Brain tissue water comes in two pools: evidence from diffusion and R2' measurements with USPIOs in non human primates

Diffusion-weighted MRI of non-human primates revealed that USPIO Bulk Magnetic Susceptibility (BMS) T2' effects of Ultrasmall Superparamagnetic Particles with Iron Oxide (USPIO) in the brain cannot be explained by a single compartment model, as diffusion and T2' effects appear coupled: Apparent Diffusion Coefficient (ADC) values depend on USPIO concentration and relaxivity effects of USPIO decrease with the b value. On the other hand, USPIO and diffusion effects could be well uncoupled using a model consisting in a fast and a slow diffusion pool with different relaxivities. Diffusion-weighting acts as a filter which emphasizes the contribution of the slow pool when increasing b values (apparent decrease in ADC and R2'). Those results have implications for human studies using BMS contrast agents, as well as BOLD and diffusion fMRI.