The resolution dependence of MRI vessel size index varies across brain regions.

Motivation: Vessel size index (VSI) MRI of the rodent brain is measured at various in-plane resolutions, but the resolution dependence of VSI has not yet been explored. Goal(s): Here, we investigated the differences in rat brain VSI at mostly measured in-plane resolutions of 125μm2 and 250μm2. Approach: Resolution-dependent differences in VSI across brain regions were investigated in in-vivo rat experiments via a steady-state susceptibility contrast method by injection of monocrystalline iron oxide nanoparticles and validated through Monte Carlo simulations. Results: In the white matter and hippocampus regions, the VSI was measured to be 12% larger as the resolution was lowered from 125μm2 to 250μm2. Impact: Because the resolution dependence of VSI quantification varies across brain regions depending on the vascular configuration within MRI voxels, caution is required when comparing and analyzing brain VSI MRI obtained at different resolutions.