Neuroimaging studies for the diagnosis of carotid stenosis

Atherosclerotic occlusive disease of the carotids can be evaluated by the use of carotid duplex sonography, MRA, CTA or angiography. Non-invasive studies play an essential role in the diagnosis. However, all techniques require validation and quality control within individual centers to guarantee high specificity and sensitivity.

Carotid duplex examination for evaluation of the major extracranial arteries is performed by realtime B-mode studies and duplex and color flow imaging and has the ability to detect and characterize atherothrombotic plaque, measure intimal media thickness (IMT) and to determine the degree of vascular stenosis and collateral supply. IMT measurements, electron-beam computed tomography (EBCT,) magnetic resonance coronary angiography and flow-mediated arterial dilation by ultrasound are used to evaluate patients with subclinical atherosclerosis at risk for ischemic events early and to monitor the effectiveness of medical interventions [8],

Carotid duplex studies in combination with TCD provide information on flow dynamics, the presence or absence of collateral circulation and its effectiveness as well as an estimation of the cerebral vascular reserve. Furthermore they help detect em-boli which may originate from the heart, the aortic arch, or the carotid itself. In the near future advancement of this technology will allow the analysis of plaque constituents and markers of inflammation in order to assess the risk of an embolic stroke [7]. Current carotid and TCD studies are supplementary to MRA or CTA and are excellent tools when validated against cerebral angiography at the institution where the studies are performed and when used under strict guidelines provided by credentialing bodies and technical quality control. However, ultrasound is operator dependent and can overestimate the degree of carotid stenosis if a contralateral high grade stenosis or occlusion is present [15].

Cerebral angiography is now performed less frequently because it does not give exact assessment of plaque size and morphology which newer MRA and CTA based techniques are capable of producing. Cerebral angiography is associated with a combined mortality and morbidity of 0.5-4% in patients with atherosclerosis so that many medical centers today perform CEA based on MRA, CTA or carotid duplex studies alone. MRA has some limitations due to possible over-estimation of the degree of stenosis and the production of flow artifacts which might result in misclassification of patients. CTA is advantageous because it is not dependent on blood flow velocity providing more precise assessment of high grade stenosis but it requires intravenous contrast which can have serious side effects.

In the near future the development of higher resolution MRI, new contrast materials and biomark-ers for detecting chronic inflammatory changes in patients with substantial atherosclerotic disease will improve the assessment of patients at risk for ische-mic events [62], [69], [12], [70]. However, at this time the indication for CEA depends on the patient's co-morbidity, life expectancy, gender, symptoms and the degree of carotid stenosis measured by appropriate imaging studies. Precise measurement of the degree of carotid stenosis is essential to decide which patient benefits the most from CEA.

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