Assessment of hemodynamic effects in functional diffusion-weighted MRI

Introduction: Recently, diffusion-weighted MRI methods to detect stimulus induced neuronal activation have been proposed. These techniques are expected to be less susceptible to hemodynamic changes. Though efforts are underway to understand the physiological origins of the observed diffusion changes in the brain, there have been indications that in studies focusing on gray matter (GM), it would be hard to separate vascular and extra-vascular changes, even at high b-values (2400s/mm). But, given that vasculature is less pronounced in white matter (WM), and that WM is more directionally organized, diffusion related protocols may be gainfully employed there. One such functional DTI (fDTI) study in WM has been reported. But, there is little known about the possible vascular effects in WM on such fDTI measurements. This study aimed at testing the possible vascular contributions in fDTI in WM and GM with relatively high b-value and with differing diffusionweighting directions. This is important in order to distinguish hemodynamic responses from possible extra-vascular responses to stimuli. We chose carbogen (5%CO2+ 95%O2) challenge, which induces vasodilation without neuronal alterations, as a paradigm to test the influence of hemodynamic changes on diffusion measurements.