Resolution dependence of vessel size index across various brain regions

Magnetic resonance imaging (MRI) excels at detecting quantitative changes in microvascular parameters such as cerebral blood volume, cerebral blood flow, and vessel size index (VSI), which are essential for diagnosing and monitoring cerebrovascular diseases. Absolute VSI estimation, often utilizing superparamagnetic iron oxide nanoparticles as contrast agents, relies on measuring transverse relaxation rates (∆R₂^⁎ and ∆R₂). This study systematically investigates the spatial resolution dependence of VSI using Monte Carlo simulations and in vivo rat brain MRI experiments. Monte Carlo simulations modeled randomly oriented vasculatures with various vessel sizes, revealing that ∆R₂ values are significantly higher at an in-plane spatial resolution of 125 × 125 µm² compared to lower resolutions, particularly for smaller vessels. In vivo experiments on 13 rats using a 7 T MRI scanner compared VSI measurements at spatial resolutions of 125 µm² and 250 µm² across different brain regions. Results indicated region-specific VSI variations, with smaller vessels showing more pronounced resolution-dependent changes. The corpus callosum and hippocampal formation regions in particular exhibited significant increases in VSI at lower resolutions. The observed variability is attributed to the differing sensitivities of ΔR₂^⁎ and ΔR₂ to vessel size. Corroboration between the experimental data and simulation findings emphasizes the necessity of optimizing spatial resolution to ensure accurate VSI quantification and enhance the precision of neuro-microvascular imaging techniques.