Arjen Lindenholz

192 CHAPTER 9 between the larger and smaller intracranial arteries that – although they differ on an anatomical and pathophysiological level – comprise one vessel bed that is physiologically connected. Implementing intracranial vessel wall MRI in clinical decision-making Considering several remaining technical and clinical challenges, intracranial vessel wall MRI could have tremendous potential for neurovascular imaging, by directly visualizing vessel wall pathology and not just the lumen. As mounting evidence suggests a causative relation between intracranial vessel wall lesions and cerebral (vascular) damage like ischemic stroke, the question arises what role vessel wall MRI should have in the clinical diagnostic process and how this may help patients in a clinical setting. 53 Currently, intracranial vessel wall MRI can function as a diagnostic tool to assess the actual status of the intracranial vessel wall, i.e., the (longitudinal) result of the complete set of risk factors on the vessel wall. Consequently, it could be used for individual risk assessment of intracranial atherosclerosis and in the future may guide patient-tailored (medical) treatment for preventive cerebrovascular management. It might also aid in determining the cause of ischemic stroke, especially in young patients or in those with a previously undetermined cause of ischemic stroke (or TIA): a recent study has shown that the conceived cause of ischemic stroke or TIA is often adapted when applying intracranial vessel wall MRI. 54-56 Furthermore, ongoing efforts to visualize atherosclerotic plaque components – like calcifications, intraplaque hemorrhage (IPH), lipid core and fibrous caps – to identify predictors of atherosclerotic plaque vulnerability could aid in deciding treatment aggressiveness. 2,21,24,28,57-59 Currently, most data on vulnerable plaques stem from carotid artery imaging studies and ex vivo intracranial atherosclerotic plaque characterization studies, as in vivo spatial resolution of intracranial vessel wall MRI sequences is often too low for accurate delineation of these individual components. 24,60,61 However, recently several studies have shown examples of signal intensity differences within vessel wall lesions that potentially reflect these individual plaque components, albeit without histopathological validation of these signal intensity differences. 2,21,57,58,62-64 A meta-analysis that included only intracranial vessel wall MRI studies showed that contrast enhancement of the atherosclerotic plaque, arterial remodeling and plaque irregularity were associated with a higher risk of stroke; remarkably, well- established vulnerability predictors for extracranial carotid plaques such as IPH or large lipid core were not associated with vulnerability. 58 These studies suggest a potential role of intracranial vessel wall MRI in predicting future cerebral ischemic events; nonetheless, the varying results, mostly based on small sample sizes and retrospective designs, currently preclude systematically applied in vivo plaque characterization in clinical practice. 1,2,58