2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

Temporal Dynamics of BOLD fMRI at the Microscopic Level

C.K. Hadjistassou, K. Moyle and Y. Ventikos
University of Oxford, UK

Keywords:
microscopic, oxygen transport, haemodynamics, convection, diffusion

Abstract:
With the prospect of elucidating the Blood Oxygenation Level Dependent (BOLD) fMRI effect we consider cerebral oxygen transport at the microscopic scale. We concentrate on the transient response of a capillary-tissue system following brain activity. Model findings cast new light on the dynamics of BOLD fMRI. Diffusion and convection in the axial and radial directions are accounted for, over a range of blood velocities and cerebral oxygen utilisations (CMRO2). The model consists of a capillary, measuring 8 m in diameter by 200 m in length, surrounded tissue 25 m thick. The non-linear kinetics of oxygen release are modelled using the oxygen dissociation curve. Global oxygen conservation is ensured by coupling the oxy/deoxyhaemoglobin and free oxygen concentrations. Model sensitivity is quantified through a systematic study considering a range of blood velocities and CMRO2 values, with results showing good agreement with earlier studies. The transient response of the system to neuronal activation is quantitatively mapped and used to investigate various hypotheses from the literature. We show that an increase in CMRO2 requires a concomitant rise in blood velocity, suggesting a coupled CMRO2-blood flow rate dependence. The time lag between the stimulus onset and the haemodynamic response (and diffusion and convection) appear promising in explaining BOLD.

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