Arjen Lindenholz

49 The Use and Pitfalls of Intracranial Vessel Wall Imaging 3 Field strength A further consideration is what field strength to use. In our experience and that of others, 2 1.5T does not achieve sufficient SNR and CNR within a reasonable acquisition time. Therefore, a field strength of 3T or higher is mandatory. In light of the increased availability of 3T MRI platforms, this makes intracranial vessel wall MRI feasible in many hospitals worldwide. The advantage of a 7T field strength is an increased SNR and CNR compared with 3T, which enables better intrinsic image contrast and a higher spatial resolution within reasonable acquisition times. 16,18 Specifically for vessel wall MRI, the higher SNR also enables the use of the inversion recovery pulse mentioned above to completely suppress CSF, and a large FOV for whole-brain coverage; however, this does come at the cost of a lower spatial resolution to keep acquisition times within limits. Nonetheless, although current 7T vessel wall MRI pulse sequences have a lower or comparable spatial resolution compared with published (and our own) 3T sequences, visualization of the vessel wall was still found superior at 7T, mainly due to higher CNR and more optimal CSF suppression. 45 However, only approximately 60 7T MRI platforms exist worldwide. Also, the inhomogeneous B 0 and B 1 fields can still hamper assessment of peripheral parts of the brain, such as in the Sylvian fissure, although this is improving over time due to the continuous development of the technology. Combined with the fact that 7T MRI has not yet been officially approved for clinical diagnostic imaging, 7T vessel wall MRI sequences are mainly used for research purposes and specific challenging clinical cases (e.g., cerebral vasculitis). Image contrast weighting Mirroring pulse sequence development in the extracranial carotid artery, pulse sequences with different image contrast weighting (e.g., T 1 -, T 2 - and PD-weighted) have also been developed for assessing the intracranial vessel wall. Theoretically, the ideal vessel wall MRI protocol would include images of all three image contrast weightings and a T 1 -weighted sequence after contrast administration to assess intracranial vessel wall pathology in the same fashion as has been done for years for their extracranial counterparts. 46 However, no clear evidence yet exists on the clinical relevance of multicontrast MRI protocols for intracranial vessel wall pathology. With the relatively long acquisition times of these vessel wall sequences, most of the currently used intracranial vessel wall MRI protocols rely on T 1 -weighted imaging, because of its benefits in the detectability of contrast enhancement after contrast administration and favorable imaging characteristics when distinguishing the vessel wall with its surrounding tissue and (if present) plaque components. 47