Standardized ultrasound evaluation of carotid stenosis for clinical trials: University of Washington Ultrasound Reading Center
© Beach et al; licensee BioMed Central Ltd. 2010
Received: 13 August 2010
Accepted: 7 September 2010
Published: 7 September 2010
Serial monitoring of patients participating in clinical trials of carotid artery therapy requires noninvasive precision methods that are inexpensive, safe and widely available. Noninvasive ultrasonic duplex Doppler velocimetry provides a precision method that can be used for recruitment qualification, pre-treatment classification and post treatment surveillance for remodeling and restenosis. The University of Washington Ultrasound Reading Center (UWURC) provides a uniform examination protocol and interpretation of duplex Doppler velocity measurements.
Doppler waveforms from 6 locations along the common carotid and internal carotid artery path to the brain plus the external carotid and vertebral arteries on each side using a Doppler examination angle of 60 degrees are evaluated. The UWURC verifies all measurements against the images and waveforms for the database, which includes pre-procedure, post-procedure and annual follow-up examinations. Doppler angle alignment errors greater than 3 degrees and Doppler velocity measurement errors greater than 0.05 m/s are corrected.
Angle adjusted Doppler velocity measurements produce higher values when higher Doppler examination angles are used. The definition of peak systolic velocity varies between examiners when spectral broadening due to turbulence is present. Examples of measurements are shown.
Although ultrasonic duplex Doppler methods are widely used in carotid artery diagnosis, there is disagreement about how the examinations should be performed and how the results should be validated. In clinical trails, a centralized reading center can unify the methods. Because the goals of research examinations are different from those of clinical examinations, screening and diagnostic clinical examinations may require fewer velocity measurements.
Repair of carotid artery stenoses (carotid revascularization) has been shown to be effective in reducing the chance of embolic stroke from carotid plaque rupture and embolization to the brain . Clinical trials of carotid artery revascularization methods such as carotid endarterectomy and carotid artery stenting are in progress to provide guidance to clinicians about the choice of therapy.
Noninvasive ultrasonic duplex Doppler examination has been a standard method for the clinical evaluation of the carotid arteries for a third of a century [2, 3]. Doppler velocity waveforms are gathered from the common and internal carotid arteries to detect local elevated blood velocity as a marker of arterial stenosis allowing categorical classification of the right and left common and internal carotid arteries into clinically useful categories. One often used classification scheme is: 1) no significant stenosis (< 50%DR), 2) moderate stenosis (50%-79%DR), 3) severe stenosis (80%-99%DR), and 4) occluded. The method and associated criteria for stenosis classification were developed in the decade prior to 1990 [3–10]. The reference standard for the classification method is X-ray contrast angiography. Some publications use other angiographic categories with divisions at 60%, 70% or other values.
A variety of Doppler velocity measurement methods are used to classify arteries into the proper angiographic categories. However, detailed publications demonstrate that although satisfactory sensitivities and specificities can be obtained by associating selected angiographic classifications with particular Doppler measurements, the relationship between Doppler measurements and angiography is not a narrow monotonic line,  but a multivariate relationship. The additional variables include: the presence of a moderate or severe contralateral stenosis [12–15], cerebral territory perfused , completeness of the circle of Willis [17–19], ipsilateral collateral flow , vertebral flow  and method of revascularization .
All carotid ultrasound duplex Doppler examinations are performed by field centers under IRB approval at the field center institutions. A duplex Doppler Ultrasound Protocol Manual is provided to each participating ultrasound laboratory by the University of Washington Ultrasound Reading Center (UWURC). Anonymized images and worksheets from each examination are sent to the UWURC.
For each of the 16 or more spectral waveforms, systolic and diastolic velocities are measured and transcribed along with the Doppler angles (from the associated B-mode images) to a standard worksheet. The worksheet is submitted with paper, film, photocopied or electronic versions of the images to the UWURC. Studies on video tape recordings are discouraged because of the excessive time required for video processing.
Turbulence or complicated oscillating flow is most likely to occur during temporal deceleration in the late phase of systole and during spatial deceleration just distal to a stenosis. This turbulence causes bruits or murmurs that can be heard with a stethoscope, and appears as spectral broadening that can be visualized in the spectral waveform. Application of "angle correction" to the Doppler frequency measurement based on the Doppler equation by measuring the Doppler angle between the ultrasound beam and the artery axis is not appropriate for turbulent wavforms because the heading of the velocity vector is random or at least chaotic during spectral broadening. Thus, some examiners differentiate Peak Systolic Velocity (PSV), which is measured during spectral broadening, from End Acceleration Velocity (EAV), which is measured just before the onset of turbulence (Figure 4). Because the PSV is often greater than the EAV and there is no guidance in the literature on which to choose, the UWURC enters both values on the review form for later analysis to provide a basis for selecting one or the other.
The resulting data form (Figure 2B) accommodates five final numeric values for each of the 16 recommended and 2 optional (additional distal ICA) measurements: 1) "Machine Set Angle" (MSA), 2) "Hand Measured Angle" (HMA), 3) "Peak Systolic Velocity" (PSV), 4) "End Acceleration Velocity" (EAV), 5) "End Diastolic Velocity" (EDV). There are also ten categorical values: 1) Waveform Missing, 2) Other Can't Verify (when the anatomic location cannot be established), 3) Angle should be Protocol (when a Doppler angle other than 60 degrees is used but a 60 degree angle could have been used), 4) Variable Angle Alignment (when the Doppler sample volume is located in a curve or other anatomic location in which the angle could have been measured differently) 5) ?PSV (when due to arrhythmia or to turbulence (spectral broadening) the systolic velocity value is uncertain, 6) PSV remeasured (used as an interim variable for marking EAV on a prior version of the review form), 7) PSV or EAV (marks whether the examiner measured the PSV or EAV), 8) H or F (marks whether the Doppler cursor was angled toward the head or the foot), 9) Velocity in Stent (provides an indication of stent location), 10) Ratio View (marks the CCA and ICA values used in computing the velocity ratio).
Each of these variables is designed either to document a feature of the measurement or to provide the basis of testing specific hypotheses in future publications. For values marked "Waveform Missing" or "Can't Verify", the values may contain errors not detected by the review process because of missing or obscured images. For instance, if a study was submitted as a clinical report, with some velocity values reported as text, but no images or waveforms were provided, the clinical values were entered on the Review Form but the corresponding review form lines were marked "Waveform Missing".
After the "Reader" checked all of the data on each form against the source data, the "Reviewer" rechecked all of the data and each reader entry against the source data. Disagreements between Reader and Reviewer were adjudicated by committee to assure uniform reading and reviewing.
All entries keyed into the database are logged to document the Reader, Reviewer, Adjudicator, and Keyer.
The data have been compiled into a database that can be configured for analysis by patient, side, treatment side, and/or time point to allow longitudinal or cross sectional comparisons.
The most objective and comprehensive survey of carotid artery examination methods is the 2002 Carotid Ultrasound Consensus Conference . In 1997, the University of Washington Ultrasound Reading Center designed the ultrasound protocol which complies with the recommendations later adopted by the consensus conference, with three exceptions: 1) The UWURC recommends the consistent use of a Doppler examination angle of 60 degrees; the Consensus Conference reports disagreement, with some members recommending 60 degrees and some recommending < 60 degrees; 2) the UWURC Doppler diastolic velocity criterion for severe stenosis is 1.4 m/s rather than 1.0 m/s as recommended by the Consensus Conference; and 3) the UWURC makes no recommendation about the evaluation of B-mode or color Doppler images, except for the identification of the location of a stent at the Doppler sample location; the Consensus Conference recommends the evaluation of these images, but provides no quantitative method of reporting the evaluation.
The Consensus Conference explains "the ability of Doppler ultrasound to ... estimate the degree of stenosis [has] been disappointing." so "Doppler ultrasound cannot be used to predict a single percentage of stenosis." but "..criteria should be consistently applied." "Published literature is replete with velocity thresholds.." "The panel suggested that ICA PSV and the presence of plaque on ... images ... should be used when diagnosing and grading ICA stenosis." "The ICA PSV is easy to obtain, has good reproducibility, and should be used in conjunction with grayscale and color Doppler.." "Two additional parameters, ICA-to-CCA PSV ratio and ICA EDV are useful .... " A summary of recommended criteria are included in Table 3." of the consensus paper .
The UW classifications, established prior to 1990, were based on a lower boundary for severe stenosis of 80% DR by angiography: (NLD-MLD)/NLD where MLD is minimum lumen diameter and NLD is the normal lumen diameter of the carotid bulb (ACAS method). Subsequently, others have adopted a 70% lower boundary where NLD is the normal lumen diameter of the ICA distal to the stenosis (NASCET method). Generally the bulb diameter is 1.5 times the normal distal ICA diameter, thus 70% NASCET stenosis = 80% ACAS stenosis.
The consensus paper offers two criteria for the 70% stenosis: PSV = 2.3 m/s and EDV = 1.0 m/s. The UWURC recommends EDV = 1.4 m/s. Because the UWURC includes the velocity values in the database, future analyses can elect to use any of these criteria.
There is also a philosophical difference between the consensus document and the UWURC recommendations. While the consensus document recommends that diagnosis be based on a combination of observations from the grayscale B-mode image, color Doppler and spectral Doppler, the exact method of combination is unclear and the use of multiple variables or observations can lead to conflicting results. The two alternate methods used in the summary portion of the UWURC review form--one based on highest PSV(ICA) with EDV(ICA) and the other based on PSV(ICA)/PSV(CCA) ratio -- will not necessarily agree, and should not be used together, but rather, one method should be selected and used consistently.
The relationship between Doppler velocity and angiographic stenosis within the significant stenosis range of interventional trials is poor. The sensitivity and specificity of the test only improves when a large number of cases with minimal or no stenosis are included in the tabulation. Perhaps we have been naïve in the quest for a linear relationship between Doppler velocity and stenotic diameter. Although each hemisphere of the brain does demand a constant average blood supply, independent of intelligence or occupation, a stenosis is likely to induce flow diversion to other potential collateral pathways (contralateral arteries or the ipsilateral external carotid or vertebral arteries), reducing the trans-stenotic flow and velocity by an unpredictable amount. The pattern of the flow diversion might provide important information for the velocity stenosis relationship, and in addition might allow inferences about recruited collaterals which might serve to reduce the risk of stroke below the chance predicted by the stenosis alone. Some sonographers do report the ratio ((ipsilateral CCA PSV)/(contralateral CCA PSV)) to support the diagnosis of ICA stenosis. However, a value less than 1.0 which indicates stenosis also indicates intracranial collateralization, which might be protective against stroke. Thus, although carotid Doppler has been used clinically for a third of a century, puzzles remain and opportunities to improve the method invite exploration.
The geometry of the Doppler equation predicts that the Doppler frequency shift will be zero if the Doppler angle is perpendicular (90 degrees). However, because of transit-time spectral broadening, helical (laminar) flow and complicated turbulent or eddy flow, even at 90 degrees the envelope of the Doppler frequency shift spectrum is not zero. This broadening affects all of the Doppler measurements except those made at a Doppler angle of zero degrees. Unfortunately, a Doppler angle of zero degrees is not possible in ultrasound examination of peripheral arteries and veins. As a result, all "angle corrected" Doppler velocity measurements monotonically increase with Doppler angle from zero to 90 degrees. If the Doppler frequency in the Doppler equation is held constant, and the Doppler angle is changed from 40 degrees to 60 degrees, the computed Doppler velocity increases by 42% or 2.1% per degree. In Figure 6, the velocity value increases by about 1.5% per degree between the 40 degree measurement and the 60 degree measurement in PSV and one EDV, and in the other EDV measurement by 0.8%. Note that in Figure 1 4, the best fit line for systolic velocity measurement increases by 1.8% per degree and the diastolic velocity measurement increased by 1.27% per degree. These values are consistent with the values that can be estimated from the Figure 1 1.30 in Primozich  of 2% per degree. It remains to be determined whether the statistically significant dependence on angle is an important factor affecting surveillance precision.
Although angle adjusted Doppler velocity measurements can be used to classify the severity of carotid stenosis and to monitor the changes in carotid stenosis over time, these velocity values computed from the measurement of a vector component of the velocity vector adjusted by geometric angle projection are not equal to the velocity components parallel to the vessel axis which contributes to the volumetric flow along the artery. The angle adjusted velocity values can only be used for empirical classification based on published standards, and for time to time comparisons of values within each patient. The classifications are only valid when the acquisition protocol is consistent with the standard.
How much of a change in estimated ICA stenosis should be considered significant?
What criteria should be used to assess patients after ICA revascularization?
Does the degree of contralateral stenosis affect the ipsilateral diagnostic criteria?
A detailed analysis of the data in the future will address these questions and the results will be published.
Because the classification of stenosis into angiographic categories by Doppler has limitations, using this categorical variable for surveillance of a revascularized artery to measure durability can lead to erroneous results. In this case, if a stenosis changes from a "moderate stenosis (50%-79%DR)" to a "severe stenosis (80%-99%DR)", the change in classification might be due to an increase in EDV from 1.38 m/s to 1.42 m/s. Such a small change in measurement might not indicate a change in arterial morphology. An alternative might be to require a change in classification from "no significant stenosis (< 50%DR)" to "severe stenosis (80%-99%DR)", which would be a change from PSV < 1.25 m/s to an EDV > 1.4 m/s. Important progression of a stenosis might not be detected if that were the criteria. If, however, the standard deviation (SD) of the difference in PSV or EDV between visits is measured, then an increase in value more then 3 SD would provide a 99% confidence that the stenosis has become more severe. In the absence of treatment, a decrease in value more than 3 SD would be surprising. However, in a trial of 1000 cases, that rare event would be expected in 10 cases, due t measurement variability rather than stenosis regression.
Are three velocity measurements in the CCA necessary to:
identify CCA disease?
provide a reference denominator for ICA/CCA ratio calculation?
Are measurements in the ECA and VA important to the clinical evaluation?
Do contralateral velocities decrease when an ipsilateral stenosis is treated suggesting that:
intracranial cross-collaterals are present?
ipsilateral intra-stenotic velocities might be reduced due to collateral flow?
Are particular velocity values or ratios predictive of complications during revascularization?
The first two questions relate to potentially simplifying the clinical examination by omitting superfluous measurements. The third question addresses a cofactor in the correlation between Doppler velocities and angiographic arterial diameter measurements. The fourth question suggests that additional inferences might be derived from a complete clinical examination including modulating the predicted risk of stroke.
Of course clinical carotid examination should be divided into two examinations: 1) screening examinations with a high sensitivity and acceptable specificity for internal carotid artery stenosis which can be carried out in a non-specialist primary care setting, and 2) diagnostic examinations with high specificity for severe carotid stenosis with "vulnerable" plaque to assure that high risk patients are directed to appropriate treatment.
When carotid examinations according to protocol have not been available, the UWURC has accepted data from "clinical examinations" to complete time points in the data set. The minimum data included in the studies have been single velocity measurements from the ICA and CCA on the evaluated side. Demonstration of a single end diastolic carotid velocity exceeding 1.4 m/s is universally accepted as proof of carotid stenotic disease, but verifying a non-stenotic carotid bifurcation requires more documentation.
Kirk W. Beach, Ph.D., M.D. Emeritus Professor of Surgery and Bioengineering.
Robert O. Bergelin, M.S., Director of Departmental Computing
Daniel F. Leotta, Ph.D., Research Engineer, Applied Physics Laboratory
Jean F. Primozich, B.S., R.V.T., Lead Vascular Technologist
P. Max Sevareid, M.P.H., Project Manager
Edward T. Stutzman, B.S., R.V.T, Vascular Technologist
R Eugene Zierler, M.D., R.V.T., Professor of Vascular Surgery
Department of Surgery
University of Washington,
Seattle, WA 98195
Asymptomatic Carotid Atherosclerosis Study
Common Carotid Artery
Angiographic stenotic Diameter Reduction
End Acceleration Velocity
External Carotid Artery
End Diastolic Velocity
Doppler beam directed toward the feet, normal flow "toward" transducer
Doppler beam directed toward the head, normal flow "away" from transducer
Hand Measured Angle by the UWURC from the B-mode image
Internal Carotid Artery
Machine Set Angle of the sonographer selected Doppler cursor
North American Symptomatic Carotid Endarterectomy Trial
Peak Systolic Velocity
University of Washington Ultrasound Reading Center
Value uncertain due to arrhythmia.
We would like to thank Abbott Vascular, Boston Scientific, Guidant, Medtronic, NIH-NINDS R01 NS 38384 (CREST) and private donations for support of this work.
- Howell SJ: Carotid Endarterectomy. Br J Anaesth 2007,99(1):119-131. 10.1093/bja/aem137View ArticlePubMed
- Barber FE, Baker DW, Nation AW, Strandness DE Jr, Reid JM: Ultrasonic duplex echo-Doppler scanner. IEEE Trans Biomed Eng 1974,21(2):109-113. 10.1109/TBME.1974.324295View ArticlePubMed
- Blackshear WM Jr, Phillips DJ, Thiele BL, Hirsch JH, Chikos PM, Marinelli MR, Ward KJ, Strandness DE Jr: Detection of carotid occlusive disease by ultrasonic imaging and pulsed Doppler spectrum analysis. Surgery 1979,86(5):698-706.PubMed
- Knox RA, Breslau PJ, Strandness DE Jr: A simple parameter for accurate detection of severe carotid disease. Br J Surg 1982,69(4):230-233. 10.1002/bjs.1800690421View ArticlePubMed
- Breslau PJ, Fell G, Phillips DJ, Thiele BL, Strandness DE Jr: Evaluation of carotid bifurcation disease: the role of common carotid artery velocity patterns. Arch Surg 1982,117(1):58-60.View ArticlePubMed
- Langlois Y, Roederer GO, Chan A, Strandness DE Jr: The use of common carotid waveform analysis in the diagnosis of carotid occlusive disease. Angiology 1983,34(10):679-687. 10.1177/000331978303401006View ArticlePubMed
- Langlois Y, Roederer GO, Chan A, Phillips DJ, Beach KW, Martin D, Chikos PM, Strandness DE Jr: Evaluating carotid artery disease. The concordance between pulsed Doppler/spectrum analysis and angiography. Ultrasound Med Biol 1983,9(1):51-63. 10.1016/0301-5629(83)90109-6View ArticlePubMed
- Kohler T, Langlois Y, Roederer GO, Phillips DJ, Beach KW, Primozich J, Lawrence R, Nicholls SC, Strandness DE Jr: Sources of variability in carotid duplex examination: a prospective study. Ultrasound Med Biol 1985,11(4):571-576. 10.1016/0301-5629(85)90027-4View ArticlePubMed
- Kohler TR, Langlois Y, Roederer GO, Phillips DJ, Beach KW, Primozich J, Lawrence R, Nicholls SC, Strandness DE Jr: Variability in measurement of specific parameters for carotid duplex examination. Ultrasound Med Biol 1987,13(10):637-642. 10.1016/0301-5629(87)90061-5View ArticlePubMed
- Taylor DC, Strandness DE Jr: Carotid artery duplex scanning. J Clin Ultrasound 1987,15(9):635-644. 10.1002/jcu.1870150906View ArticlePubMed
- Moneta GL, Edwards JM, Papanicolaou G, Hatsukami T, Taylor LM, Strandness DE, Porter JM: Screening for asymptomatic internal carotid artery stenosis: Duplex criteris for discriminating 60% to 99% stenosis. J Vasc Surg 1995, 21: 989-994. 10.1016/S0741-5214(95)70228-8View ArticlePubMed
- Grajo JR, Barr RG: Duplex Doppler sonography of the carotid artery: velocity measurements in an artery with contralateral stenosis. Ultrasound Q 2007,23(3):199-202. 10.1097/RUQ.0b013e31814fb469View ArticlePubMed
- Sachar R, Yadav JS, Roffi M, Cho L, Reginelli JP, Aböu-Chebl A, Bhatt DL, Bajzer CT: Severe bilateral carotid stenosis: the impact of ipsilateral stenting on Doppler-defined contralateral stenosis. J Am Coll Cardiol 2004,43(8):1358-1362. 10.1016/j.jacc.2003.11.049View ArticlePubMed
- AbuRahma AF, Richmond BK, Robinson PA, Khan S, Pollack JA, Alberts S: Effect of contralateral severe stenosis or carotid occlusion on duplex criteria of ipsilateral stenoses: comparative study of various duplex parameters. J Vasc Surg 1995,22(6):751-61. discussion 761-762 10.1016/S0741-5214(95)70066-8View ArticlePubMed
- Hayes AC, Johnston KW, Baker WH, Kupper C, Poole MA, Keagy B, Burnham S: The effect of contralateral disease on carotid Doppler frequency. Surgery 1988,103(1):19-23.PubMed
- Van Laar PJ, Hendrikse J, Mali WP, Moll FL, van der Worp HB, van Osch MJ, van der Grond J: Altered flow territories after carotid stenting and carotid endarterectomy. J Vasc Surg 2007,45(6):1155-1161. 10.1016/j.jvs.2006.11.067View ArticlePubMed
- Papantchev V, Hristov S, Todorova D, Naydenov E, Paloff A, Nikolov D, Tschirkov A, Ovtscharoff W: Some variations of the circle of Willis, important for cerebral protection in aortic surgery--a study in Eastern Europeans. Eur J Cardiothorac Surg 2007,31(6):982-989. 10.1016/j.ejcts.2007.03.020View ArticlePubMed
- Kablak-Ziembicka A, Przewlocki T, Pieniazek P, Musialek P, Motyl R, Moczulski Z, Tracz W: Assessment of flow changes in the circle of Willis after stenting for severe internal carotid artery stenosis. J Endovasc Ther 2006,13(2):205-213. 10.1583/05-1700R.1View ArticlePubMed
- de Nie AJ, Blankensteijn JD, Visser GH, van der Grond J, Eikelboom BC: Cerebral blood flow in relation to contralateral carotid disease, an MRA and TCD study. Eur J Vasc Endovasc Surg 2001,21(3):220-226. 10.1053/ejvs.2000.1308View ArticlePubMed
- Fujioka S, Karashima K, Nakagawa H, Saito Y, Nishikawa N: Classification of ophthalmic artery flow in patients with occlusive carotid artery disease. Jpn J Ophthalmol 2006,50(3):224-228. 10.1007/s10384-005-0312-yView ArticlePubMed
- Welch HJ, Murphy MC, Raftery KB, Jewell ER: Carotid duplex with contralateral disease: the influence of vertebral artery blood flow. Ann Vasc Surg 2000,14(1):82-88. 10.1007/s100169910015View ArticlePubMed
- Lal BK, Hobson RW, Tofighi B, Kapadia I, Cuadra S, Jamil Z: Duplex ultrasound velocity criteria for the stented carotid artery. J Vasc Surg 2008,47(1):63-73. 10.1016/j.jvs.2007.09.038View ArticlePubMed
- Moneta GL, Edwards JM, Chitwood RW, Taylor LM Jr, Lee RW, Cummings CA, Porter JM: Correlation of North American Symptomatic Carotid Endarterectomy Trial (NASCET) angiographic definition of 70% to 99% internal carotid artery stenosis with duplex scanning. J Vasc Surg 1993,17(1):152-159. 10.1067/mva.1993.42888View ArticlePubMed
- Primozich JF: Extracranial Arterial System. In Duplex Scanning in Vascular Disorders. 3rd edition. Edited by: Strandness DE. Philadelphia: Lippincott Williams and Wilkins; 2002:191-231.
- Zierler RE: Basic and practical aspects of cerebrovascular testing. In Vascular Diagnosis. 4th edition. Edited by: Berstein EF. St Louis: Mosby; 1993:308-314.
- Back MR, Wilson JS, Rushing G, Stordahl N, Linden C, Johnson BL, Bandyk DF: Magnetic resonance angiography is an accurate imaging adjunct to duplex ultrasound scanning in patient selection for carotid endarterectomy. J Vasc Surg 2000, 32: 429-440. 10.1067/mva.2000.109330View ArticlePubMed
- Moneta GL, Taylor DC, Zierler RE, Kazmers A, Beach K, Strandness DE Jr: Asymptomatic high-grade internal carotid artery stenosis: Is stratification according to risk factors or duplex spectral analysis possible? J Vasc Surg 1989,10(5):475-483. 10.1067/mva.1989.15589View ArticlePubMed
- Grant EG, Benson CB, Moneta GL, Alexandrov AV, Baker JD, Bluth EI, Carroll BA, Eliasziw M, Gocke J, Hertzberg BS, Katarick S, Needleman L, Pellerito J, Polak JF, Rholl KS, Wooster DL, Zierler E, Society of Radiologists in Ultrasound: Carotid artery stenosis: grayscale and Doppler ultrasound diagnosis--Society of Radiologists in Ultrasound consensus conference. Ultrasound Q 2003,19(4):190-198. 10.1097/00013644-200312000-00005View ArticlePubMed
- Ku DN, Giddens DP: Pulsatile flow in a model carotid bifurcation. Arteriosclerosis 1983, 3: 31-39.View ArticlePubMed
- Iconico Screen Protractor [http://iconico.com/protractor/]
- Kliewer MA, Hertzberg BS, Kim DH, Bowie JD, Courneya DL, Carroll BA: Vertebral artery Doppler waveform changes indicating subclavian steal physiology. AJR Am J Roentgenol 2000,174(3):815-819. Erratum in: AJR Am J Roentgenol 2000, 174(5): 1464View ArticlePubMed
- Sevareid PM: Multi-Center, Diagnostic Duplex Doppler Ultrasound vs. Carotid Artery Angiogram Correlation in Patients Screening for Carotid Artery Stenting Trials, MPH thesis. Seattle, USA: University of Washington, Health Services and Health Policy Research; 2008.
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