|Year : 2012 | Volume
| Issue : 2 | Page : 137-143
Colour Doppler evaluation of extracranial carotid artery in patients presenting with features of cerebrovascular disease: A clinical and radiological correlation
Sanjeev Sehrawat, Sarabjit Singh Thind, Vikram Singh, Rajesh Kuber, Satish Naware, Harshawardhan Shrotri
Department of Radio Diagnosis, Padmashree Dr. D. Y. Patil Medical College, Sant Tukaram Nagar, Pune, Maharashtra, India
|Date of Web Publication||10-Nov-2012|
270, Tarun Enclave, Pitampura, Delhi
Source of Support: None, Conflict of Interest: None
Aim: To evaluate the morphological and hemodynamic changes that take place in carotid arteries by colour Doppler in patients presenting with features of stroke. Background and Objectives: Cerebrovascular accidents constitute a major cause of adult mortality. The principal indication for cerebrovascular Doppler examination is stroke prevention. Colour Doppler sonography is a sensitive method for detection of atherosclerotic plaque and provides considerable information about the extent and severity of plaque as well as the resulting diminution of arterial lumen. The main strengths of sonography of carotid arteries are patient comfort, lack of risk and accuracy in detecting carotid stenosis. Material and Methods: A prospective study of Colour Doppler in carotid arteries was carried out for 12 months from 1 st July 2009 to 1 st July 2010. The study was carried out on 40 individuals, suspected of cerebrovascular insufficiency and having one or the other risk factors for cerebrovascular disease. A detailed clinical history, CNS examination findings and evidence of hypertension, diabetes mellitus, hyperlipidemia and ischemic heart disease were noted. Carotid Doppler evaluation was done by using Siemens Antares Ultrasound system. The data gathered were grey scale and Doppler findings of common carotid artery, internal carotid artery and external carotid arteries. Doppler findings were correlated with clinical features and risk factors. Results: In our study of 40 patients, the commonest lesion found was the atherosclerotic plaque. Highest incidence of plaque was seen in males 41% in the age group of 60-70 years and in females 37% in age group of 70-80 years. Cigarette smoking was the most common risk factor (60%) associated with stroke/ Transient Ischaemic Attacks (TIA). Hemiparesis was the most common presenting symptom (35%) among the symptomatic cases. Atheromatous plaque was most commonly found in the right carotid system (60%). Most common site for atheromatous plaque was found to be carotid bifurcation (33.3%). Significant stenosis, i.e., >50% was found in 12 (60%) of the cases. Peak systolic velocity ratio showed significant stenosis in 12 (60%) of symptomatic cases and in 4 (20%) of asymptomatic controls. Plaque characteristics showed 9 hyperechogenic, 8 calcific, 4 low echogenic and 5 moderate or heterogeneous plaques. Conclusion: The role of carotid Doppler in detecting the site and morphology of atherosclerotic plaque with quantifying the amount of stenosis is very well-justified. In addition carotid Doppler can also be used to assess the prognosis in potential symptomatic and asymptomatic patients with one or the other risk factors for cerebrovascular disease.
Keywords: Carotid artery, carotid Doppler, peak systolic velocity ratio
|How to cite this article:|
Sehrawat S, Thind SS, Singh V, Kuber R, Naware S, Shrotri H. Colour Doppler evaluation of extracranial carotid artery in patients presenting with features of cerebrovascular disease: A clinical and radiological correlation. Med J DY Patil Univ 2012;5:137-43
|How to cite this URL:|
Sehrawat S, Thind SS, Singh V, Kuber R, Naware S, Shrotri H. Colour Doppler evaluation of extracranial carotid artery in patients presenting with features of cerebrovascular disease: A clinical and radiological correlation. Med J DY Patil Univ [serial online] 2012 [cited 2017 Sep 20];5:137-43. Available from: http://www.mjdrdypu.org/text.asp?2012/5/2/137/103344
| Introduction|| |
Stroke is one of the leading causes of death. One-third of cases are fatal, and survivors usually have prolonged or irreversible disabilities.  An estimated 80% of strokes are thromboembolic in origin, often with carotid plaque as embolic source.  For more than a decade various sonographic techniques have been used for the assessment of carotid arteries in cerebrovascular disease. Of these, continuous-wave Doppler and single-gate pulsed-wave Doppler sonography incorporated in duplex systems are reported to be highly accurate relative to angiography for the detection and classification of the degree of obstruction producing a narrowing of lumen more than 50%. 
B-mode gray scale sonography allows for imaging of atherosclerotic plaques and intima-media thickness of carotid arteries.  Increase in intima-media thickness of an artery has been used as a surrogate marker of the early atherosclerotic process.  In addition, the use of high-resolution B-mode real-time sonography makes it possible to identify small, nonstenotic (<50%) plaques and to describe the echo morphology of carotid arteries. 
Besides estimating the degree of stenosis, the biggest advantage of sonography is its ability to characterize plaque and identify plaques with higher risk of embolization with high-resolution ultrasound. Plaque can be characterized into relative risk groups for containing intraplaque hemorrhage which is thought by many to be precursor plaque ulceration.  Duplex sonography is a popular, noninvasive, accurate and cost-effective means of detecting and assessing carotid disease. Carotid sonography has largely replaced angiography for suspected extracranial carotid atherosclerosis. 
| Material and Methods|| |
This study was carried out using real time ultrasound equipment capable of B-mode imaging, pulsed-wave duplex scanning, colour Doppler flow imaging and power Doppler imaging (Siemens Acuson Antares). All patients of adult age group presenting with clinical features of stroke or patients presenting with history suggestive of transient ischaemic attacks were taken up for the study. Detailed clinical history was taken from each patient. This included: age, sex, pre/post menopausal state and risk factors including hypertension, diabetes mellitus, history suggestive of coronary artery disease, smoking, alcohol intake, previous stroke or TIA, peripheral arterial disease, hyperlipidemia.
Visual Inspection of Gray Scale Images
Vessel wall thickness: Thickening of I-M complex greater than 0.8 mm was considered abnormal , [Figure 1] and [Figure 2].
|Figure 1: B mode ultrasound axial image of common carotid artery for assessment of intima media complex measured by calipers|
Click here to view
|Figure 2: Colour Doppler ultrasound longitudinal scan of the common carotid, carotid bulb and proximal internal and external carotid arteries|
Click here to view
Plaque characterization: Plaque texture was classified as being hypoechoic, echogenic or calcified. The uniform hyperechoic acoustic texture corresponds pathologically to dense, fibrous and connective tissue [Figure 3]. Calcified plaque produces posterior acoustic shadowing and is common in asymptomatic individuals. Hypoechoic plaque is characterized pathologically by containing deposits of lipid and cholesterol [Figure 4].  Lipid and cholesterol plaques can ulcerate and lead to thromboembolic episodes [Figure 5].
|Figure 3: Longitudinal section of the common carotid artery reveals a plaque with uniform hyper echoic appearance corresponds pathologically to dense, fi brous and connective tissue|
Click here to view
|Figure 4: Axial image reveals a hypo echoic plaque is in the ICA characterized pathologically by containing deposits of lipid and cholesterol|
Click here to view
|Figure 5: Longitudinal scan reveals a hypo echoic plaque is in the proximal ICA characterized with surface ulceration|
Click here to view
Doppler measurements were obtained in the stenotic portion of the carotid lumen. Parameters measured included: 
Carotid occlusion was diagnosed by absence of arterial pulsations, occlusion of lumen by echogenic material, absence of Doppler flow signals, and subnormal vessel size (chronic occlusion). North American Symptomatic Carotid Endarterectomy Trial (NASCET) and Asymptomatic Carotid Atherosclerosis Study (ACAS) stenosis measurement method was used. The residual lumen diameter compared with diameter free of disease. Vertebral artery was evaluated for their patency and direction of flow. Results of colour Doppler flow imaging, duplex scanning were compared and perfect agreement between these was calculated.
- Peak systolic velocity (PSV)
- Peak end diastolic velocity (EDV)
- Systolic and diastolic ratios
Limitations of Duplex Scanning and Sources of Error
There are many possible sources of error and limitation while using duplex scanning to assess carotid disease. These relate both to the examination techniques and criteria used to classify the degree of stenosis. One potential drawback of the technique is its inability to image vessels other than those in the neck. This has not been a major problem particularly as significant occlusive disease of the intracranial arteries and the siphon is not a common problem. Inability to image the vessel of interest properly may lead to an incorrect estimation of Doppler angle. This most often occurs when the vessel is tortuous. Calcified plaque may obscure an area of stenosis from proper interrogation because of its interference with sound transmission. ,
The high velocity jet of a severe stenosis may be missed if not carefully searched for. Although flow velocities remain quite constant in the carotid arteries from patients to patients, certain disease states may affect carotid flow. Low cardiac output or proximal stenosis may decrease flow velocities, leading to an underestimation of the degree of stenosis. Aortic insufficiency will change the character of carotid waveform making it more pulsatile and make the differentiation of internal and external carotid arteries more difficult. This technique is of little value for evaluating the vertebrobasilar circulation because of the smaller caliber and tortuosity of vertebral artery. ,
| Result|| |
The study was carried out on 40 patients. Out of which 29 (72.5%) were males and 11 (27.5%) were females. Age distribution was as follows: 14 patients (35%) were in age group of 50-60 years, 17 (42.5%) in age group of 61-70 years, 6 (15%) in age group of 71-80 years and 3 (7.5%) in age group of above 81 years. Majority of patients were in the age group of 61-70 years.
Out of 40 patients hemiparesis was the most common presenting clinical symptom 9 (22.5%), followed by giddiness 4 (10%), hemiplegia 3 (7.5%), hemisensory disturbances (HSD) 3 (7.5%), and transient monocular blindness (TMB) 1 (2.5%). Asymptomatic patients presented with known risk factors of DM 3 (7.5%), HTN 4 (10%), smoking >10 years 7 (17.5%), hyperlipidemia 3 (7.5%) and 3 (7.5%) patients with TIA. [Table 1]
Among the 40 patients, atheromatous plaques was most commonly found on right side 24 (60%) cases followed by right left 17 (42.5%) and bilateral, i.e., in both the carotids in 11 cases (27.5%).
Out of 24 patients with atheromatous plaque on right side 6 patients (25%) had plaque in CCA, 8 (33.3%) in ICA segment, 7 (29.1%) in right carotid bifurcation and only 2 (8.3%) patients had plaque in ECA. Common site of plaque was ICA followed by carotid bifurcation. On left side out of 17 patients with plaque 7 (41.7%) had plaque in ICA segment, 5 (29.4%) in Carotid bifurcation followed by 3 (17.6%) in CCA and 2 patients (11.7%) had plaque in ECA segment.
Among 40 cases, significant stenosis was in 15 patients. Maximum number of patients with significant stenosis, i.e., >50% were found in 61-70 years age group, i.e., 7 (46.6%) followed by 4 (26.6%) in 50-60, 3 (20%) in 71-80 years age group and one (6.6%) in above 81 years.
According to plaque characterization on ultrasound. Majority of the plaque were low echogenicity plaques in 18 (45%), calcified plaque 9 (23%), moderately echogenic 3 (7%) and 10 (25%) hyperechogenic plaque [Figure 6].
6 Correlation was done in the risk factors and degree of stenosis in carotid arteries. Out of 22 patients with history of smoking 7 (31.8%) had significant stenosis of >50%. Out of 15 patients of hypertension 3 (20%) had significant stenosis. In 18 patients with DM 3 (16%) had significant stenosis and out of 12 patients with risk factor of hyperlipidemia 2 (16.6%) had significant stenosis. Among 40 patients with smoking history and male patients was found most common risk factor which showed significant stenosis of >50%.
Correlation was done between intima-media thickness (IMT) and risk factors [Table 2]. More than 0.9 mm was considered abnormal and with increased risk of plaque formation. In our study out of 29 male patients 14 (48.2%) had abnormal IMT that is >0.9 mm, followed by patients with DM 6 (33%) out of 18, in HTN out of 15 patients only 4 (26.6%) had abnormal IMT.
Correlation between percentage stenosis and spectral velocity changes in stenosed areas was made in study group. Peak systolic velocity (PSV) range was also increased in proportion to stenosis. Highest PSV of over 140 cms/sec was found at the point of most severe stenosis [Table 3].
80-years-old female patient with complaints of left-sided weakness and history of diabetes mellitus and hypertension since 10 years [Figure 5].
| Discussion|| |
The study was conducted on total of 40 patients. All the patients were scanned for both right and left CCA, ICA and ECA. B-mode, colour mode and spectral wave features were studied. The use of linear array with band width of 7-11 MHz was found as optimal vascular probe in our study as the carotid vascular system is situated superficially. The higher frequency allows for better resolution and an adequate penetration with an acceptable field of view. Kreb and Giyanani  also advised the use of high-frequency transducer for imaging of carotid vessels. They also recommended that colour flow imaging and spectral wave pattern are must for proper interpretation and diagnosis of pathology.
The main role of carotid Doppler examination in the carotid artery disease is the detection of occlusive lesions in the vicinity of carotid bifurcation and internal carotid artery (ICA). The accurate diagnosis of critical stenosis (more or equal to 70% decrease in diameter) is important because these are the patients who carry increased risk of cerebral infarction [Figure 4].
Subcritical stenotic lesions are also important as cereberal thromboembolism may occur in this setting as mentioned by Chambers and Norris.  Moneta et al. have noted that the carotid lesions with less than 70% stenosis are generally treated medically with drugs that inhibit platelet aggregation and thrombus formation. 
The North American Symptomatic Carotid Endarterectomy Trial (NASCET) study, and the Asymptomatic Carotid Atherosclerosis Study (ACAS) as quoted by Biller, William et al.  have demonstrated that the surgical treatment of endarterectomy should be reserved for those patients with carotid stenosis of more than 70%.
Angiography as the "Gold Standard" has amply been replaced by duplex ultrasonography in most diagnostic circumstances. Positive experience with carotid duplex ultrasonography scanning unassisted by angiography in the setting of an impending surgical intervention has been embraced by a growing number of doctors. 
So the present study was done to evaluate extracranial carotid arterial system by carotid Doppler in the population who presented with cerebrovascular insufficiency and comparing them with asymptomatic individuals.
Out of 40 patients which were studied, 29 were male patients (72.5%) and 11 were females (27.5%). These finding correlated well with the North American Symptomatic Carotid Endarterectomy Trial (NASCET) studies, where NASCET had 72% male and 28% female patients.  Maximum patients who presented with cerebrovascular symptoms were between the age group of 50-60 and 61-70 years which is correlating with the findings of Fabris et al. prevalence of plaque of 38.9% in general population and increase in number and severity of plaque with increasing age. 
The commonest lesion was atherosclerotic plaque. Of the 40 patients examined, 10 patients had no lesion in carotid arteries (25%) and 29 patients (72%) had atherosclerotic plaques. The patients were evaluated for risk factors, anatomical site of involvement of plaque and the percentage stenosis it caused.
The anatomical sites were classified as proximal and distal CCA, carotid bulb, ICA and ECA. The right side was affected more than the left side. Twenty four plaques were present on right side as compared to 17 on left side, 11 were present bilaterally. In carotid arteries the commonest site of involvement was ICA (18 plaques) followed by carotid bulb (15 plaques). The ECA was least affected, with 2 plaques on each side. Kerb et al also found that commonest site of involvement was carotid bifurcation followed by ICA and intracranial portions of ICA. The bifurcation of the common carotid artery is the commonest source of plaque in the extracranial carotid artery, which is in accordance with Philips et al., where they found that the areas of transient reversal of flow, flow separation and eddy formation after referred as "boundary layer separation" corresponds to segments in the carotid bifurcation where atherosclerosis developed first. 
The traditional risk factors for stroke/TIA include diabetes mellitus, hypertension, smoking, hyperlipidemia and previous stroke. We found in our study that cigarette smoking was the commonest risk factor (55%), followed by diabetes mellitus (45%), hypertension (37.5%) and hyperlipidemia (30%) and previous stroke (15%). According to Whisnant et al. chronic cigarette smoking is a strong predictor for severe extracranial carotid atherosclerosis.  Iadecola et al.  had proved that control of blood pressure leads to a substantially lower risk of stroke. Diabetes mellitus is another risk factor for atherosclerosis. Lindberg Pertu and Roine Risto  in their study had observed that two thirds of all ischemic stroke types on admission had diabetes mellitus. George Howard  showed that most risk factors continue to be associated with increased atherosclerosis at older ages, possibly suggesting a continued value in investigation of strategies to reduce atherosclerosis by reducing risk factors at older age. In patients with high risk factor increase in intima-media thickness (>0.9 mm) of carotid arteries was also seen. Out of 29 male patients 14 (48.2%) had abnormal IMT that is >0.9 mm, followed by patients with DM 6 (33%) out of 18, in HTN out of 15 patients only 4 (26.6%) had abnormal IMT. These findings were correlated with Sun, Cheng-Huai et al.  who stated that higher IMT was associated with male gender. They also stated that plaque prevalence was positively associated with IMT and blood pressure, fasting blood sugar were independent risk factors related to both carotid atherosclerosis and thick IMT. Our findings also correlate with those of Jadhav and Kadam.  who found IMT >1.1 mm in diabetics in Indian population and also that males are more commonly affected than females.
The ultrasound characterization of the plaque showed that of the 40 plaques present, majority of the plaque were low echogenicity plaques. The principle arterial pathology detected in the carotid artery disease by B-mode ultrasonography is "Atherosclerotic plaque". This is seen as "Echogenic" material that encroaches on the arterial lumen and produces a flow void. This was emphasized by Erickson et al According to Libby  characterization is very important as low echogenic plaques are prone to ulceration and provide a nidus for formation of platelet thrombi. This may lodge into cerebral circulation and cause transient ischemic attack. In our study we found most of plaques (45%) as low echogenic plaques.
The patients were then classified according to the degree of stenosis that was present. This helps in evaluating treatment modalities in patients. The patients were classified on the bases of presence of symptoms as 20 (50%) symptomatic and 20 (50%) asymptomatic patients. Out of 20 symptomatic patients, 4 (20%) had severe stenosis that is >70%, 8 (40%) had 51%-70% stenosis, 3 (15%) had 31%-50% stenosis, 2 (10%) had 1%-30% stenosis and 4 (20%) were normal with no stenosis. In 20 asymptomatic patients 14 (70%) were normal with no stenosis, 3 (15%) patients with 1%-30% stenosis, 2 (10%) had 31%-50% stenosis, 2 (10%) had 51%-70% stenosis and only one (5%) had >70% stenosis. Brown et al.  categorized the duplex ultrasound findings in carotid arteries of stenosis with respect to presence of symptoms as symptomatic, asymptomatic and symptomatic with previous history of ischemic symptoms. Prevalence of severe stenosis (>70%) was 20% in symptomatic patients and 5% in asymptomatic, which correlates with our study.
According to Robinson et al.,  the duplex imaging of complete carotid occlusion was based on the following observation, as absence of arterial pulsation, lumen filled with echogenic material, subnormal vessel size (chronic occlusion) and absence of Doppler flow signals or weak Doppler signals. In our study, one case of complete carotid stenosis was noted, which showed all the above four characteristics of Doppler study.
The spectral waveform of the stenosed vessel was also analyzed to access the changes which occurred. In our study PSV increases as stenosis increases, this is similar to the study of Grant et al.  (PSV >140 cm/sec in >70% stenosis). PSV is best parameter because of ease of measurement and increasing specificity with increasing stenosis. Grant et al , also found that mean PSV increased with increased stenosis level. They found a specificity of 90.6% and concluded that Doppler ultrasound is excellent method to diagnose stenosis.
EDV also increased with the degree of stenosis in our study. EDV increased in range of 60-100 cm/sec in 80% stenosis. This concurs with the findings of Staikov et al.  who stated that optimal PSV and EDV for diagnosis of 80% or greater stenosis was 190 cm/sec and 65 cm/sec.
In our study most of the findings were in conformity with the literature. Doppler can be an accurate imaging modality to quantify and diagnose carotid artery disease.
| References|| |
|1.||Gaitini D, Soudack M. Diagnosing carotid stenosis by Doppler sonography. J Ultrasound Med 2005;24:1127-36. |
|2.||Rumack CM, Wilson SR, William Charboneau J, Johnson JM. Diagnostic ultrasound. 3 rd ed. Vol. 1. Uttar Pradesh, India: Elsevier; 2009. p. 943. |
|3.||Steinke W, Kloetzsch C, Hennerici M. Carotid artery disease assessed by colour Doppler flow imaging: Correlation with standard Doppler sonography and angiography. AJR Am J Roentgenol 1990;154:1061-8. |
|4.||Poli A, Tremoli E, Colombo A. Ultrasonographic measurement of the common carotid artery wall thickness in hypercholesterolemic patients: A new model for the quantification and follow up of preclinical atherosclerosis in living human subjects. Atherosclerosis 1988;70:253-61. |
|5.||Lusby RJ, Ferrel LD, Ehrenfeld WK, Stoney RJ, Wylie EJ. Carotid plaque hemorrhage. Arch Surg 1982;117:1479-88. |
|6.||Bluth EI, Stavros AT, Marich KW, Wetzner SM, Aufrichtig D, Baker JD. Carotid duplex sonography multicenter recommendation for standardized imaging and Doppler criteria. Radiographics 1988;8:487-506. |
|7.||Polak JF. Carotid ultrasound. Radiol Clin North Am 2001;39:569-89. |
|8.||Jadhav UM, Kadam NN. Carotid Intima-Media thickness as an independent predictor of coronary artery disease. Indian Heart J 2001;53:458-62. |
|9.||Carroll BA. Carotid ultrasound. Neuroimaging Clin N Am 1996;6:875-97. |
|10.||Ricotta JJ, Bryan FA, Bond MG, Kurtz A, O'Leary DH, Raines JK, et al. A correlative analysis of the human carotid bifurcation by B-mode ultrasound, angiography and pathological examination of endarterectomy specimens. J Vasc Surg 1987;6:512-20. |
|11.||Riley WA, Barnes RW, Applegate WB, Dempsey R, Hartwell T, Davis VG, et al. Reproducibility of non-invasive ultrasonic measurement of carotid atherosclerosis: The asymptomatic carotid artery plaque study. Stroke 1992;23:1062-8. |
|12.||Polak JF, Kalina P, Donaldson MC, O'Leary DH, Whittemore AD, Mannick JA. Carotid endarterectomy: Preoperative evaluation of candidates with combined Doppler sonography and MR angiography. Radiology 1993;186:333-8. |
|13.||O'Donnel RF. Correlation of B-mode ultrasound imaging and arteriography with pathologic findings at carotid endarterectomy. Arch Surg 1985;120:443-8. |
|14.||Krebs CA, Giyanani VL, Eisenberg RL. Ultrasound atlas for vascular diseases. 1 st ed. 1999. p. 53-102. |
|15.||Chambers BR, Norris JW. Outcome in patients with asymptomatic neck bruits. N Engl J Med 1986;315:860-5. |
|16.||Moneta GL, Taylor DC, Nicholls L, Bergelin RO, Zierler RE, Kazmers A, et al. Operative versus non-operative management of asymptomatic high grade internal carotid stenosis: Improved results with endarterectomy. Stroke 1987;18:1005-10. |
|17.||Biller J, William FM, Castaldo JE, Whittemore AD, Harbaugh RE, Dempsey RJ, et al. Guidelines for carotid endarterectomy: A statement for healthcare professionals from a special writing group of the stroke council, American Heart Association. Circulation 1998;97:501-9. |
|18.||Cartier R, Cartier P, Fontaine A. Carotid endarterectomy without angiography. The reliability of Doppler ultrasonography and duplex scanning in preoperative assessment. Can J Surg 1993;36:411-6. |
|19.||North American Symptomatic Carotid Endarterectomy Trial collaborators interim results. N Eng Med 1999;323:215-23. |
|20.||Fabris F, Zanocchi M, Smazal SF, Stavros AT, Daigle RJ. Duplex sonography in evaluation of carotid artery disease. AJNR Am J Neuroradiol 1983;4:678-80. |
|21.||Philips DJ, Green FM Jr, Langlois Y, Roederer GO, Strandness DE Jr. Flow velocities in carotid bifurcation of young presumed normal subjects. Ultrasound Med Biol 1983;9:39-9. |
|22.||Whisnant JP, Homer D, Ingall TJ, Baker HL Jr, O'Fallon WM, Wievers DO. Duration of cigarette smoking is strongest predictor of severe extracranial carotid artery atherosclerosis. Stroke 1990;21:707-14. |
|23.||Costantino I, Gorelick Philip B. Hypertension, angiotensin and stroke: Beyond blood pressure. Stroke 2003;35:348-50. |
|24.||Perttu L, Risto R. Hyperglycaemia in acute stroke. Stroke 2004;35:363-4. |
|25.||Howard G, Burke GL, Monolio TA, Wolfson SK, O'Leary DH. Does the association of risk factors and atherosclerosis change with age. Stroke 1997;28:1693-701. |
|26.||Sun Y, Lin CH, Lu CJ, Yip PK, Chen RC. Carotid atherosclerosis, intima media thickness and risk factors-an analysis of 1781 asymptomatic subjects in Taiwan. Atherosclerosis 2002;164:89-94. |
|27.||Erickson SJ, Mewissen MW, Foley WD, Lawson TL, Middleton WD, Quiroz FA, et al. Stenosis of the internal carotid artery. Assessment using colour Doppler imaging compared with angiography. AJR Am J Roentgenol 1989;152:1299-305. |
|28.||Libby P. Vascular Disease. Harrison's principles of internal medicine. 14 th ed. In: Fauci AS, Braunwald E, Isselbacher KJ, Wilson JD, Martin JB, Kasper DL, et al. Editors. New York, USA: McGraw Hill; 1998. p. 1345-51. |
|29.||Brown MD, Zwiebel WJ, Call GK. Degree of cervical carotid stenosis and hemispheric stroke. Duplex US findings. Radiology 1989;170:541-3. |
|30.||Robinson ML, Sacks D, Perlmutter GS, Marinelli DL. Diagnostic criteria for carotid duplex sonography. AJR Am J Roentgenol 1988;151:1045-9. |
|31.||Grant EG, Duerinckx AJ, El Saden SM, Melany ML, Hathout GM, Zimmerman PT, et al. Ability to use duplex ultrasound to quantify ICA stenosis: Fact or fiction. Radiology 2002;214:247-52. |
|32.||Staikov IN, Nedeltchev K, Arnold M, Remonda L, Schroth G, Sturzenegger M, et al. Duplex sonographic criteria for measuring carotid stenosis. J Clin Ultrasound 2002;30:275-81. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]