How the brain is fed and drained: cerebral vascular geometry and its implications for BOLD spatial and temporal resolution
The Wellcome Department of Imaging Neuroscience, UCL , UK
The vascular supply and drainage systems of the brain have an intimate relationship with the neuronal and glial tissue that they irrigate. Changes in localized neuronal activity generate localized vascular responses, regulated by chemical messengers consequent upon neuronal activity rather than by metabolic signals, although these responses also ensure metabolic homeostasis. Their temporal and spatial precision, critical to their use in functional neuroimaging, depend on the co-ordinated mechanisms of cerebrovascular coupling (Segal and Duling, 1986), and on the geometry of the cerebral vasculature. This geometry, which is continuously self-optimizing via demand-driven angiogenesis (Lamanna et al, 2004), results in differing spatiotemporal characteristics for changes in perfusion, blood velocity, blood volume and blood oxygenation.
Measurements of typical human brain surface vasculature were combined with general mathematical considerations (Herman et al, 2001, Grasman et al, 2003) for optimal supply and drainage of a two-dimensional manifold supplied and drained by one-dimensional pipes, to provide a model for the cortical surface vasculature (Turner 2002). This model allows estimation of the downstream dilution of local blood oxygenation and velocity changes, and of the downstream changes in dispersion of the haemodynamic response (de Zwart et al, 2005). How far BOLD signal can extend beyond an area of changed neuronal activity is shown to depend on the size of that area, and the haemodynamic response becomes slower and broader downstream. Principles for improved design of BOLD fMRI experiments are deduced.