Arjen Lindenholz

58 CHAPTER 3 A2, P1 and (very) proximal P2 segments. For all currently published vessel wall MRI sequences, including our own sequences, voxel size exceeds the vessel wall thickness of all arteries, and will result in overestimation of wall thickness. Lesions are visible, though, as they generally have a higher intensity or thicker appearance than the adjacent, apparently normal vessel wall. In our experience, most intracranial vessel wall lesions are detectable when scrutinizing the source data of the vessel wall MRI sequences, that is, when the images are acquired in a transverse (7T) or transverse oblique plane (3T). 43 However, multiplanar reformatting – when applicable – may increase diagnostic confidence of some lesions, including lesions of the M1 segment of the MCA, which is one of the few circle of Willis vessels visualized solely parallel to its orientation in a transverse slice ( Figure 1 ). Before using intracranial vessel wall MRI as a diagnostic tool in patients, one should become acquainted with the normal appearance of the arterial wall on vessel wall images ( Figure 1 and Figure 3 ). The normal vessel wall can be seen as a thin line surrounding the vessel lumen and is isointense to the brain parenchyma (white and grey matter have approximately the same signal intensity on vessel wall images because sequence parameters are specifically optimized for the vessel wall). Vessel wall MRI of healthy intracranial vasculature frequently suggests a variance in wall thickness that is most likely not related to vessel wall disease but due to partial volume effects. With normal ageing, the vessel wall has been shown to increase in thickness; however, the question remains whether this small increase in thickness can be visualized with the currently used spatial resolutions. 45 In younger patients without significant brain atrophy, the MCA is often located directly adjacent to the brain parenchyma, with little or no visible CSF in between. Because its signal intensity is similar to that of brain parenchyma in most sequences, it is often difficult to assess all segments of the MCA vessel wall (including possible lesions) ( Figure 3 ).