Evaluating the Static Dephasing Regime Assumption for Gradient Echo Signal in Brain Cells

Motivation: The static dephasing regime, which neglects diffusion effects, is typically assumed for modeling magnetic susceptibility effects within a voxel. Given the complexity of brain microstructure, this assumption warrants further investigation. Goal(s): This work aims to assess the validity of the static dephasing regime under varying diffusion conditions within two brain cell types. Approach: Monte Carlo simulations were performed to model spin diffusion around myelinated axons and astrocytes under different diffusion conditions. Results: Spin phase accumulation and decay rates around astrocytes and axons show significant deviations from the static dephasing assumption, particularly in regions with restricted/hindered diffusion and high axonal volume fractions. Impact: The static dephasing assumption, commonly used to model magnetic susceptibility effect within a voxel, may fail in brain microstructure, particularly in regions with restricted/hindered diffusion and high axonal volume fraction, highlighting the necessity of incorporating diffusion effects.