Arjen Lindenholz

45 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 Introduction In recent years, intracranial vessel wall magnetic resonance imaging (MRI) has seen an exponential increase in popularity and clinical applicability. 1,2 Several years ago, vessel wall imaging was restricted to extracranial (peripheral) arteries, such as the carotid arteries, in which atherosclerotic ‘vulnerable plaques’ – prone to causing embolism and subsequent ischemic stroke – could be assessed using different dedicatedMRI sequences. However, increasing evidence shows that not extracranial but intracranial atherosclerosis (ICAS) is the leading causes of ischemic stroke worldwide. 3-6 In addition, ICAS has been associated with an increased (recurrent) stroke risk and vascular dementia. 4,7-9 This change of insight into causes of ischemic stroke has led to a trend towards imaging the intracranial vasculature. Traditional imaging methods for visualizing the intracranial arteries include intra- arterial digital subtraction angiography (DSA), computed tomography angiography (CT/CTA), Doppler ultrasonography (US) and magnetic resonance angiography (MRA). DSA can detect stenotic lesions in large, as well as small arteries, including the A2 segment of the anterior cerebral artery, M2-M3 segments of the middle cerebral artery (MCA), P2 segment of the posterior cerebral artery, and even more peripheral segments of the intracranial arteries. With unenhanced CT arterial vessel wall calcifications can be detected that are associated with future stroke risk. 10-13 CTA is an easily accessible and fast procedure in patients with acute stroke and can depict stenotic lesions very accurately in the more proximal intracranial arteries and its branches. 14 Doppler ultrasonography can depict the vessel wall of primarily the middle cerebral artery (MCA), thereby providing (single-artery restricted) information on vessel wall pathology. Finally contrast material- enhanced and time-of-flight MRA, like CTA, can depict stenotic lesions in proximal cerebral arteries, less quickly but without additional radiation dose compared with CTA. However, all of these techniques – except for Doppler US – have the disadvantage of only depicting the arterial lumen (including possible stenoses) and not the culprit vessel wall itself. Intracranial vessel wall MRI has two major advantages over DSA, CTA and MRA: I. It can depict non-stenotic lesions; and II. It can further characterize stenotic lesions that have already been detected with common angiographic methods. In this ‘How I do It’ article, we describe the utility and practical steps for performing and interpreting intracranial vessel wall MRI scans. These practical steps are based on our teaching experience of intracranial vessel wall MRI to radiologists and neuroradiologists who have started using this method in daily clinical practice. The key questions that are often raised during their learning curve will also be addressed in this article. Key steps and common vessel wall pathology will be shown in illustrative figures of patient examples.