The ability to visualize the perfusion territories of the brain feeding arteries is important for many clinical applications. Recently, selective arterial spin labeling (ASL) MRI has been introduced as the first method capable to visualize the perfusion territories of the brain in-vivo. In this thesis we describe the possibilities of selective ASL MRI and show the importance of perfusion territory information in studying the cerebral circulation in both patients with and without steno-occlusive disease. This most important findings of this thesis are twofold. Firstly, we demonstrate that the perfusion territories of the brain feeding arteries are considerably variable. Secondly, the variation in perfusion territories is mainly caused by anatomical variants of the circle of Willis, large artery steno-occlusive disease, or the combination of both. Thus far, the cerebral vascular territories are generally described as relatively invariant. Numerous standard atlases and textbooks show schematic drawings of the ‘normal’ territorial distribution. Most of these drawings are based on combinations of postmortem studies and assume a symmetrical and negligible variable territorial distribution. In contrast to these post-mortem studies, we demonstrated in-vivo that the variability of the cerebral perfusion territories is significantly greater than was previously assumed. The finding that the configuration of the circle of Willis strongly affects the extent of the cerebral perfusion territories seems relevant since up to 65% of healthy control subjects have an anatomical variant type. In addition to the large variability at the level of the circle of Willis, our results demonstrated that the presence of a severe stenosis or occlusion at the level of the arteries in the neck has major consequences for the distribution of the cerebral perfusion territories. Obstructive arterial disease at the level of the arteries in the neck is found in about 8% in the general healthy population, up to 30% in patients with symptomatic cerebral ischemia. The large variability of the cerebral territorial distribution demonstrated in this thesis has major implications for the clinical diagnosis and treatment of stroke. For example, physicians considering whether to treat acute stroke often use anatomic CT or MR images to assess affected vascular territories, and to determine whether infarction is embolic or hypotensive in nature. However, the results of this thesis demonstrate that neither the territories affected nor the nature of stroke can be accurately diagnosed on the basis of such anatomic studies. Currently used schematic drawings of the cerebral flow territories are based on standard atlases, and therefore give no certainty on the extent of the territories in the individual patient. To know more accurately the location of the perfusion territories, one should visualize them. In conclusion, the interaction of stenosis severity, multi-vessel disease, and vascular anatomy defines the location and the extent of the perfusion territories of the brain feeding arteries. To relate focal brain lesions to underlying perfusion territories in individual cases, knowledge of the territorial distribution is essentia
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