212 M. Günther Introduction The human arterial tree system is crucial for the deliv- ery of oxygen and nutrients to the cells. For critical or- gans like the brain and the spinal cord the vascular sys- tem is designed in a way to ensure sufficient oxygen transportation to this particular tissue. This is usually achieved by having backup vessels (so-called collater- als) and connections (communicating arteries) be- tween major arteries like in the circle of Willis at the base of the human brain. In equilibrium, only a small amount of blood is transported through those collater- al and communicating vessels. But in case of an acute pressure drop in one region blood flow through this backup system will increase and ensure sufficient sup- ply of the tissue. Even in case of non-acute changes of pressure relationships in cross-connected vessels this auto-regulation will adapt to the actual flow pattern and can help to level under-supply in certain regions. Figure 1 shows a schematic drawing of the circle of Willis as it is commonly presented in educational books. However, this is not the whole truth. Although the human arterial tree is very similar in most individ- uals, there exist several variants, which differ in (sometimes important) details. Studies of the circle of Willis based on magnetic resonance angiography (MRA) have revealed a huge amount of variants (Krabbe-Hartkamp, van der Grond et al. 1998), where certain connecting vessels are missing or very small (hypo-plastic). How can state-of-the-art techniques be employed for proper assessment of an individual’s vascular tree anatomy and function? The obvious approach is to im- age the arterial tree of individuals to acquire knowl- edge about the actual realization of the vessel struc- ture. As a non-invasive medical imaging technique, which does not employ ionizing radiation, magnetic resonance imaging (MRI) is a tool routinely used in such cases. Due to its flexibility and broad range of contrasts MRI enables a proper depiction of the arteri- al tree even without the use of endogenous contrast agents. This article will briefly summarize recent de- velopments in noninvasive measurement of macro- Magnetic resonance techniques to measure distribution of cerebral blood flow M. Günther 1,2 1 mediri GmbH, Heidelberg, Germany; 2 Neurologische Klinik, Universitätsklinikum Mannheim, Universität Hei- delberg, Germany Applied Cardiopulmonary Pathophysiology 13: 212-218, 2009 A2 ACA A1 ACo MCA PCA BA P1 P2 PCo ICA A2 ACA A1 ACo MCA PCA BA P1 P2 PCo ICA Figure 1. Schematic diagram of the vessels that form the circle of Willis: The system is fed by left and right internal carotid arteries (ICA) and by the basilar ar- tery (BA). "The pre-communicating segments (Al) of the right and left anterior cerebral arteries (ACA) and an anterior communicating artery (ACo) between them form the anterior part of the circle. The pre-com- municating segments (P1) of the right and left posteri- or cerebral arteries (PCA) form the posterior part of the circle together with the right and left posterior communicating arteries (PCo). The right and left pos- terior communicating arteries originate from the right and left internal carotid arteries (ICAs). The post- communicating portions of the anterior and posterior cerebral arteries are A2 and P2, respectively. MCA = middle cerebral artery". (Krabbe-Hartkamp, van der Grond et al. 1998)