waveform obtained from the proximal tortuous CCA (increased velocity) or the distal CCA that may include the bulb (decreased velocity) is used in the calculation of the ratio. At the level of the CCA bifurcation, the carotid bulb can be visualized characterized by a slight dilation of the vessel. This increase in diameter causes the bulb to produce a change in blood flow hemodynamics from that of the CCA, as blood velocities are lower and flow is more turbulent ( Figure 37-4 , C ). Boundary layer separation, which is a normal flow disturbance, may also be detected as a transient reversal of blood flow along the posterior wall of the bulb. Boundary layer separation may be visualized in the CD display and SD waveforms. Sample volumes for the ICA and ECA should start just distal to this area.
Proper identification of the ICA and ECA is not a problem in most patients, but it is essential for performing accurate carotid duplex imaging. The most reliable method used to distinguish the ICA from the ECA is the SD signal. The SD waveforms obtained in the ICA have hemodynamic characteristics of low resistance and the absence of spectral broadening. The ICA also maintains continuous antegrade flow throughout the cardiac cycle, caused by the low peripheral resistance of the brain ( Figure 37-4 , D ). This is demonstrated on SD by the waveforms remaining above the baseline and on CD by a continuous color pattern being displayed. Blood flow PSV may slightly decrease as evaluation progresses distally within the ICA, but SD waveform should remain consistent.
The ECA demonstrates a more pulsatile SD signal (high resistive, minimal diastolic flow) because it supplies blood to the skin and muscular bed of the scalp and face, which contain high resistant vascular beds ( Figure 37-5 ). On SD interrogation, the ECA usually has a steep slope to peak systole and an end-diastolic velocity that is very close to zero or absent. The normal color flow pattern of the ECA will reflect the higher resistance of the vascular beds of the scalp and face, as it will decrease during diastole and may disappear at end-diastole. Additionally the ECA is smaller in diameter, as it transports a lower blood volume, and typically originates anterior and medial at the carotid bifurcation. The ECA has extracranial (cervical) branches. The first branch of the ECA, the superior thyroid artery, is often visualized during a carotid duplex imaging examination. This can also aid in its identification. Last, a temporal tap maneuver can be used to confirm ECA and ICA differentiation, which is performed by “tapping” on the superficial temporal artery while obtaining an SD waveform. Tapping will cause small deflections in the end-diastolic component of the ECA waveform but will cause no changes to an ICA waveform (see Figure 37-5 ).