How Vascular Effects Contribute to Heavily Diffusion-Weighted FMRI Signal

Denis Le Bihan^1,2, Toshihiko Aso^1,2, Shin-ichi Urayama², Cyril Poupon¹, N Sawamoto², Kenji Aso², Hidenao Fukuyama²

Using heavily diffusion-sensitized MRI a transient decrease in water diffusion has been reported in the activated visual cortex of human subjects. The steep onset of the diffusion response and its temporal precedence relative to the BOLD fMRI response suggest a non-vascular origin. However, this assumption has been challenged by recent studies. The purpose of this work is to provide a theoretical framework which resolves the controversy. It is shown that during neuronal activation residual vascular effects only represent a fraction of the DfMRI signal. The diffusion component largely dominates at very large b values before hemodynamic events take place. The vascular-related tissue relaxivity component dominates the DfMRI signal at the end of the activation window.