Imaging of Cerebral Arterial Occlusive Disease: Do We Really Look for the Right Things?

Stroke is the main cause of disability in adults and represents the secondmost prevalentcause of death in the Western world. Cerebral arterial occlusive disease (CAOD) of the internal carotid artery (ICA) has been identified as a major trigger for stroke. Based on large-scale randomized trials published in the 1990s, the basis for determining the appropriate therapeutic approach and regimen for this disease is exact ICA stenosis grading, even though only 20% to 30% of strokes stem from high-grade stenoses. In those studies, digital subtraction angiography (DSA) served as the gold standard. Since then, numerous studies have compared all novel imaging techniques, such as color-coded Doppler sonography, magnetic resonance (MR) angiography, and computed tomographic (CT) angiography, as noninvasive alternatives to DSA. In general, it is accepted that these techniques provide high sensitivity and specificity for the detection and exact grading of ICA stenosis. However, for almost 20 years we have put our focus almost entirely on the morphological grading of ICA stenoses, despite the fact that most of the strokes arise from embolic material from the plaque and not from hypoperfusion due to the grade of the stenosis. A few years ago, modern imaging techniques began to be used to explore atherosclerotic plaques because correlations between plaque composition and final outcome in thesepatients had been reported. All available modalities have been used for this purpose, and small case series have initially reported enthusiastic results. However, no high-grade evidence trial has ever confirmed these preliminary results. To date, the real value of plaque imaging is still a matter of debate, mainly because actual techniques are not yet able to provide detailed information on the plaque composition with an image resolution acceptable for clinical use. An interesting approach to risk stratification is the calcium scoring of the ICA and the circle of Willis. Analogous to coronary plaque assessment, a higher degree of plaque calcification detected on CT without contrast medium would suggest a higher risk for potential cardiovascular or cerebrovascular events. The value of this approach is not yet determined, as data are not complete for coronary arteries let alone for the carotid arteries. Risk stratification for stroke should also include the hemodynamic effects caused by ICA stenosis. The evaluation of contrast enhancement in the brain can be performed by CT or MR imaging, yielding data regarding cerebral blood volume, cerebral blood flow, mean transit time, and time to peak mapping. The acquired information details the hemodynamic effects of ICA stenoses and offers a prognosis of therapeutic success in symptomatic patients based on the concept of cerebral hypoperfusion. Additionally, the diffusion-perfusion mismatch in symptomatic patients represents an interesting concept in clinical application. In theory, areas of reduced perfusion without diffusion flow voids are considered to be ideal for revasculariza-