Investigating and understanding the complex blood supply of the brain in health and disease has been a major challenge for scientists for many centuries, as in-depth, detailed knowledge of the intracranial vascular anatomy and cerebrovascular physiology is ultimately essential for accurate diagnosis and therapy. With the emerging availability of medical imaging and rapid technological advances, in particular concerning non-ionising magnetic resonance (MR) imaging techniques, brain imaging has become a firmly established tool for this purpose in both the clinical and research settings.

However, current knowledge of the cerebral circulation still remains limited. Clear evidence of the anatomical differences between distinctive ethnic populations, particularly relating to the morphologic heterogeneity of the intracranial arterial network and the impact of its vari- able structure on intracranial haemodynamics, cerebral blood flow, and the pathogenesis of cerebrovascular disease is still lacking, while the continuous adoption and increasing avail- ability of non-invasive neuroimaging techniques has raised specific new challenges, such as the identification of incidental imaging findings.

This thesis presents the first comprehensive evaluation of the vascular architecture of the Circle of Willis and its variation in a single tri-ethnic population-based sample including a total of 750 individuals, using standardised high-resolution 3-tesla time-of-flight MR angiography. In addition, this work investigates the population prevalence and risk factor profiles of incidental MR angiographic intracranial arterial imaging findings in a multi- ethnic cohort. In a subsample of 360 subjects, pseudo-continuous arterial spin labelling MR is further used to investigate global cerebral perfusion by evaluating blood flow and the pres- ence of arterial transit artefacts, and the impact of the vertebrobasilar vascular morphology and other cardiovascular factors on cerebral blood flow and artefact manifestation in the posterior circulation are explored.

As a whole, this work delivers novel and unique insights into the intracranial vascular anatomy and haemodynamic physiology as well as into the prevalence of large vessel cerebrovascular disease as part of a multi-ethnic population-based investigation, presenting new evidence for morphological and pathophysiological differences between women and men, and between different ethnic populations.