Arjen Lindenholz

25 Clinical Vascular Imaging in the Brain at 7T 2 In a considerable proportion of patients, however, the origin of stroke may be related to intracranial arterial lesions, evaluation of which may especially benefit from high-resolution MR imaging. Not unlike arterial lesions in the neck, intracranial arterial lesions include atherosclerotic plaques, and, more rarely, dissection or vasculitis. Intracranial atherosclerosis Arterial TOF-MRA is routinely performed in stroke patients to detect intracranial arterial stenosis, and is usually preferred above angiographic techniques (CTA and DSA) because of its non-invasiveness, the lack of ionizing radiation, and no need to administer a contrast medium ( Figure 2 ). Also, it can be acquired in the same imaging session as DWI, the most sensitive technique to detect acute infarction. Compared to 1.5T and 3T, arterial TOF-MRA at 7T allows the visualization of much smaller intracranial arteries, such as the lenticulostriate arteries ( Figure 2 ), while adding a contrast agent may enable the visualization of even more distal arteries in addition to veins and venules. 2,4,5 The presence of intracranial arterial narrowing alone, however, does not necessarily correspond to a stroke origin; arterial stenosis may already have been present a long time before the onset of stroke, and may be compensated by adequate primary (circle of Willis) or secondary (leptomeningeal) collaterals. 6,7 In addition, atherosclerotic plaques may be present without luminal narrowing due to arterial remodeling. 8 Because of the shortcomings of lumenography, there is a need for intracranial vessel wall imaging to link cerebral infarction with intracranial arterial lesions, such as symptomatic plaques, dissection or vasculitis. Because of the small caliber of intracranial arteries, a high SNR which can be transferred into high spatial resolution, as well as a high CNR are required for visualization of the pathologic vessel wall, and even more so for the healthy vessel wall ( Figure 3 ). 3 Since spatial resolution increases with field strength, (ultra-)high field imaging techniques are required to visualize wall thickening of arteries of the circle of Willis and beyond. 3 Also, for optimal vessel wall visualization, signal suppression of the arterial lumen (black blood imaging techniques including double inversion recovery and techniques based on motion-sensitizing prepulses) is required for delineation of the inner vessel wall ( Figure 3, A and B ), while cerebrospinal fluid (CSF) suppression facilitates the demarcation of the outer vessel wall ( Figure 3B ), especially for the more peripheral vessels surrounded by subarachnoid (CSF) spaces in case of cerebral atrophy. 3 However, acquisition of 3D isotropic sequences with high resolution and sufficient brain coverage result in relatively long scan times ( Figure 3 ). 3,9 Compared to 3T imaging, the increased signal-to-noise ratio (SNR) of 7T leads to an overall better vessel wall visibility, visualizes more atherosclerotic plaques, and thus offers the highest potential to identify the total burden of intracranial atherosclerosis. 10,11 Recently, several studies investigating the relationship between intracranial vessel