Arjen Lindenholz

188 CHAPTER 9 Over the last two decades, intracranial vessel wall MRI has seen a steady increase in popularity, both in research and clinical practice. While before 2010 very few articles addressing intracranial vessel wall MRI were published annually, since that time publications have increased exponentially, with more than 80 published articles in 2019. 1 Initially, efforts were set on technical developments to actually visualize the intracranial vessel wall with sufficient image quality, but when this was achieved a shift soon occurred towards potential clinical applications of intracranial vessel wall MRI, for instance in central nervous vasculitis (CNS) vasculitis and intracranial aneurysms. 2-8 The real value of intracranial vessel wall MRI in clinical practice, however, has still not been entirely elucidated, thereby limiting its routine application in the clinic, mainly because of a lack of validation studies and difficulties in translating imaging findings to pathology. The goal of this thesis was to explore the current state of technical developments and clinical indications of intracranial vessel wall MR imaging, and to make the first steps from research-based knowledge towards interpretation of the value of vessel wall imaging findings in the clinical setting. Optimizing vessel wall sequences for clinical use Intracranial vessel wall MRI sequences suffer from a relative long acquisition time: most clinically applicable 3T sequences have acquisition times varying between 5 to 10 min. 9 When both pre- and postcontrast vessel wall imaging is required, acquisition time is doubled which makes it even more difficult to implement vessel wall sequences in existing clinical imaging protocols with predetermined time slot lengths. Further, motion artefacts will deteriorate image quality when acquisition times are too long, especially in neurologically compromised patients for whom it may be more difficult to lie still. However, as image quality is a balance between acquisition time, spatial resolution, signal-to-noise ratio (SNR) and contrast-to- noise ratio (CNR), time reduction is in conflict with the highly-demanding technical requirements for vessel wall MRI. For clinical applicability, it is important that the trade-offs in sequence parameters (and, for that matter, acquisition of pre- and/ or postcontrast images) are chosen in such way that the clinical question can be answered. This implies that clinical vessel wall MR sequences may be patient- tailored or disease-tailored. In Chapter 4 a comparison was made between several 3T vessel wall MRI sequences, showing that acquisition time could be reduced by 30% compared with existing clinical sequences. Although the overall technical image quality of the most optimized sequence was lower, qualitative image assessment was still acceptable, which makes this vessel wall MRI sequence more clinically applicable to implement in existing protocols. Ongoing technical developments may allow for higher spatial resolution without significant acquisition time extension in the future. Hardware developments like more advanced receiver coils with a larger field-of-view and a higher number of receiver