1. Recognize anatomy and physiology related to a
carotid duplex examination
2. Understand common and uncommon carotid pathology
3. Distinguish normal and abnormal carotid duplex
Stephanie Wilson BS, RVT, RDMS
Coordinator of the Vascular Sonography Program
South Hills Institute of Business and Technology
State College, PA
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Estimated Time for Completion: approximately 1 hour
Date of Release and Review: November 13, 2015
Expiration Date: November 12, 2018
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Carotid duplex examination is a well established diagnostic imaging tool to assess the status of a patient’s cerebrovascular circulation. It has been utilized since the late 1970s where its clinical application was first developed at the University of Washington. Duplex can categorize disease based on various ranges of the severity of stenosis as opposed to contrast angiography that defines stenosis based on diameter reduction. The results of carotid duplex examination can assist the clinician in determining a patient’s relative risk of stroke and also assist in determining treatment using stenting or surgical techniques. This examination relies on strong knowledge of cerebrovascular anatomy and pathophysiology, proper examination technique and quality instrumentation which will be presented in this online review course.
The most common cause for carotid artery disease is atherosclerosis. Atherosclerotic disease is the accumulation of lipids, cholesterol, and/or triglycerides to cause an inflammatory response within the tunica intima of the artery. The arterial walls thicken, harden and lose their elasticity. Plaque composition can include lipids, complex carbohydrates, blood byproducts, and fibrous tissue. These deposits within the intimal wall cause an inflammatory response with smooth muscle cell proliferation into the tunica media. As the plaque advances and ages, it may become altered by hemorrhage, cell necrosis, or ulceration and it is categorized based on its appearance and composition. A plaque that is homogeneous, with low level echoes from a thin layer of lipid material on the intima of the artery is known as a fatty streak. A fibrous plaque is homogeneous with low to medium level echoes from additional accumulations of lipid deposits, collagen and elastic fibers. A complicated lesion is heterogeneous with bright echoes causing
acoustic shadows from the progressive fibrotic plaque. The deposits that cause this include additional collagen, calcium and cellular debris. An ulcerative lesion can occur from the deterioration of smooth fibrous plaque and may result in distal embolization. Ulceration is associated with intraplaque hemorrhage and can be seen sometimes as swirls of color Doppler within the plaque. As the plaque protrudes into the arterial lumen and becomes altered, it is at a higher risk of embolization. The most common sites for atherosclerosis include branches, bifurcations and origins of vessels. The vast majority of carotid duplex examinations will be to assess the presence and amount of plaque with in the cerebrovascular vessels to aid in determining a patient’s relative stroke risk.
Ischemic strokes are the most common type of stroke and are the result of an embolic event or large vessel atherosclerosis. An embolic stroke is a territory infarction usually from a cardiac source and patients with dysrhythmia such as atrial fibrillation. The embolic source for a stroke could also be from the carotid arteries; however, this is less common. Some authors have reported close to 15% of strokes are the result of emboli originating from atherosclerotic plaque in the carotid bifurcation. Large vessel atherosclerotic disease can also be the etiology for an ischemic stroke. This would occur if there are intra or extracranial vessels that demonstrate stenosis >50 percent. A carotid duplex exam, if done properly, has a very high rate of accuracy in detecting extracranial cerebrovascular stenoses >50 percent. These atherosclerotic lesions are most likely to occur at the bifurcation of the common carotid into the internal and external carotid arteries.
Aside from strokes caused by atherosclerotic disease, there are a few other, less common pathologies that may be encountered during carotid examination worth mentioning. They include fibromuscular dysplasia, carotid dissection, carotid body tumors, aneurysmal disease, radiation injury and vasculitis.
1. Fibromuscular dysplasia (FMD) is a rare, non-atherosclerotic disease of the vessel wall that causes intermittent aneurysmal dilation and stenosis of the mid to distal segments of the internal carotid arteries. The alerting sign for fibromuscular dysplasia will be the location of the stenosis because it will occur in the mid to distal segments of the internal carotid arteries. Figure 1 is an example of the elevated velocities and flow disturbance observed in the distal segment of the internal carotid artery. It is a classic presentation of a patient with fibromuscular dysplasia. This disease is more common in younger patients ages 25-50 and is almost three times more likely to occur in females compared to males. The renal arteries are the most common location for FMD and will cause renal hypertension. The internal carotid arteries are the second most common location and in both cases, FMD changes in the vessel wall occur bilaterally. The areas of narrowing in the vessel followed by slight aneurysmal dilatation give this pathology its characteristic appearance where the vessel walls looks like a string of beads.
2. Carotid dissection is another uncommon condition to encounter and can be seen as the result of trauma or of aortic dissection. A dissection occurs when the intima of the artery tears away from the media and there are two visible lumens within one vessel. Dissections that arise from the aorta may not cause any cerebral symptoms or pain. They can be suspected in younger patients that present without any risk factors for atherosclerotic disease and especially if there is a history of trauma. Trauma can be as dramatic as a motor vehicle crash or as subtle as a cough or sharp turning of the head. Both lumens should be documented in short and long axis views as well as spectral Doppler analysis of the waveforms from both lumens.
3. Carotid body tumors or paragangliomas are identified as highly vascularize masses nestled between the internal and external carotid arteries that cause them to splay far apart. The external carotid artery is the most common source of blood supply to this benign tumor. Most carotid body tumors are asymptomatic.
4. Carotid aneurysm or pseudoaneurysms are also rare to encounter. True aneurysms of the carotid vessels appear as bulging or expansion of all three layers of the vessel walls. The most common patient presentation will be a pulsatile mass in the neck that is not tender. A pseudoaneurysm could occur from infection, suture deterioration at a bypass anastomosis, or weakening/injury to the vessel wall. The disruption of the vessel wall causes blood to escape the arterial wall into the surrounding tissue. The blood presents as a circular mass with actively swirling blood flow as shown in cineloop 1.
Figure 1: Distal Internal Carotid Artery IMD
There will also be a neck of the pseudoaneurysm that connects the swirling blood to the adjacent artery. Flow within the neck has a characteristic oscillating flow pattern. This is demonstrated in figure 2. Patients can present with a palpable and pulsatile neck mass and a carotid bruit.
Figure 2: Spectral Doppler of Pseudoaneurysm Pedicle
5. Radiation injury can occur in patients that have various types of cancer and undergo radiation therapy in the neck. The radiation causes vessel wall necrosis from radiation injury. The radiation damages the vessel wall to cause narrowing/stenosis at those segments. Patients do not present with typical atherosclerotic risk factors and will also have absence of atherosclerotic plaque in the carotid bifurcation. The lesions are usually located in the mid common carotid arteries and appear circumferential in nature. Another characteristic sign is that the narrowed portion of the vessel is often much longer than atherosclerotic lesions.
6. Vasculitis or Arteritis is inflammation of the arterial wall and is sometimes encountered as a result of Takayasu’s arteritis or temporal arteritis. The etiology is unknown and there is a variety of clinical presentations. Inflammation of the vessel walls causes the distinguishing appearance of concentric narrowing/stenosis.
This examination requires strong knowledge of the extra and intracranial carotid vessels. The carotid anatomy begins with the innominate artery which is the first branch off the aortic arch. The innominate artery is a short vessel that quickly bifurcates into the right common and right subclavian arteries. The next branch off the aortic arch is the left common carotid artery and the last major aortic arch branch is the left subclavian artery. The right and left common carotid arteries travel on the anterolateral side of the neck and travel superiorly until they bifurcate into the external and internal carotid arteries at approximately c2-c3. Normal common carotid artery diameters are 1.0cm + 0.25cm. The external carotidartery travels superiorly and has several extracranial branches to supply the face, neck and skull. These branches include the superior thyroid, lingual, facial, occipital, auricular, ascending branch and the last two are the superficial temporal and internal maxillary arteries, which are the terminating branches. These extracranial branches can serve as important collateral pathways in the event of significant carotid artery disease. The size of the external carotid artery is variable; however, it should be smaller than the common carotid artery. In contrast, the internal carotid artery should be slightly larger than the external carotid artery and does not have extracranial branches. The internal carotid artery courses posterior and lateral as it travels superiorly in the neck, crosses the cranium and terminates into the middle and anterior cerebral arteries intracranially. The bilateral internal carotid arteries are responsible for the majority of blood supply to both cerebral hemispheres via the anterior circulation of the cerebrum. The extracranial portion of the internal carotid artery is call the cervical segment. A dilated segment of vessel near the carotid bifurcation can be observed and is referred to as the carotid bulb or sinus. It contains sensory nerve endings that are baroreceptors, whose function is to sense pressure changes and decrease the heart rate. The carotid bulb has a highly variable location and could include sections of the distal common, proximal internal and proximal external carotid artery. The intracranial portions of the internal carotid artery include the petrous, cavernous and cerebral segments; however, the intracranial segments are not routinely part of a carotid duplex
examination. The first intracranial branch of the internal carotid artery is the ophthalmic artery and it supplies flow to the eye. The ophthalmic artery serves as an important collateral pathway in the event of significant carotid disease. The major vessels included in a carotid exam will be the common, cervical internal and external carotid arteries throughout their course. The bilateral vertebral arteries, which are responsible for supplying flow to the posterior circulation, are also routinely included in an extracranial carotid duplex examination. The vertebral arteries are the first major branches off the right and left subclavian arteries. They travel superiorly and posteriorly toward the vertebrae. In most cases they enter the transverse process at c6. As the vertebral arteries move superiorly, they enter the skull through the foramen magnum and will join to form the basilar artery in the brain.
Carotid Duplex Exam
A carotid duplex examination can be indicated in patients who are having stroke like symptoms such as visual disturbances, transient ischemic attacks, amaurosis fugax, or hemispheric paralysis/paraplegia. A carotid bruit heard during a physical exam may also be an indication as well as suspected subclavian steal or follow up from carotid endarterectomy or stenting. Contraindications may include neck bandages and active internal jugular vein intravenous access, arterial wall calcification causing shadowing and patients that are unable to be positioned adequately. Instrumentation for this examination should include a high resolution duplex ultrasound system with excellent b-mode, color, and spectral Doppler capabilities. A linear array transducer with a range of frequencies of 5-12MHz is most often adequate; however, lower frequency transducers may be helpful when looking at vessels that are either unusually deep or superficial in the neck. Brachial pressures should be obtained and recorded to help determine if the patient may have subclavian steal syndrome. The examination begins with the patient in the supine position and the head turned slightly away from the side being examined. To begin the duplex exam, the common carotid artery (CCA) is located in short axis of the mid neck, lateral to the thyroid and medial to the internal jugular vein (IJV). Next, the transducer should be swept caudad to identify the common carotid origin. If on the right side, the CCA can be observed arising from the innominate artery and on the left, arising from the aorta. Sometime, the origin of the left CCA is not visible due to the depth of this vessel, which arises from the aortic arch. Attempts should be made to see as far proximally as is reasonable, but not visualizing the true origin on the left is fairly common. The transducer is swept throughout the CCA and cephalad through the internal and external carotid bifurcation. Representative images can be obtained demonstrating the short axis views of the mid CCA, proximal internal carotid artery (ICA) and proximal external carotid artery (ECA) as demonstrated in figure 3.
Figure 3: R short axis ICA ECA
These images can also be obtained using color Doppler to demonstrate patency of each vessel. The same vessels should also be evaluated throughout their course in long axis using b-mode and color Doppler to record representative images at the CCA, ICA and ECA. These sample images are shown in figures 4 and 5. Figure 4 demonstrates the long axis view of the mid CCA in b-mode imaging.
Figure 4: L MCCA B-mode
Equipment settings should be optimized throughout by showing the vessel centered on the image from top to bottom and left to right. There should be even echogenicity throughout the image and this can be adjusted using the time gain compensation (TGC) and overall gain. The focal position should be set at theposterior wall of the main vessel and the highest frequency utilized for the best imaging resolution but without compromising penetration. All equipment features available should be optimized such as compound imaging and speckle reduction. Each manufacturer will have various applications and the equipment should be optimized based on its capabilities. Figure 5 demonstrates a long axis view of the distal CCA and extending into the proximal ICA.
Figure 5: RICA proximal B-mode
This is an important view because it is in the area of the carotid bifurcation and is a common location for atherosclerotic plaque to form. Color Doppler can also be used to take representative images at these locations. After the initial b-mode and color Doppler sweeps and representative images are completed, the exam should focus on obtaining spectral Doppler signals throughout all segments of each vessel. An angle of < 60 degrees should be used at all times as well as aligning the cursor parallel to the vessel walls. This will ensure accurate velocity measurements. Waveforms should be obtained and recorded at the proximal, mid and distal CCA, proximal external carotid artery, proximal mid and distal ICA and the vertebral artery. Some departments will also record proximal subclavian artery waveforms. At each location, the peak systolic and end diastolic velocities should be recorded. Any area that plaque is visualized or if the vessel appears narrowed should prompt further investigation by sweeping or walking the spectral Doppler sample gate through the narrowed area to identify the highest velocity. Figure 6 demonstrates a still image where the sonographer swept the sample gate through the narrowed area and captured the pre-stenotic and stenotic velocities. The first measured velocity on the left of the image is only around 100cm/s and then the velocity clearly elevates significantly to the right of the spectral tracing until it reaches over 400cm/s. Recording these pre-stenotic and stenotic velocities is the diagnostic tool to identify the percent stenosis.
Figure 6: Demo of walking the Doppler through the stenosis
To summarize, if there is a high velocity present then the sample gate should be walked through the narrowed area to identify the highest velocity and additional waveforms should be obtained to document pre-stenotic velocities and post-stenotic turbulence. If there is a suspected occlusion, the documentation should include color and spectral Doppler images and waveforms to demonstrate the absence of flow. If the equipment has power Doppler capabilities, that should be used also. Careful attention should be paid to the equipment settings to lower the velocity scale and increase the gain so the
machine is set up to detect slower flow rates. Additional color and grayscale images should be obtained to further delineate areas of pathology such as short and long axis views with and without color Doppler. Some facilities will also include Doppler evaluation of the proximal subclavian artery as part of their carotid duplex protocol. This will help determine the presence of subclavian steal phenomenon. This is where the vertebral artery steels blood from the brain by either oscillating or becoming retrograde to provide circulation to the arm and hand. This occurs if the stenosis is proximal to the vertebral artery origin and occurs more often on the left side. Bilateral systolic brachial pressures, which should be approximately equal, will also assist in determining the presence of a subclavian steal phenomenon. The examination is then repeated for the contralateral side.
For examinations that are performed after a carotid endarterectomy, bypass or stent, the first exam is often done within the first month. The above protocol can be followed; however, post-procedure complications have an alternate appearance on ultrasound. The type of procedure will dictate the post-op ultrasound appearance. Carotid endarterectomies can be performed with and without a patch. If a patch is used, it can be either autogenous vein or synthetic material, and the vessel will be more dilated in the patched area as compared to the native vessel. Vein patches appear similar in wall structure to native vessels. Dacron patches will have a ‘saw like’ or woven appearance and PTFE will show two brightly echogenic lines as the wall of the material. Careful attention should be paid to look for residual plaque, tissue flaps, hyperplasia or perivascular masses or fluid. Although restenosis rates are low, follow up duplex is recommended annually and should always be performed bilaterally to aid in early diagnosis of contralateral disease.
For the evaluation of stents, the standard carotid protocol can be used as described above with a few additional images. The native vessel proximal and distal to the stent, the proximal and distal ends of the stent and throughout the course of the stent should be evaluated fully using short and long axis views in b-mode, color Doppler. Spectral Doppler velocities are obtained throughout and the sample volume should be swept or walked throughout stented site. Each subsequent examination should be compared to the prior exam to document any changes.
The velocities obtained in the internal carotid arteries should be compared to validated diagnostic criteria. The Intersocietal Accreditation Commission for Vascular Testing recommends the use of the “Carotid Artery Stenosis: Grayscale and Doppler Ultrasound Diagnosis – Society of Radiologists in Ultrasound (SRU) Consensus Conference” criteria for carotid interpretation as shown in table 1.
Table 1: Modified from: Grant EG, Benson CB, Moneta GL, et al:
Carotid Artery Stenosis: Gray-scale and Doppler US Diagnosis – Society of
Radiologists in Ultrasound Consensus Conference. Radiology 2003; 229: 340-346.
The amount of ICA disease can be categorized based on peak systolic and end diastolic velocities as well as the ICA/CCA peak systolic velocity ratio. This table should only be used to categorize the degree of ICA stenosis. The ICA/CCA ratio is obtained using the CCA PSV in the distal segment, or within approximately 3cm from the bulb and compare it to the highest ICA velocity. Once the ICA stenosis has been categorized, the plaque can also be described sonographically using the following terms as examples: homogeneous or heterogeneous, smooth or irregular surfaced, hyperechoic or hypoechoic. Figure 7 demonstrates a very classic appearance of a complex internal carotid artery plaque. It demonstrates a long axis view of the right proximal internal carotid artery. The plaque characteristics are heterogeneous and irregular surfaced. The hyperechoic plaque is well demonstrated by the posterior shadowing that occurs. Although plaque might look very significant based on the b-mode image, remember that spectral Doppler analysis is the diagnostic gold standard for determination of percent stenosis. Despite what is visible on B-mode, the diagnostic criteria relies on the peak systolic and end diastolic velocities obtained using spectral Doppler analysis. A good example to demonstrate this concept is by comparing Figures 7 and 8. They are images of the right and left proximal internal carotid arteries to demonstrate the atherosclerotic plaque. Based on b-mode appearance alone, there appears to be more plaque within the right internal carotid artery as compared to the left.
Figure 7: B-mode R PICA plaque
Figure 8: B-mode L PICA plaque
Figures 9 and 10 demonstrate the proximal internal carotid artery spectral Doppler waveforms in those vessels. Figure 9 is of the right internal carotid artery and demonstrates peak systolic velocities at 364cm/s and end diastolic velocities at 84cm/s. Compare this to the SRU criteria listed in table 1 and the stenosis will be >70 percent. The peak systolic velocity in the left internal carotid artery is 484cm/s and the end diastolic velocity is 168cm/s as shown in figure 10.
Figure 9: RICA Spectral Doppler
Figure 10: LICA Spectral Doppler
This also would place the degree of stenosis in the left internal carotid artery at >70% stenosis based on the SRU criteria; however, both the peak and end diastolic velocities are significantly higher on the left compared to the right. This is a good example to demonstrate the need to rely on spectral Doppler analysis rather than b-mode imaging alone to determine the severity of the disease.
There are some challenges that might be encountered as part of the exam when the anatomy is altered or there might be an atypical presentation of disease. It is important to remember that b-mode and color Doppler imaging can guide the path of the exam and help demonstrate anything that appears atypical. Representative images of the anatomy can spatially correlate where the spectral Doppler analysis is performed. Any area that demonstrates an elevated velocity should be carefully interrogated with the sample volume to determine the discrete velocity information using spectral analysis. Regardless of the anatomic presentation or the appearance on b-mode, the determination of internal carotid artery stenosis should only be done using spectral analysis. One example of this is when there is kinking or tortuosity of the internal carotid artery. This kinking or tortuosity can occur in the embryonic development stages but can also be acquired with age. Studies show that women may be affected more than men and symptoms, if present, occur at an elderly age. Tortuosity is usually asymptomatic; however if symptoms are present they would manifest as transient ischemic attacks (TIAs) and often occur during neck rotation. The sharp angles encountered from these tortuous vessels make proper angle correction sometimes difficult to obtain. Higher velocities might be encountered along a sharply curved vessel and a notation should be made that the higher velocities were obtained at a curvature – which makes the standard diagnostic criteria from table 1 less accurate. B-mode and color images demonstrating that the elevated velocities are obtained at the curvature can assist in determine stenosis caused by plaque versus kinking. If the stenosis is truly hemodynamically significant, than post-stenotic turbulence and tardus parvus signals will also be present.
In addition to categorizing ICA stenosis, the vertebral artery should be assessed for flow direction and stenosis, and the ECA, CCA and subclavian artery can also be evaluated for stenosis. Alternate criteria should be used to identify stenosis in the common carotid, external carotid, vertebral or subclavian arteries. For the CCA, ECA, vertebral or subclavian arteries, basic principles of arterial hemodynamics can be used. A doubling of velocities compared to the proximal normal segment, with post stenotic turbulence, is good evidence to demonstrate a >50% hemodynamically significant stenosis.
Post-procedure interpretation should include making adjustment to the interpretation criteria if needed. Some facilities modify their carotid interpretation criteria for patients who have had carotid endarterectomies, but most will use the same velocities as the native circulation. Flow within the patched vessel segment that is slightly dilated may have a swirling appearance or slower velocity, which is expected. Many authors report that the normal peak systolic velocities within a stent are higher (ranging from 175-200cm/s) than the native vessels. Finding these velocities at baseline should not necessarily raise concern if the b-mode and color images show widely patent vessels; however, if velocities increase significantly from one exam to the next, that will raise greater concern for intent restenosis. Comparing velocities from one exam to the next for post-procedure evaluation is a crucial element to correct interpretation.
Carotid duplex ultrasound is the best non-invasive tool that can be used by health care providers to determine the presence of carotid stenosis. Spectral Doppler analysis along with b-mode and color Doppler are well known to be accurate and reliable testing methods when proper testing and interpretation techniques are used. Atherosclerotic plaque is easily identified and evaluated along with several other, lesser known carotid pathologies as described earlier in this online course. Duplex evaluation of patients who have had carotid endarterectomy or stenting is also an important tool in follow-up for these patients. It is the most cost-effective and reliable way to monitor post-procedure patients. The techniques and applications described in this course allow clinicians to reliably diagnose, follow and hopefully help prevent strokes which is the most serious consequence of carotid atherosclerotic diseases.
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