Arjen Lindenholz

33 Clinical Vascular Imaging in the Brain at 7T 2 one hand and blood flow in the perforating arteries on the other hand. 91,92 BOLD fMRI, which is typically used for task-related and resting state fMRI experiments, may be adapted to measure cerebral perfusion and cerebrovascular reactivity (CVR) before and after a certain vascular challenge, such as the inhalation of carbon dioxide, breath-holding or medication administration (acetazolamide). 72 These stimuli all result in an increased blood flow to the brain and a decreased BOLD signal, and it is generally assumed that brain regions with a decreased signal change upon a certain challenge have a lower cerebrovascular reserve and are at an increased risk of future ischemia. 72 At 7T, BOLD fMRI may detect differences in cerebrovascular reactivity at a millimeter scale, and may be applied to study the association of CVR with lacunes, white matter hyperintensities and microbleeds. 93-96 Despite all technical advancements, 7T still faces some practical challenges. First, technical improvements are needed to counter the increased magnetic field (B0) and radiofrequency field (B1) inhomogeneities ( Figure 7 ). These inhomogeneities cause signal loss and a variable flip angle, most prominently in the temporal ( Figure 7 ) and cerebellar regions, which not only hinders the evaluation of these regions in individual subjects, but also hampers group analyses of 7T datasets. The use of dielectric pads may partly overcome this problem. Upgraded head and neck coil technology with larger coverage and more homogenous magnetic fields are required for arterial spin labeling (ASL) perfusion imaging, which at present is still preferentially performed at 3T. 72 Whole brain diffusion MRI also is still challenging at 7T due to magnetic field inhomogeneities, shorter relaxation times, and increased power deposition (specific absorption rate (SAR)), although progress is currently being made to acquire high-quality and reproducible DWI and diffusion tensor imaging (DTI) datasets. 4,97-99 Finally, promising results on implant safety – which has long been a major drawback for clinical application – have shown no increased risk of radiofrequency heating of metallic implants. 100 These safety results and the growing evidence of an additional clinical value of 7 T MRI will pave the way towards certification for (regular) clinical applications in the near future. 4