Arjen Lindenholz

189 General Discussion 9 channels, or advanced pulse sequence designs, such as compressed sensing, different blood and cerebrospinal fluid suppression techniques or more efficient k -space sampling trajectories may further optimize intracranial vessel wall MRI for use in clinical MRI protocols. 4,8,10-14 Validation of intracranial vessel wall findings When assessing intracranial vessel wall MR images, the vessel wall is basically being appreciated on two characteristics: thickening and enhancement. Vessel wall thickening is further specified by configuration (eccentric or concentric) and whether it has led to stenosis. Enhancement is similarly specified by configuration, and whether it follows a specific pattern (e.g., entire vessel wall, interleaving normal vessel wall segments or partly enhancing vessel wall thickening). 4 For clinical implementation, it is essential that these vessel wall changes correlate with true abnormalities; histopathological validation of the detected vessel wall lesions is therefore crucial. Contrary to most extracranial arteries, tissue samples from intracranial arteries (aside from surgically removed aneurysms and sporadic vessel biopsies) cannot be obtained while the patient is alive. 15-18 The most ideal alternative is histopathological validation using tissue from deceased patients who consented to an autopsy procedure and underwent in vivo intracranial vessel wall MRI while still alive. So far, only a few studies using this setup have been published; with the increasing use of intracranial vessel wall MRI, this number may increase in the near future. 19-21 A less ideal but an easier to perform alternative uses solely ex vivo MRI and histopathological examination, variably performed at 1.5T, 3T and 7TMRI. 21-26 Studies using this approach showed a clear correlation between intracranial atherosclerotic plaques at histopathology, and the presence of vessel wall lesions on MR images. Its advantages are a higher achievable spatial resolution and lack of pulsatility ormotion artefacts; however, fixation effects (such as cell shrinkage) and tissue temperature can change the MRI characteristics of the tissue. In addition, functional measures of the vessel wall like enhancement cannot be performed in postmortem tissue. This makes histopathological validation of certain vessel wall pathologies, such as inflammatory vasculopathies that show predominantly vessel wall enhancement in typical patterns, more difficult. Furthermore, other vasculopathies like intracranial aneurysms, moyamoya disease and post intra-arterial thrombectomy changes like those reported in Chapter 7 have not been histopathologically validated in this way yet. A third validation option is to extrapolate findings from extracranial vessel wall MRI, for which histopathological validation is much easier, for example because of the availability of carotid endarterectomy specimens. 21,27 This however needs to be done with caution because the intracranial vessel walls are anatomically and histologically different from extracranial arteries. 28