About Carotid Duplex Imaging
Objectives
- Know the stroke risk factors, warning signs, and symtoms
- Learn about the anatomy of the carotid arteries
- Describe the technical approach to imaging the carotid arteries with ultrasound
- Define the criteria utilized in assessing the degree of carotid artery stenosis present
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Introduction
The life of an individual is often dramatically affected by having a stroke. Not only is stroke a leading cause of adult disability in the United States, but approximately 160,000 of the 700,000 strokes that occur each year result in death.1 Additionally, the direct (hospital, physician, rehabilitation, etc.) and indirect (lost productivity etc) costs associated with stroke tally more then $30 billion per year.1
A stroke or "brain attack" is caused by an interruption of blood flow to the brain (ischemic stroke) or by a ruptured intracranial blood vessel (intracranial hemorrhage). Approximately 80% of all known strokes are ischemic and the remaining 20% are hemorrhagic. Because extracranial carotid artery disease is responsible for more than 50% of all the strokes, carotid ultrasound becomes an important imaging modality to identify disease that may be the potential cause of a stroke.
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Stroke Risk Factors, Warning Signs, And Symptoms
Several risk factors have been identified for stroke. Risk factors may be categorized into those that are not modifiable and those that are changeable or controllable.
Non-modifiable Risk Factors
- Age (risk increases with increasing age)
- Sex (incidence is higher in males, although females generally have a more severe deficit)
- Race (African-Americans have a higher stroke risk)
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Modifiable or Controllable Risk Factors
- Atrial fibrillation & other cardiac diseases
- Obesity (patients have more hypertension, lipid abnormalities, etc.)
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Warning Signs The five warning signs of stroke are:
- 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
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Classification of Symptoms The classification of cerebrovascular symptoms include the following:
- stroke or cerebrovascular accident (CVA): permanent ischemic neurological deficit
- transient ischemic attack (TIA): ischemic neurological deficit that lasts less than 24 hours
- amuarosis fugax: transient partial or complete loss of vision in one eye
- hemiparesis: unilateral partial or complete paralysis
- dysarthria: difficulty with speech
- aphasia: inability to communicate (speech, writing)
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Anatomy
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. It descends to the left of the trachea and esophagus. Three main arteries arise from the superior convexity of the arch in its normal configuration. The brachiocephalic trunk (innominate artery) is the first branch, the left common carotid artery the second, and the left subclavian artery the third branch in approximately 70% of cases.
The innominate 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 common variant (approximately 10%) is the left common carotid artery forming a common origin with or originating directly from the innominate artery.
Common Carotid Artery Each common carotid artery (CCA) ascends through the superior mediastinum anterolaterally in the neck and lies medial to the jugular vein. The left common carotid is usually longer than the right, because it originates from the aortic arch. In the neck, the carotid artery, jugular vein, and vagus nerve are enclosed in connective tissue called the carotid sheath. The vagus nerve lies between and dorsal to the artery and vein. The common carotid artery usually does not have branches, but occasionally it is the origin to the superior thyroid artery. The termination of the common carotid artery is the carotid bifurcation, which is the origin of the internal carotid artery (ICA) and the external carotid artery (ECA). The CCA bifurcates in the vicinity of the superior border of the thyroid cartilage (approximately C4) in 70% of the cases, and the level of the CCA bifurcations may be asymmetrical. The CCA bifurcation, however, has been described as low as T2 and as high as C1.
External Carotid Artery The external carotid artery originates at the mid-cervical level and is usually the smaller of the two terminal branches of the CCA. Initially, it lies anteromedial to the internal carotid artery, but as the ECA ascends, it courses posterolaterally. In approximately 15% of the population, the external carotid artery originates lateral to the internal carotid artery. This anatomic variation occurs more frequently on the right (3:1).
There are eight named branches of the external carotid artery: the superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, and the terminal branches, the superficial temporal, and the internal maxillary artery. The abundant number of anatomoses between the branches of the ECA and the intracranial circulation underscore its clinical significance as a collateral pathway for cerebral perfusion when significant disease is present in the internal carotid artery.
Internal Carotid Artery The internal carotid artery is usually the larger of the common carotid artery terminal branches. The ICA is divided into four main segments: the cervical, petrous, cavernous, and the cerebral.
The cervical portion of the internal carotid is evaluated during carotid duplex imaging examinations. The cervical portion of the ICA begins at the common carotid artery bifurcation and extends to the base of the skull. The ICA lies in the carotid sheath and runs deep to the sternocleidomastoid muscle. In the majority of individuals, the internal carotid artery lies posterolateral to the external carotid artery and courses medially as it ascends in the neck. At its origin, the cervical internal carotid artery normally has a slight dilation, termed the carotid bulb and/or the carotid sinus. The cervical ICA usually does not have branches. With age and progressive disease, the cervical ICA may become tortuous, coiled, or kinked.
Vertebral Artery The vertebral arteries (VA) are large branches of the subclavian arteries. Atherosclerotic changes usually occur at the origin of the vertebral arteries. 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 equal in size in approximately 25% of the cases; therefore, size asymmetry is common. In the majority of the cases, the left vertebral artery is the dominant artery. The vertebral artery can be divided into four segments: the extravertebral, intervertebral, horizontal, and the intracranial.
The extravertebral segment is evaluated during the duplex imaging examination. This segment courses superiorly and medially from its subclavicular origin to enter the transverse foramen of the sixth cervical vertebra. The proximal segment of the vertebral artery is approximately 4-5 cm in length and usually there are no branches.
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Technical Aspects Of Carotid Duplex Imaging
The examination is explained and a history (risk factors, symptoms) is obtained from the patient. Arm pressures are recorded bilaterally (<20 mmHg difference is within normal limits) and the presence of cervical bruits are documented.
Suggested instrument set-ups for carotid duplex imaging are as follows: (1) use a high frequency (7-10MHz) linear array transducer, (2) image orientation: head to the left of the monitor, (3) color assignment: although color is based on the direction of blood flow (toward or away) in relation to the transducer, red is usually assigned to arteries and blue to venous blood flow, (4) the color scale (PRF) should be adjusted throughout the examination to evaluate the changing velocity patterns, (5) the wall filter is set low, (6) the color "box" width affects frame rates (number of image frames per second) so the color display should be kept as small as possible, and (7) the color gain should be adjusted throughout the examination as the signal strength changes.
Carotid imaging is performed with the patient lying in the supine position on an examination table. The patient's head is placed on pillow and turned slightly away from the side being scanned. After applying ultrasound gel to the neck, the transducer is placed above the clavicle in a longitudinal plane. The CCA is located and is followed proximally as far as the clavicle will permit. Although the origin of the right CCA is often located as it arises from the innominate artery, the left CCA originates from the arch and is usually not accessible to ultrasound imaging. The transducer is moved cephalad following the CCA to the level of the carotid bifurcation (thyroid cartilage). |
Figure 1 - Longitudinal image of the normal carotid artery and bifurcation into the internal and external carotid arteries.
The internal and external carotid arteries are individually followed distally to the angle of the mandible. Multiple longitudinal views (anterior, lateral, and posterior to the sternocledomastoid muscle) and a transverse view are required to completely assess the cervical carotid arteries. Although the lateral approach provides the best visualization of the carotid system, the distal ICA is usually best visualized from a posterior approach.
Proper identification of the internal and external carotid arteries is not a problem in most patients. The most reliable method to distinguish the ICA from the ECA is by the Doppler signal. The ICA has diastolic flow due to the low peripheral resistance of the brain, and the ECA demonstrates a more pulsatile Doppler signal (minimal diastolic flow) because it supplies blood to the muscular bed of the jaw and face. Additionally, the ECA is usually smaller in diameter, has cervical branches, and usually originates anterior and medial at the carotid bifurcation.
The vertebral arteries are located by angling the transducer slightly laterally from a longitudinal view of the mid/proximal CCA. The vertebral artery lies deeper then the CCA. To reliably identify the vertebral artery, it should be followed distally, and periodic shadowing should be visualized from the transverse processes of the vertebrae. The vertebral artery is accompanied by a the vertebral vein and proper identification of the artery is made by the Doppler signal. Once the vertebral artery has been correctly identified, it should be followed as far proximally as possible. The use of color Doppler will greatly assist in locating the vertebral artery, its origin, and evaluating the direction of blood flow.
Doppler interrogation of the carotid system is performed in the longitudinal plane using a 60 degree angle between the ultrasound beam and the vessel walls (placement of the Doppler sample volume parallel to the color jet has not undergone extensive validation criteria). Using a constant Doppler angle permits comparison of repeated studies in the same individual. The sample volume size should be small and placed in the center of the artery (or center stream). The Doppler volume is moved slowly through the artery searching for the highest velocity. The color Doppler display will help to guide the proper placement of the sample volume and is useful in locating sites of disease (aliasing).
Doppler signals are recorded from the proximal, mid, and distal common carotid artery, the origin of the external carotid artery, the proximal, mid, and distal internal carotid artery, the origin of the vertebral artery, and the subclavian artery bilaterally. In addition, Doppler signals are obtained from any area of stenosis. (See Fig. 2 and 3). It is important to evaluate all Doppler signals bilaterally to correctly perform a carotid duplex imaging examination in an individual patient.
The location of any plaque visualized during the examination should be described, as well as its surface characteristics (smooth vs irregular) and echogencity (calcification).
It is very important for patient management to differentiate between a high grade stenosis versus an occlusion of the ICA. An ICA occlusion is not amenable to surgical intervention. To characterize an ICA as occluded, the artery should be evaluated with Doppler, color Doppler, and power Doppler to rule out the presence of trickle flow. When determining if the ICA is occluded, the color PRF should be decreased to document the presence of any slow moving blood flow and the color gain should be increased to enhance any blood flow. Secondary ultrasound characteristics of an ICA occlusion are: echogenic material filling the lumen, lack of arterial pulsations, reversed color blood flow near the origin of the occlusion, and the loss of diastolic blood flow in the ipsilateral CCA.
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Figure 2 - High grade stenosis is shown just distal to the bifucation of the common carotid artery. (see area of narrowing) |
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Figure 3 - Color Doppler image shows aliasing (blue/yellow color); the Doppler sample volume is placed within the area of highest velocity (turbulence) and the spectral waveform shows an abnormal waveform of 4.5m/sec in this patient with internal carotid artery stenosis. |
A new development in the evaluation of blood flow is called B-Flow imaging. B-Flow allows the flow pattern to be shown in shades of gray and provides excellent visualization of the walls of the carotid and vertebral arteries. (Fig. 4 and 5) Two short real time clips will allow the reader to compare the difference between color Doppler and B-Flow imaging in two different patients. The first patient presents with an internal carotid artery stenosis and the second patient has a complete occlusion of the ICA.
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Figure 4 - Color Doppler shows a longitudinal image of a patient with internal carotid artery stenosis. The internal carotid artery shows the area of narrowing with increased (aliased) flow denoted by the blue/yellow pattern. |
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Figure 5 - B-Flow in the same patient with internal carotid artery stenosis shows the walls of the vessel as well as the area of narrowing. |
B-Flow Clips - click on image to view flow with Quicktime
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ICA stenosis (511kB) |
ICA occlusion (795kB) |
In addition to documenting the presence of atherosclerotic disease during a carotid duplex imaging examination, the sonographer should also be aware that carotid aneurysms, dissections, and carotid body tumors may be encountered (Fig. 6).
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Figure 6 - A patient with a carotid artery dissection shows the "false"channel (yellow-orange) as separate from the normal lumen (red). |
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Interpretation Of Carotid Duplex Imaging
The accurate interpretation of a carotid duplex imaging examination depends upon the quality and the completeness of the evaluation. Often the patient's body habitus will affect the quality of the image and the sonographer's ability to search the entire carotid system with Doppler. The sonographer must be prepared to switch transducers if necessary to complete the carotid examination and have a complete understanding of the equipment controls to optimize the duplex imaging system. In addition to the peak systolic velocity, end diastolic velocity, direction of blood flow, the shape of the Doppler spectral waveforms should be evaluated bilaterally. Abnormal waveform shape (increased or decreased pulsatility) may be an indicator of more proximal (innominate, subclavian) or distal (intracranial) disease.
To determine the degree of stenosis present, a complete Doppler evaluation of the artery is necessary. There should be an elevated velocity through the narrowed segment and post-stenotic disturbances distal to the stenosis. The highest velocity obtained from a stenosis is used to classify the degree of narrowing. Doppler signals obtained distal to the area of post-stenotic flow disturbance may be normal or diminished, and the upstroke of the distal Doppler spectral waveform may be slowed.
The information in the following table (Carotid Research Laboratory, University of Washington) is used to categorize disease from the origin of the internal carotid artery:
Diameter reduction |
Peak systolic velocity |
End diastolic velocity |
<50% |
<125 cm/sec |
50-79% |
>= 125 cm/sec |
80-99% |
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>=140 cm/sec |
Occlusion |
no signal |
no signal |
Because the recent carotid endarterectomy trials [North American Symptomatic Carotid Endarterectomy Trial (NASCET)2, Asymptomatic Carotid Atherosclerosis Study (ACAS)3, European Carotid Surgery Trial (ECST)]4 used specific thresholds for surgical treatment, ultrasound criteria for ICA stenosis >70% and >60% were needed to classify patients. Investigators have found that an ICA/CCA peak systolic velocity (PSV) ratio useful in grading ICA stenosis >70% and >60%. The following ratios are calculated using the highest PSV from the origin of the ICA divided by the highest PSV from the CCA (distal):
Diameter reduction |
ICA/CCA PSV ratio |
70-99% |
>45 |
60-99% |
>3.26 |
In the presence of a contralateral ICA occlusion the velocity from the ipsilateral ICA may be elevated. This may lead to overreading the extent of the ipsilateral ICA disease.7,8 To avoid overestimation of the ICA stenosis new velocity criteria have been suggested. A peak systolic velocity of > 140 cm/sec is used for a stenosis greater than 50% diameter reduction and an end diastolic velocity of >155 cm/sec for a stenosis greater than 80% diameter reduction of the lumen.7
The evaluation of normal vertebral arteries produce a wide variation in velocities. Absolute velocities are not useful to diagnose a stenosis. Post-stenotic disturbances in the vertebral artery and changes in the waveform shape may suggest a proximal obstruction. It is important to note the direction of blood flow in the vertebral artery. A subclavian steal is present if there is reversal of vertebral artery blood flow direction secondary to a significant obstruction in the ipsilateral subclavian or innominate artery. The arterial obstruction must be located proximal to the origin of the vertebral artery. A vertebral artery Doppler signal can also display an alternating (toward and away) pattern. If an alternating pattern occurs in the vertebral artery it may change to reversal of blood flow direction with arm exercise or after reactive hypermia of the ipsilateral arm induced by maintaining a blood pressure cuff inflated above systole for approximately 3 minutes.
The evaluation of normal subclavian arteries produce multiphasic high-resistance Doppler signals. If there is a significant stenosis or oclusion of the proximal segment of this vessel then the Doppler signal distal to the stenosis is monophasic. A difference of blood pressure in the arms greater than 20mmHg is usually associated with significant disease of the subclavian artery on the side with the lower blood pressure.
Other important information to include in the interpretation of a carotid duplex imaging examination is: (1) the location of the stenosis, (2) the extent of the plaque and patency of the distal ICA, (3) the presence of tortuousity or kinking of the vessels, and (4) plaque characteristics (smooth vs irregular surface, calcification).
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Intraoperative Carotid Duplex Imaging
The assessment of the carotid endarterectomy site by duplex imaging for technical adequacy is an effective method to improve the results of the operation. Intraoperative duplex imaging identifies disturbed blood flow and anatomic abnormalities such as residual plaque, thrombus, and platelet aggregation. The detection of peak systolic velocities >150 cm/sec with the presence of an anatomic defect warrants correction because of its potential to progress.9 Investigators have reported that the use of intraoperative carotid duplex imaging has had a favorable impact on the stroke rate and incidence of restenosis of the carotid artery.9,10
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Summary
The best carotid duplex imaging examinations will be achieved by the following suggestions: (1) optimize the gray scale image and know how each color control affects the image and how the different instrument controls affect each other, (2) compare the right and left side, (3) be aware of the spectral Doppler waveform configuration which may suggest proximal or distal pathology, (4) use a consistent Doppler angle, (5) use the color Doppler and power Doppler display as a guide to the Doppler examination, and (6) each institution should establish their own diagnostic criteria.11
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References
- National Stroke Association, 1999. www.stroke.org
- North American Symptomatic Carotid Endarterectomy Trial Collaborators: Beneficial effect of carotid endarterectomy in symptomatic patients with high grade carotid stenosis. N Engl J Med 1991; 325:445-453.
- Executive Committee Asymptomatic Carotid Atherosclerosis Study. Endarterectomy for asymptomatic carotid artery stenosis. JAMA 1995;273:1421.
- Randomized trial of endarterectomy for recently symptomatic carotid stenosis: final results of the MRC European Carotid Surgery Trial (ECST). Lancet 1998;351:1379-1387.
- Moneta GH, Edwards JM, Chitwood RW et al. Correlation of North America Symptomatic Carotid Endarterectomy Trial (NASCET) angiographic definition of 70-99% internal carotid artery stenosis with duplex scanning. J Vasc Surg 1995; 17:152-159.
- Moneta GH, Edwards JM, Papanicolaou G et al. Screening for asymptomatic carotid internal carotid artery stenosis: duplex criteria for discriminating 60-99% stenosis. J Vasc Surg1995; 21:989-994.
- Fujitani Rm, Mills JL, Wang LM, Taylor SM. The effect of unilateral internal carotid artery occlusion upon contralateral duplex study: Criteria for accurate interpretation. J Vasc Surg 1992;16:459-468.
- Spadone DP, Barkmeier LD, Hodgson KJ, et al. Contralateral internal carotid artery stenosis or occlusion: pitfall of correct ipsilateral classification- A study performed with color-flow imaging. J Vasc Surg 1990; 11:642-649.
- Bandyk DF, Mills JL, Gahtan V, Esses GE. Intraoperative duplex scanning of arterial reconstructions: Fate of repaired and unrepaired defects. J Vasc Surg 1994;20:426-433.
- Baker WH, Koustas AG, Burke K, et al. Intraoperative duplex scanning and late carotid artery stenosis. J Vasc Surg 1994;19:829-833.
- Kuntz KM, Polak JF, Whittemore AD, et al. Duplex ultrasound criteria for the identification of carotid stenosis should be laboratory specific. Stroke 1997;28:597-602.
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