Arjen Lindenholz

10 CHAPTER 1 Intracranial vessel wall magnetic resonance imaging (MRI) is an emerging technique that has gained an increasing popularity in the last decade because of its potential applications in a variety of intracranial vascular diseases. 1-4 This thesis will address both technical aspects and developments of intracranial vessel wall MRI as well as indications and implications of this technique for clinical practice. A history of vessel wall MR imaging At the time when vessel wall MR imaging of the intracranial arteries was first introduced in 1995 using a 1.5 tesla (T) MRI platform, the technique had been limited to the larger extracranial arteries such as the vertebral and cervical carotid arteries. 5 These arteries could be readily assessed with vessel wall MRI due to their relatively large size, and they were of specific interest because of being a predilection site for atherosclerotic plaques, especially the carotids. 6,7 With vessel wall MRI important information beyond the lumen of the extracranial arteries could be perceived: not only the exact location and severity of the (stenotic) atherosclerotic plaque, but also the extent of the non-stenotic atheromatous part of the plaque that had not (yet) resulted in luminal narrowing due to outward arterial remodeling of the affected artery. 8,9 It had already been recognized through pathology and lumenographic studies that the presence of (unstable) atherosclerotic plaques could cause hemodynamically significant stenosis and thromboembolism, potentially leading to ischemic stroke or transient ischemic attack (TIA). The reliability of this MR imaging technique was further validated by histopathological examination of the material obtained through carotid endarterectomy procedures. 10,11 Over the subsequent decade, higher magnetic field strength MRI platforms like 3T – due to their higher attainable signal-to-noise ratio (SNR) and spatial resolution – facilitated more detailed atherosclerotic plaque imaging and the recognition of the ‘vulnerable’ plaque as atherosclerotic culprit lesion, further supporting the importance of vessel wall MR imaging in the diagnostic workup of cerebral ischemic events. Simultaneously, interest in visualizing the vessel walls of the intracranial arteries was growing as increasing evidence of predominantly lumenographic studies showed that intracranial atherosclerosis (ICAS) and stenosis also play an important causative role in ischemic stroke and TIA, especially in the Asian population. 12 The intracranial arterial vessel wall, however, was still a relatively unexplored area because visualizing these vessel walls posed several challenges given the deep location, the smaller size and tortuosity of the intracranial arteries. With the advent of 3T platforms and the development of higher-resolution extracranial vessel wall MRI sequences, new opportunities arose to visualize the sub-millimeter thin intracranial vessel wall as well. 13-15