Direction-Dependent Diffusion fMRI Signals During Hypercapnia & Hyperoxia

Tao Jin¹, Seong-Gi Kim¹

Recently, diffusion-weighted fMRI (DfMRI) signals of brain water were reported to increase with the degree of diffusion sensitization during human visual stimulation, indicating a decrease of the apparent water diffusivity; however, the interpretation of the signal origin was controversial. Le Bihan et al. attributed the activation-induced change of the apparent water mobility to a functional expansion of neuronal cell membrane. Miller et al. found similar DfMRI signal change during a hypercapnia challenge, which was also dependent on the direction of diffusion-sensitizing gradients, thus suggesting that the DfMRI signal change might be due to residue intravascular signals. In this preliminary study, we measured the direction-dependent change of DfMRI signal i) during intravascular susceptibility change without changes in vascular physiology by the intravascular injection of a small amount of iron oxide nanoparticle, and ii) during global hypercapnia and hyperoxia stimulations in anesthetized rats after the suppression of the intravascular signals.