Arjen Lindenholz

23 Clinical Vascular Imaging in the Brain at 7T 2 Introduction Vascular disorders of the brain, including stroke, are a major cause of death in addition to physical and cognitive disability. During the past decades, imaging has become an indispensable tool in the work-up, treatment planning and follow-up of ischemic and hemorrhagic stroke, as well as in the identification of cerebrovascular anomalies predisposing to stroke. In addition, imaging may record the burden of incidental cerebrovascular lesions that may lead to pathologic brain aging. Compared to CT and other imaging modalities, brain magnetic resonance imaging (MRI) is by far the best technique to assess the total extent of cerebrovascular diseases in individual patients, as it allows for the accurate visualization of acute and chronic manifestations of large- and small-vessel disease in both the supra- and infra-tentorial regions. Although routine clinical practice is currently still limited to standard (1.5T) and high-field (3T) MRI, cerebrovascular disease evaluation at ultra- high-field (7T) MRI may benefit from a high signal to noise ratio (SNR) which can be transferred into high spatial resolution, as well as a high contrast to noise ratio (CNR). By enabling the evaluation of the brain parenchyma on a submillimeter scale, very small cerebrovascular lesions, such as cortical microinfarcts, have come within the detection limit of 7T and to a lesser degree 3T MRI. 1 Also, compared to 1.5T arterial MR angiography (MRA), the use of high-field (3T) and evenmore so ultra-high field MRI (7T), has enabled the visualization of the lumen of much more peripheral intracranial vessels, and of the intracranial vessel walls of the circle of Willis (CoW) and beyond. 2,3 Finally, MR perfusion weighted imaging (PWI) may benefit from 7T, for instance by the increased susceptibility effects and the lower amount of contrast agent required for dynamic susceptibility contrast (DSC) perfusion at 7T. Thus, the advent of 7T has during the past decade resulted in a wave of research exploring new developments in cerebrovascular imaging, which are now increasingly finding their way into clinical practice. In the current article, we will review the emerging clinical applications and future directions of vascular imaging in the brain at 7T. Clinical applications A clinically feasible stroke imaging protocol at 7T has already been proposed and investigated for subacute and chronic stroke patients. 4 This imaging protocol includes T1-weighted 3D Magnetization-Prepared Rapid-Acquired Gradient-Echo (3D-MPRAGE), T2-weighted 2D Fluid Attenuated Inversion Recovery (2D-FLAIR), T2-weighted 2D Turbo Spin Echo (2D-T2-TSE), T2*-weighted 2D Fast Low Angle Shot Gradient Echo (2D-HemoFLASH) and 3D arterial Time-of-Flight (TOF) MRA. However, the imaging protocol excludes diffusion weighted imaging (DWI), the most sensitive imaging technique to detect acute infarctions. 4 With improvements being made to DWI at 7T, it may be expected that clinical stroke protocols at 7T imaging will be extended to include acute stroke patients as well.