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| Carotid Arteries: Use of Ultrasound Duplex Imaging for Detection of Plaque By Misty Edwards |
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Abstract
This article describes the normal anatomy, hemodynamics, and sonographic appearance of carotid arteries. It describes the proper instrumentation, equipment, patient and transducer position for a carotid duplex exam. Abnormal hemodynamics of a diseased vessel are explained. The use of endarectomy instead of medical therapy is also discussed and what happens during the procedure. This article describes how carotid ultrasound helps to reduce the risk of a stroke. |
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Introduction
Carotid duplex imaging is a direct noninvasive test that provides prudent information regarding the carotid arteries. Ultrasound is used to examine the carotid arteries for the detection and assessment of possible underlying arterial disease. By using gray scale and spectral Doppler imaging, an evaluation of the vessel and the hemodynamics can be made. Evaluation of the carotid arteries helps to decrease the risk of a future stroke. |
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Anatomy and Physiology
The carotid arteries supply blood to the brain and originate from the aortic arch. The right side of the head and neck are supplied by the brachiocephalic artery, which branches to become the right subclavian and right common carotid arteries (CCA). On the left, the left subclavian and left common carotid arteries branch directly from the aortic arch without an intervening brachiocephalic artery. Neither common carotid artery has collateral branches, but each divides into the internal and external carotid arteries (ICA and ECA) at the level of the upper boarder of the thyroid cartilage. The ICAs supply the brain with 75% of its blood. Two vertebral arteries also supply the brain with a small amount of blood, but can become more important if the carotid arteries become narrowed or blocked. The ECAs feed the neck and face. However, many branches of the ECA may become collaterals of occlusion occurs within the internal carotid or vertebral arteries.
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Like all other arteries, the walls of the carotids consist of three layers – the intima, the media, and the adventitia. All three layers can be visualized on ultrasound as a series of parallel stripes. Intimal thickening of > 1.2mm may be an indication of plaque. The carotid arteries pulsate with systole and diastole like other normal arteries. A change in pulsatility may be the only indication of an abnormality. Atherosclerosis decreases pulsatility because the vessels are hardened from the deposition of plaque within the lumen.
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The blood flow in the CCAs is usually described as laminar, meaning the blood is moving in parallel lines. Blood flow within the carotid bulb may be described as disturbed or turbulent. Vortex and eddy currents are normal findings within the bulb caused by the bifurcation and bends of the vessel. Plaque formation most commonly occurs in the bulb resulting from areas of stagnation. Normal velocities of the carotid arteries have not been established. The reported mean peak systolic value (PSV) of the ICA is from 54 to 88 cm/sec, but it is not viewed as abnormal until PSV exceeds 100cm/sec. The mean PSV of the ECA is reported as 77cm/sec with the maximum velocity not exceeding 115cm/sec.
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Differentiating between the ICA and ECA is probably the most difficult part of a carotid exam. Anatomically, the ICA usually runs more posterior and lateral, while the ECA runs more anterior and medial. The ICA is normally larger than the ECA and slowly tapers as it travels toward the head. The extracranial ICA has no branches. The ECA has many branches to feed the neck and face. Vessel identity can be aided by spectral and color Doppler. The ICA has a low resistance waveform with a blunted systolic upstroke, less pronounced diacrotic notch, and more diastolic flow. The ECA waveform is more high resistance with a steeper systolic upstroke, very pronounced diacrotic notch, and little to no diastolic flow. Differences in pulsatility between the ICA and ECA are also visible with color flow Doppler. Blood flow continues throughout the entire cardiac cycle in the ICA resulting in continuous color fill. In the ECA, color flickers or flashes because flow is diminshed or absent in diastole.
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Instrumentation
Carotid duplex scanning should be performed with appropriate equipment: a high frequency transducer (5-10mHz); color flow imaging; pulsed and directional Doppler with velocity measurement capabilities; and Doppler spectral analysis. Many instruments are available that provides these features |
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Examination
When examining the carotid arteries, the patient is normally supine with the examiner at the patient’s head. It may sometimes be necessary to tilt and rotate the patient’s head to enhance neck exposure. Also, having the patient move the arm and shoulder down may facilitate visualization of the vessel. Many different scan planes may be used to examine the carotid arteries. Generally the more posterior-lateral positions are more useful for imaging the carotid bulb and ICA. When scanning in the longitudinal plane, the patient’s head should be located on the left side of the image. Transverse images are normally oriented as viewed from the feet, with the patient’s right on the left side of the image.
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| Always record images sequentially starting on the right side. Record B-mode images in longitudinal and transverse planes starting in the CCA (image subclavian) and proceed into the ICA and then the ECA. Record color Doppler images in the longitudinal plane. Obtain spectral Doppler waveforms in the CCA (proximal and distal), within the carotid bulb, ICA (proximal, mid, and distal), ECA (proximal), vertebral, and subclavian arteries. |
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Advantages and Limitations
Duplex ultrasound has many advantages. Duplex scanning can localize arterial disease in the extracranial carotid arteries. It can differentiate between a tight stenosis and occlusion. With the use of ultrasound, non-hemodynamically significant lesions can be documented and disease progression can be followed. This exam provides information about the surface character of plaque and can also evaluate pulsatile masses in the carotid.
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| There are also some limitations to duplex imaging of the carotids. Patients may have dressings, skin staples, or sutures that interfere with the exam. The size and contour of the neck can also make the exam difficult. Acoustic shadowing from calcified vessels may impair visualization. Disease can be over estimated when an artifact is mistaken for plaque. Disease may also be underestimated by failing to appreciate the low level echoes of soft plaque. Using the wrong Doppler angle can either over or under estimate the severity and extent of the disease. |
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Plaque Characteristics
The primary purpose of carotid scanning is the detection and assessment of carotid stenosis. Ultrasound has been used to image and characterize plaque within the carotids. It is characterized as low, medium, or high in echogenicity and as homogenous or heterogenous. Low echo plaque contains a large amount of lipid material and is difficult to image. Moderate echo plaque is fibrous plaque made up of collagen and lipids. Strong echo plaque has strong reflections caused by vessel calcification. Calcified plaque can also be termed as heterogenous. Homogenous plaque is more uniform in texture.
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When disease is detected in the carotid arteries, it is important to measure prestenotic, stenosic, and post stenotic velocities. It is very important that velocities are taken at the highest velocity. There is only a small increase in velocity until the stenosis exceeds 50% decrease in area. When the stenosis reaches 70% decrease in area, the amount of flow begins to diminish rapidly. Peek systolic velocity (PSV) starts to drop off when the stenosis starts to exceed 90% area reduction. Spectral broadening is proportional to the amount of stenosis and may be the only sign of disease.
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Risk Factors
Atherosclerosis is a disease that causes hardening and narrowing of arteries that can occur anywhere in the body. When atherosclerosis affects the carotid arteries, it can decrease or completely block the blood flow to the brain, which may result in a stroke. Atherosclerosis is caused by a combination of factors including smoking, high cholesterol, obesity, lack of exercise, and family history. |
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Treatment
Carotid endarterectomy is performed to remove the plaque from the vessel to reduce the chance of a stroke. An incision is made in the neck so that the intimal layer can be removed. To be a candidate for an endarterectomy, patient must be symptomatic with 70% or greater area reduction. Endarterectomy patients are scanned 3, 6, and 12 months to look for post-surgical complications. Endarterectomy has a 9.8 to 23.2% restenosis occurrence. Imaging of the post-endarterectomy carotid bifurcation shows the absence of the intimal reflection. Several studies have shown carotid endarterectomy can be more effective than medical therapy in reducing the incidence of stroke. |
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Conclusion
Noninvasive vascular diagnostic technology has improved over the past 20 years. Carotid duplex scanning is a direct procedure that evaluates the flow patterns within the vessel. This is performed with B-mode imaging of the vessel with Doppler velocity spectral analysis of blood flow patterns. This noninvasive test provides valuable information about possible underlying carotid arterial disease and helps to decrease the chance of a future stroke.
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References
Zwiebel, William J., Introduction of Vascular Ultrasonography 4th Edition. 105-149 Philadelphia: W.B. Saunders Company., 2000
Ridgway, Donald P., Introduction to Vascular Scanning: A Guide for the Complete Beginner 2nd Edition. 87-122 Pasadena: Davies Publishing Co., 2001
Patton, Kevin T. and Thibodeau, Gary J., Principles of Anatomy and Physiology. 562-570 St. Louis: Moby Inc., 1999
Curry, Reva Arnez and Tempkin, Betty Bates., Ultrasound: An Introduction to Normal Structural and Functional Anatomy. 421-427 Philadelphia: W.B. Saunders Company., 1995
Runwell, Claudia and McPharlin., Vascular Technology. 121-150 Pasadena: Davies Publishing Co., 2000
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Misty Edwards 09/17/02 VAS 135
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Copyright CVTC 2003. All Rights Reserved.