ProSono                                                                 Copyright 2006                       Carotid Arter...
ProSono                                                               Copyright 2006Patients with stenosis of 70-99% showe...
ProSono                                                                Copyright 2006        obstructed, the symptoms may ...
ProSono                                                               Copyright 2006          ICAVL Standards –Extracrania...
ProSono                                                                   Copyright 2006 Gray Scale Imaging  The ultrasou...
ProSono                                                                             Copyright 2006        The following ta...
ProSono                                                                       Copyright 2006 Color Doppler Protocol      ...
ProSono                                                                          Copyright 2006    Site       Recorded Sam...
ProSono                                                                Copyright 2006 Society of Radiologists in Ultrasou...
ProSono                                                                  Copyright 2006Diameter           Peak            ...
ProSono                                                                      Copyright 2006        Diagnostic criteria inc...
ProSono                                                                            Copyright 20061  Lindgren A, Roijer A, ...
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Microsoft Word - vasc_chapter_carotid_artery_triplex

  1. 1. ProSono Copyright 2006 Carotid Artery Triplex Ultrasound The primary role of triplex sonography in assessing patients with suspectedextracranial cerebrovascular disease is to first, detect the presence ofatherosclerotic disease and, second, to determine the degree to which theinternal carotid artery is stenosed. Vascular surgeons and interventionalists relyvery heavily on the results of carotid triplex ultrasound in managing their patientswith carotid disease. It is not uncommon for surgeons to take a patient directly tothe operating room based on the results of a carotid triplex exam from a lab thathas a proven track record and validated results. Magnetic resonanceangiography (MRA) and catheter-angiography also play important roles inworking up a patient with suspected critical carotid artery disease. However,each modality has its limitations and technical pitfalls. Rationale for Carotid Triplex Ultrasound Carotid disease in stroke There is convincing evidence of a causal association between atheroscleroticnarrowing of the carotid arteries and cerebral infarction. Moderate to severecarotid artery stenosis occurs in approximately 15 to 30 percent of patients withcarotid artery territory (anterior cerebral circulation) stroke, and is associated withan increased risk of recurrent stroke.1 2 As the degree of stenosis increases therisk of stroke also increases and surgical or interventional reduction of moderateto severe ipsilateral carotid artery stenosis reduces the risk of future stroke insymptomatic patients.3 4 By identifying the presence and degree of carotidstenosis, imaging of the carotid arteries aids in the selection of patients who maybenefit from surgical therapy. In the early 1990s two large-scale clinical trials were conducted that sorted outthe confusing, non-standardized ways that carotid ultrasounds were beingperformed and interpreted in North America. The North AmericanSymptomatic Carotid Endarterectomy Trial (NASCET) compared the efficacyof carotid duplex ultrasound and catheter angiography in identifying patients witha ≥ 0 IAs n s w oh dsmpo o c rba i h mie e t T es d 7 % C t o i h a y tms f ee rl c e c v ns h t y e s s . ualso assessed clinical outcomes in those patients, half of whom were randomizedto surgical treatment (TEA) the other half to medical management. The studyreached some noteworthy conclusions: o Carotid duplex ultrasound, when performed in accordance with strict technical standards, equals or exceeds catheter angiography in detecting a ≥ 0 IAs n s . 7 % C t oi e s o The risk of ipsilateral stroke was reduced significantly in patients with carotid stenosis 50-69% who received carotid endarterectomy.5Carotid Artery Triplex Ultrasound (1)
  2. 2. ProSono Copyright 2006Patients with stenosis of 70-99% showed the most significant reduction in therate of ipsilateral stroke.6 7 Role of Carotid Triplex Ultrasound Evaluation o Detect presence of atherosclerotic disease o Grade ICA stenoses –significantly affects clinical management of patient o Evaluate plaque morphology- especially surface characteristics that might suggest plaque ulceration or intraplaque hemorrhage o Presence of collateral flow –associated with altered intracranial hemodynamics or vertebral/subclavian steal syndrome. o Intimal-medial thickness measurement –used by some physicians in adjusting pharmacological management of patients with hyperlipidemia, diabetes, hypertension syndrome. Clinical Indications for Carotid Triplex Ultrasound While more patients are requesting and receiving carotid triplex ultrasound examinations for screening and as part of a complete physical examinations in senior patients with cardiovascular risk factors, the primary symptomatic clinical indications remain unchanged. They include: o TIA –Transient Ischemic Attack –(See Chapter –Strokes and TIAs) o Amaurosis fugax –Amaurosis fugax is a sudden, dramatic loss of vision in one eye caused by a temporary lack of blood flow to the retina. The visual loss is frequently described as having a gray or b c “ l k shade pulled a d w ” r “a r l s arwn ” o n o a c mea e n r i over the affected eye. It usually lasts n o g several seconds but may last several minutes. Amaurosis fugax frequently results from embolization of thrombus or plaque debris from the carotid bifurcation to the retinal artery. This blocks the artery for a time and causes loss of vision in that eye for as long as its blood supply is cut off. o Ischemic oculopathy - A condition in which there is ischemia in both the anterior and posterior segments of the eye caused by atherosclerotic carotid artery disease. Posterior segment abnormalities include ischemic insults to the retina or optic nerve.8 The most common presenting symptoms are ocular pain and visual loss, either gradual or abrupt, amaurosis fugax, and afterimages. Loss of visual acuity is the most frequently encountered symptom. Atherosclerotic disease of the carotid artery is the most common cause; however, it is an infrequent manifestation of carotid. Approximately 5% of patients with marked carotid artery stenosis present with manifestations of ischemic oculopathy.9 o Central retinal vein occlusion - A condition caused by thrombosis in the vein draining the retina. The sudden obstruction of the blood flow away from the eye can lead to bleeding in the retina and loss of vision. This disorder is most commonly seen in older individuals who have significant atherosclerosis. In addition, hypertension is a risk factor as well as conditions that make the blood clot more easily. If a small vein isCarotid Artery Triplex Ultrasound (2)
  3. 3. ProSono Copyright 2006 obstructed, the symptoms may be mild. If a major vein is blocked, the symptoms are usually rapid and severe. Disorders that increase the viscosity of the blood and make it "more sticky" may lead to retinal vein occlusion. Some conditions which can produce this state are inherited disorders, smoking, use of estrogen and polycythemia. Hypertension and atherosclerosis may damage the vessels, making them more subject to developing clots. Common symptoms include blurred vision is there is bleeding within the retina or fluid immediately in front of it. If a major draining vein is obstructed, the retina will become damaged and visual loss will occur over several hours. o Follow-up to carotid surgery - thromboendarterectomy (TEA or CEA). Patients who have had open surgery for the removal of clot and plaque and repair of a severely diseased carotid artery should be followed on a regular basis. Many times these patients also have contralateral carotid artery disease which also bears following (watchful waiting). o Follow-up to carotid stenting –Carotid artery angioplasty with supportive stent placement is becoming a routine part of vascular interventional practice. It is usually reserved for patients who are at high risk for complications from an open surgical procedure under general anesthesia. It is important to note that stents alter hemodynamic states in the internal carotid artery. Typically, peak systolic velocities are higher in a stented artery than in a normal carotid artery (PSV >140 cm/sec). The sonographic criteria for determining normalcy are still being validated. o Pre-op Coronary Artery Bypass Surgery –Because of the high rate of association between carotid artery disease and coronary artery disease, patients being worked-up for bypass surgery routinely undergo carotid triplex ultrasound examination. The detection of an occult high-grade lesion in one of the internal carotid arteries might postpone the open-heart procedure until the neck can be cleaned out. In one study the finding of carotid stenosis greater than 75% in patients over 60 years of age was associated with occurrence of stroke in 15% of cases. The authors concluded that carotid screening is helpful to determine patients at increased risk of stroke and should be performed in patients greater than 60 years10 o Vertigo, dizziness, and syncope are not generally caused by carotid stenosis; patients with these symptoms who are found to have carotid stenosis should be considered "asymptomatic" regarding the stenosis.Carotid Artery Triplex Ultrasound (3)
  4. 4. ProSono Copyright 2006 ICAVL Standards –Extracranial Cerebrovascular Testing 2005 The Intersocietal Commission on the Accreditation of Vascular Labs (ICAVL)published revised Standards for carotid artery testing in 2005. A complete copyof this document is available at: A web link can be foundon the Sonography-Related Links page of the ProSono Library. The following protocol incorporates these widely accepted standards into anexamination process that produces reliable, reproducible results that is easy onboth the sonographer and the patient. General Comments: o A complete carotid triplex study is bilateral and includes evaluation of the common carotid artery (CCA), internal carotid artery (ICA), external carotid artery, vertebral artery (VA), and subclavian artery (SA) on each side. o Measurements of both arm systolic pressures are recommended to help identify vertebral-subclavian steal syndrome or hemodynamically significant subclavian artery stenosis. o Both imaging and Doppler information are used to identify vessels. o Doppler signals are obtained from each selected site at an angle of 60o or less with respect to the vessel wall or color-aided flow direction. Examination ProtocolExamination Protocol- One of the important aspects of a vascular triplexexamination protocol is that it provides a method for the sonographer to perform,record and report information obtained during a triplex procedure in an efficientand reproducible way. The three components of a triplex ultrasound exam are: o 2-D High-resolution, gray scale imaging (2D) o Color Doppler imaging (CDI) o Spectral Doppler waveform analysis (SA) Patient Position o Supine o Head turned slightly o Standard ultrasound approach (right side) o Vascular lab approach (from head) Instrumentation o High frequency (5MHz –15MHz) phased array transducer. o Range-gated Doppler with the ability to adjust position of the range gate within the area of interest. o Audio output DopplerAppropriate recording device or digital archiving capabilities.Carotid Artery Triplex Ultrasound (4)
  5. 5. ProSono Copyright 2006 Gray Scale Imaging The ultrasound examination begins with brief real- time survey of the neck vasculature. The goal of the initial survey is to: o Evaluate anatomy o Get a “ y fh l d l o te a ” a n o Observe and note presence of pathology In a sagittal (coronal) plane of section the survey begins with the transducer placed on antero- lateral neck using the sternocleidomastoid muscle as a built-i “tn of Atrd nii teC A n s d f. f i t n h C , a ” e e f g y Coronal section through the transducer is moved cephalad from the clavicle carotid bifurcation to the angle of mandible. At the bifurcation, subtle showing a large, angling of the transducer anteriorly and posteriorly heterogeneous plaque at and rotation to obtain a longitudinal image of the the origin of the ICA. vessels, the ICA and ECA can be imaged. Representative 2D images are obtained and documented of the: o CCA o ICA o ECA o Carotid bifurcation In a transverse (axial) plane of section, the carotid system is evaluated from the clavicle to the angle of the mandible. Again, the sternocleidomastoid muscle is used as a window and representative images are obtained and copied of the: Transverse section o CCA through CCA showing a focal soft plaque. o Carotid bifurcation Identifying and Differentiating Neck Vessels Correct differentiation between the ICA and the ECA at the bifurcation is of critical importance. Since the ECA provides blood to the face and scalp, structures that are easily and adequately perfused by collateral circulation in the event of a complete occlusion, the presence and extent of disease in this vessel is of little, if any, clinical importance. The ICA, however, is the main conduit for blood to the anterior and middle circulation to the brain. Disease in the ICA is of immense clinical importance and assessment of presence and severity is the primary reason for performing carotid. While the ECA typically lies anterior and medial to the ICA, variations in anatomic configuration of the neck vasculature, particularly vessel tortuosity, can complicate acquisition of spectral Doppler. Also, changes in resistivity in the cerebral circulation and some types of cardiac comorbidity can alter the spectral waveform appearance.Carotid Artery Triplex Ultrasound (5)
  6. 6. ProSono Copyright 2006 The following table provides helpful hints in correctly identifying the bifurcation vessels. Table I. Carotid Bifurcation Vessel Triplex Characteristics ICA ECA Lateral and posterior (lots of variation) Medial and anterior (lots of variation) Branches early in neck –superior thyroidal No branches present in neck artery Low-resistance waveform High-resistance waveform Alterations waveform during temporal artery N rs o s t “ mp rlr r tp i ” o e p n e o t oaat ya p g e e n tapping “ mp rlr r tp i ” as ltcn u t h ldfrnieteE A Te oaat ya p g i i e e h i e o e i e tt h C e n s mp q p f e a from the ICA. Using CDI to guide the spectral Doppler range gate, it is placed the vessel to be interrogated. While watching the spectral display and listening to the audio output, the temporal artery, as it passes just anterior to the pinna of the ear, is lightly tapped with the free hand. Since the temporal artery is a proximate branch of the ECA, the tapping is transmitted into the ECA and is displayed as an oscillation on the spectral display. It can also be heard on audio output. If there is no alteration of the Doppler signal during the tapping, then the vessel being interrogated is most likely the ICA. ECA –early branching is ECA –“ mp rl p i ” t oa t p g. e a n seen in the neck. Flow Transmitted oscillation from toward the transducer is tapping the temporal artery just demonstrated in the superior anterior to the pinna of the ear thyroidal artery at its takeoff are seen during diastole in the from the ECA ECA. Doppler Protocol –General Comments o Accurate Doppler findings are critical to correct diagnosis and grading. o Color Doppler provides global display of hemodynamic conditions present. o Color Doppler directs acquisition of spectral information. o Spectral Doppler provides flow velocities –(quantified data.) This data is the foundation for grading ICA stenoses.Carotid Artery Triplex Ultrasound (6)
  7. 7. ProSono Copyright 2006 Color Doppler Protocol Color Doppler imaging (CDI) provides a global display of hemodynamic patterns in the area being examined. It can focus th e a n r atni o e x mi ’ t t n n es e o variations in the laminar arterial flow patterns seen in normal, non-diseased carotid arteries. Using the CDI display as a guide, the sonographer can quickly and accurately obtain spectral Doppler waveforms in the areas of interest. The hemodynamic parameters displayed by CDI include: o Presence of flow –system sensitivity and gain controls must be optimized. o Direction of flow –relative to the transducer position. o Relative velocities within the CDI display area. o Flow characteristics in such as: 1. Laminarity 2. Turbulence 3. High velocity jets CDI is also of great value in identifying the presence and direction of flow in the vertebral arteries as they pass through the ossified cervical vertebrae. Detection of a short segment of a vertebral artery in an intervertebral space makes acquisition of spectral Doppler waveforms fast and accurate. Again, system sensitivity and gain settings must be optimized for adequate detection. Hemodynamic flow characteristics as displayed with Color Doppler imaging (CDI). LEFT: normal laminar –normal flow pattern with increasing velocities toward central lumen. MIDDLE: turbulent flow –multi-directional flow distal to a stenosis RIGHT: high velocity jet –stenosis profile with a high-velocity jet at point of maximum stenosis Spectral Doppler Protocol Spectral Doppler waveforms provide the foundation for grading carotid artery stenoses. While 2D and CDI images offer a wealth of information on the morphologic status of the extracranial carotid arteries, it is the spectral Doppler information that allows determination of the extent of hemodynamic significance of an ICA atherosclerotic lesion.11 12 Since hemodynamic significance of a carotid lesion is usually the determining factor in patient management, acquisition of adequate and accurate spectral Doppler waveforms is essential in a carotid triplex examination. Using CDI as guide, range-gated samples of flow patterns are obtained from various sites within the neck arteries. At a minimum, these sites should include:Carotid Artery Triplex Ultrasound (7)
  8. 8. ProSono Copyright 2006 Site Recorded Samples Demonstrating: CCA 2 sites Mid and distal PSV*, direction of flow ICA 2 sites Proximal and mid PSV and EDV* –highest values, direction of flow ECA 1 site PSV, direction of flow VA 1 site Presence and direction of flow SA 1 site Presence and phasicity (triphasic normal) PSV = peak systolic velocity EDV –end diastolic velocity When carotid disease is present, additional documentation should be made ofany abnormal Doppler findings. When a stenosis is present, spectral Dopplerwaveforms should be obtained proximal, at and distal to the area of maximumstenosis. PSV and EDV measurements should be made and recorded. Triplex demonstration of a very tight (80-99%) stenosis in the proximal ICA. The color column is narrowed by mural plaque. CDI demonstrates increased velocities at and just beyond this narrowing with turbulent flow d w s e m.T iiac si“tn s poi” o nt ar h s l s s o i rf . s a c e s l e Spectral Doppler waveforms demonstrate extremely high PSVs (~450 cm/sec) and an elevated EDV (~150 cm/sec). Standardized Interpretation Protocol Triplex sonography has been used in the evaluation of carotid artery diseasesince the introduction of the technology in the late 1970s. Technologicaladvances in digital imaging, image processing, and Doppler display formats havebeen accompanied by an increased accuracy in clinical assessment. Manyauthors over the years have published broadly validated data, however, thediagnostic criteria used in their protocols has varied and has madestandardization of examination methods and outcomes difficult. Three major developments in the standardization of triplex sonography as itrelates to the diagnosis and management of patients with carotid atheroscleroticdisease have occurred in the past decade: the North American symptomaticCarotid Endarterectomy Trial (NASCET), the Asymptomatic CarotidAtherosclerosis Study (ACAS), and the report from the Society of Radiologists inUltrasound Consensus Conference.Carotid Artery Triplex Ultrasound (8)
  9. 9. ProSono Copyright 2006 Society of Radiologists in Ultrasound Consensus Conference. In 2003, a panel of radiologists, neuroimaging specialists, vascular surgeons and vascular technologists met to establish criteria for the interpretation of carotid triplex examinations. Their recommendations: o All ICA examinations should be performed with grayscale, color Doppler and spectral Doppler ultrasound. o Categories of disease: 1. Normal 2. < 50% stenosis. 3. 50 to 69% stenosis. 4. ≥ 0 s n s t n a occlusion. 7 % t oi o e r e s o Peak systolic velocity (PSV) in the ICA and the presence of plaque are the primary diagnostic criteria. o Stratification Criteria: Diameter reduction Plaquing PS V % 0 No plaque present <125 cm/s < 50 Plaque present <125 cm/s 50-69 Plaque present 125 - 230 cm/second ≥0 7% Plaque present >230 cm/second Near occlusion Markedly narrow lumen on color Doppler images Total occlusion No detectable patent lumen on grayscale and no flow detected with spectral power and color Doppler Final reports should include specific velocity and grayscale findings with studylimitations noted when they exist.13 14 University Of Washington Criteria15 The Department of Vascular Surgery at the University of Washington Medical Center has long been the leader in acquiring, analyzing and validating spectral Doppler data. It is assumed, of course, that plaque is present on 2D imaging in all except the 0% diameter reduction category. The following table summarizes their interpretive algorithm:Carotid Artery Triplex Ultrasound (9)
  10. 10. ProSono Copyright 2006Diameter Peak End Spectralreduction systolic diastolic characteristics % velocity velocity 0 <125 cm/s Normal spectral window 1-15 <125 cm/s Spectral broadening in systolic deceleration 16-49 <125 cm/s Spectral broadening throughout systole 50-79 >125 cm/s < 140 cm/s Extensive spectral broadening 80-99 >125 cm/s >140 cm/s Extensive spectral broadeningOccluded Absent signals Dampened signal in CCA  ICA:CCA Ratio16 A variety of interpretative schema use several ratios obtained from carotid artery spectral Doppler waveforms. The most common are the: o ICA to CCA peak systolic ratio (IC:CC - PSV) o ICA to CCA end diastolic ratio (IC::CC –EDV) IC: CC ratio > 4.0 = >70% stenosis (Sensitivity: 91%; specificity: 90%; PPV = 87%; NPV=94%) Vertebral and Subclavian Interpretation Subclavian steal phenomenon (SSP) refers to a benign condition in which hemodynamically significant narrowing of the proximal subclavian artery results in flow reversal in the ipsilateral (same side) vertebral artery. It is not an uncommon finding during carotid triplex ultrasound evaluation and has been documented to occur in about 6% of patients undergoing testing.17 SSP is asymptomatic and refers to retrograde flow in the vertebral artery only. Subclavian steal syndrome (SSS), on the other hand, describes a condition characterized by flow reversal in the vertebral artery associated with transient neurological symptoms related to cerebral ischemia. The primary lesion causing vertebral artery flow reversal is proximal subclavian artery stenosis or occlusion, resulting in decreased blood pressure in the arm distal to the stenosis. A pressure reduction occurs in the arm that causes the collateral network in the Circle of Willis to kick in and shunt blood down the ipsilateral vertebral artery to the compromised vascular territory. When symptoms are present they typically are related to vertebrobasilar and posterior cerebral circulation ischemia. Specifically, symptoms may include: dizziness, unsteadiness, vertigo, and visual changes. In chronic, severe SSS, arm ischemia occurs, causing arm claudication and rest pain.18Carotid Artery Triplex Ultrasound (10)
  11. 11. ProSono Copyright 2006 Diagnostic criteria include: o Brachial systolic pressure >20 mmHG lower than contralateral side. o Diminished or absent radial pulse. o Supraclavicular bruit present on auscultation. o Flow reversal in vertebral artery. o SA spectral Doppler waveform monophasic. o Technical Considerations The primary sonographic finding in diagnosing subclavian steal is the demonstration of flow reversal in the vertebral artery. Having obtained bilateral brachial artery systolic pressures prior to beginning the ultrasound protocol, the examiner will be alerted to pay particular attention to vertebral flow direction if one arm pressure measures more than 20 mmHg lower than the other arm. A diminished or absent pulse and monophasic Doppler signals in the radial artery are further clinical evidence that a hemodynamically significant stenotic lesion is present somewhere between the heart and the wrist. Triplex ultrasound readily reveals the presence of flow reversal in the vertebral artery on both CDI and spectral Doppler waveform display. It goes without saying that, to be able to detect flow reversal, the operator must be thoroughly familiar with how directional information is displayed on his/her system. A review of the underlying physical principles of directional Doppler display see Chapter 11: Doppler Instrumentation : A practical recommendation for validating CDI demonstration of vertebral artery flow direction is to compare color flow in the vertebral to color flow in the CCA and internal jugular vein (IJV). Rarely is flow in the CCA reversed and flow reversal in the IJV is unheard of. In a normal hemodynamic state, then, color hue should be the same in the VA and the CCA. In a steal state, color hue in the VA will be the same as in the IJV. Normal vertebral artery flow pattern. The VA demonstrates flow toward the head (red = flow away). The vertebral vein immediately above it demonstrates flow toward the heart (blue –flow away). The spectral Doppler waveform obtained from the VA is plotted under the x-axis indicating flow away.Carotid Artery Triplex Ultrasound (11)
  12. 12. ProSono Copyright 20061 Lindgren A, Roijer A, Norrving B, et al. Carotid artery and heart disease in subtypes of cerebralinfarction. Stroke 1994. 25:2356-62.2 Bogousslavsky J, Van Melle G, Regli F. The Lausanne Stroke Registry: analysis of 1,000consecutive patients with first stroke. Stroke 1988. 19:1083-92.3 Barnett HJ, Taylor DW, Eliasziw M, et al. Benefit of carotid endarterectomy in patients withsymptomatic moderate or severe stenosis. North American Symptomatic Carotid EndarterectomyTrial Collaborators. N Engl J Med 1998. 339:1415-25.4 Anonymous. Randomised trial of endarterectomy for recently symptomatic carotid stenosis: finalresults of the MRC European Carotid Surgery Trial (ECST). Lancet 1998. 351:1379-87. (9113):5 Yao-Jen Chang, BA; Alexandra J. Golby, BS, et al. Detection of Carotid Stenosis: FromNASCET Results to Clinical Practice. Stroke. 1995;26:1325-13286 Ferguson GG, Eliasziw M, Barr HW, et al. The North American Symptomatic CarotidEndarterectomy Trial: surgical results in 1415 patients. Stroke. 1999 Sep;30(9):1751-8.7 Morgenstern LB, Fox AJ, Sharpe BL, et al. The risks and benefits of carotid endarterectomy inpatients with near occlusion of the carotid artery. North American Symptomatic CarotidEndarterectomy Trial (NASCET) Group. Neurology. 1997 Apr;48(4):911-5.8 Young LH, Appen RE. Ischemic oculopathy. A manifestation of carotid artery disease. ArchNeurol. 1981 Jun;38(6):358-6.9 Faggioli GL, Curl GR, Ricotta JJ. The role of carotid screening before coronary artery bypass.Vasc Surg. 1990 Dec;12(6):724-9; discussion 729-31.11 Eliasziw M, Rankin RN, Fox AJ, et al. Accuracy and prognostic consequences ofultrasonography in identifying severe carotid artery stenosis. North American SymptomaticCarotid Endarterectomy Trial (NASCET) Group. Stroke.1995 Oct;26(10):1747-52.12 Staikov IN, Arnold M, Mattle HP, et al. Comparison of the ECST, CC, and NASCET gradingmethods and ultrasound for assessing carotid stenosis. European Carotid Surgery Trial. NorthAmerican Symptomatic Carotid Endarterectomy Trial. J Neurol. 2000 Sep;247(9):681-6.13 Grant EG, Benson CB, Moneta GL, Carotid artery stenosis: grayscale and Doppler ultrasounddiagnosis--Society of Radiologists in Ultrasound consensus conference. Ultrasound Q. 2003Dec;19(4):190-8.15 Moneta GL, Edwards JM, Chitwood RW, Correlation of North American Symptomatic CarotidEndarterectomy Trial (NASCET) angiographic definition of 70% to 99% internal carotid arterystenosis with duplex scanning. J Vasc Surg. 1993 Jan;17(1):152-7; discussion 157-9.16 Hwang CS, Liao KM, Tegeler CH.J A multiple regression model of combined duplex criteria fordetecting threshold carotid stenosis and predicting the exact degree of carotid stenosis.Neuroimaging. 2003 Oct;13(4):324-9.17 Kliewer MA, Hertzberg BS, Kim DH, et al: Vertebral artery Doppler waveform changesindicating Weak or absent radial and ulnar pulse in the presence of ipsilateral reduced bloodpressure (change is >20 mm Hg) when compared to the contralateral arm suggests SSS. Am JRoentgenol 2000 Mar; 174(3): 815-9.18 Gosselin C, Walker PM: Subclavian steal syndrome: existence, clinical features, diagnosis andmanagement. Semin Vasc Surg 1996 Jun; 9(2): 93-7.Carotid Artery Triplex Ultrasound (12)