الفهرس 
ANATOMY OF THE CAROTID ARTERYOnly 14 pages are availabe for public view
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Abstract
ontrast angiography has generally been proposed as a necessary preoperative study for carotid endarterectomy. However, the cost of contrast angiography, as well as the risk of stroke associated with this procedure, has encouraged the development of alternative non invasive imaging modalities, including MRA and Doppler Ultrasound, for the detection of significant carotid disease. Accurate evaluation of the carotid bifurcation has become particularly important in view of the North American Symptomatic Carotid Endarterectomy Trial (NASCET), which confirmed a benefit of carotid endarterectomy in symptomatic patients with 70% to 99% carotid stenosis.
Over three decades ago, Doppler was introduced as the first Ultrasonographic method for evaluation of cerebrovascular disease. Since then, the development of new ultrasound techniques has revolutionized the clinical applications for extra cranial carotid disease.
Gray scale Ultrasound is very useful to demonstrate the characteristics of plaques including its extension and echogenic texture and assessing the degree of stenosis in transverse scan using surface area reduction ratio especially in hemo-dynamically insignificant degrees of stenosis.
Color Doppler ultrasound is an accurate and reproducible test providing not merely morphological but also functional information by showing areas of aliasing through the stenotic segment and hence facilitates the site to be chosen to measure the higher possible velocities and decreases the examination time.
Using spectral wave analysis, many pulsed Doppler ultrasound indices are used to evaluate stenosis; the most accurate one is the peak systolic velocity of ICA to peak systolic velocity of CCA ratio. Absolute velocity measurements are not sufficient to diagnose accurate degree of stenosis.
Limitations, to pre-operative use of Doppler ultrasound alone, are based on possible missing of an existing intra luminal thrombus, inadequate identification of ulceration, in addition Doppler ultrasound can only assess the CCA and the cervical portion of ICA. Ultrasound examination is further augmented if the patient has a short neck, a high bifurcation or heavily calcified plaques.
Carotid MRA is non invasive, simple, does not expose patient to ionizing radiation, and is not operator dependent, MRA has potential value in detecting disease outside the ultrasonic window, including its use in determining the level of a carotid stenosis and, possibly, in uncovering tandem lesions as it is a useful screening technique that reveals intracranial carotid stenosis, occlusion, add to this, it depicts intracranial vascular anomalies and aneurysms.
The use of three-dimensional MR angiogram can provide better visualization of cervical carotids from any angle desired and may be as or more accurate than DSA for determining the degree of stenosis and with the development of new techniques such as black blood imaging techniques for direct plaque imaging, MRA will have a superior capability for discriminating tissue characteristics to improve the accuracy of identifying the lipid rich necrotic core, acute intra plaque haemorrhage and improves the depiction of intimal calcifications and fibrous cap integrity or rupture.
Our results suggest that MRA has a better discriminatory power compared with DUS in recognizing 70% to 99% stenosis and is sensitive and specific test compared to DSA. For detecting carotid artery occlusion both modalities are very accurate.
The use of conventional angiography should be limited to cases where discordance between results of Doppler ultrasound and MRA occur, and not for every case.