Arjen Lindenholz

47 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 of CSF flow. 32,33 At higher field strengths, such as 7T, the higher attainable SNR enables the use of previously mentioned inversion recovery pulse, resulting in nearly optimal CSF suppression. 15 Spatial resolution Due to a lack of in vivo - ex vivo correlation studies (see chapter ‘Future Prospects’), it is currently not clear what minimum spatial resolution is necessary to accurately diagnose intracranial vessel wall pathology, nor is it known what spatial coverage is clinically relevant. Previous histopathologic studies have shown the intracranial arterial vessel wall tovary in thickness from0.2 - 0.4mmfor thedistal internal carotid artery (ICA) to 0.2 - 0.3 mm for the middle cerebral artery. 34 However, radiological measurements have shown a larger vessel wall thickness of approximately 1.0 mm for both middle cerebral artery and internal carotid artery. 35 Several reasons can account for this difference, for example underestimation in histopathologic studies due to preparation techniques (cell shrinkage (8-20%) in histopathologic preparation), 36,37 or overestimation in the radiological studies due to partial volume effects (relatively large voxels), measurement errors and/or pulsatility effects of the vessel wall during acquisition. On the basis of these considerations, and given the fact that one needs at least two voxels in an object to measure its size/ thickness accurately, 38 an MRI sequence with 0.18 mm isotropic voxels – assuming a histologically processed wall thickness of 0.3 mm and a mean shrinkage effect of 15% – would theoretically enable highly detailed assessment of the circle of Willis and, depending on the field of view (FOV), its large and small branches. 34 However, it remains to be seen whether this much detail is really necessary in clinical practice in all patient groups. 39 Also, (ultra) high resolution sequences would be met with significant time constraints, limiting their application to cooperative patients with low morbidity, or necessitating either introduction of very fast imaging techniques or compromising between spatial resolution and FOV. To avoid these limitations, intracranial vessel wall MRI sequences in current practice are mainly aimed at detecting larger lesions in the intracranial arteries proximal and just distal to the circle of Willis (M1, A1, P1 segments) and at the border of the M1-M2, A1-A2 and P1-P2 segments. The acquired in-plane spatial resolution of these sequences ranges between 0.4 x 0.4 mm 2 to 0.9 x 0.9 mm 2 . 1,18,28,31,35,40,41 For more distal arteries, however, detection reliability will decrease because the diameter of these arteries and therefore lesions at the walls of these arteries become smaller, increasing the impact of partial volume effects, 35 which needs to be taken into account when assessing the vessel walls in clinical practice (see ‘Systematic approach of vessel wall assessment’) .